Roofs Archives » Building Regulations South Africa
Nov 242013
 

Engineers Design Buildings for a Purpose

Riga Latvia Mall Roof Engineers Specs Are Vital

A roof garden can clearly be seen under construction on the roof of the mall in Riga, Latvia

Engineers around the world will design a structure based on the brief given to them by a developer. The basis for any good design is to have a structure that is “fit for the purpose” for which it is intended. Most international building codes, and this includes our South African National Building Regulations, are very specific in terms of requirements. For instance, our National Building Regulations (NBR) state:

“Any building and any structural element or component” must be designed to “provide strength, stability, serviceability and durability”.

It is also vital that buildings are designed so that if the structural system is in any way overloaded they won’t collapse with disastrous consequences.

The Regulations also state that these design requirements shall be “deemed to be satisfied” when buildings are designed in accordance with the relevant Part of SANS 10400, in this case, Structural Design.

Roof Design of Riga Mall

Even though the devastating collapse of the roof at Riga Mall in Latvia on Saturday November 23, 2013 is currently under investigation (as at 24th November) there is some certainty that a roof garden being constructed on the roof of the mall played a significant role.

The engineers who designed the mall roof clearly did not intend for the roof structure to carry the extra weight that was being imposed on it. Tons of soil, concrete pavers and possibly water features, as well as many people walking on the roof, would put tremendous additional strain on a structure that was not engineered to carry such loads.

So why are people surprised it collapsed?

 

Mar 112013
 

Fenestration and Glazing Guidelines, Procedures and Calculations.

Windows house335 Fenestration Calculations

The guidelines featured in the pdf below give an idea to anyone wanting to calculate the Fenestration Compliance Procedures in terms of Sans 10400-XA:2011 and SANS 204:2011, what is required and what is involved.

There is a step-by-step guide to area (nett floor) calculation with reference to the parts of the regulations that apply. A SGHC (solar heat gain calculator) is also supplied, to calculate the heat conducted in and out of a building. There are a couple of “real life” calculations at the end of the document that illustrate how this was achieved.

You will see that the calculations that need to be done to comply with the Regulations are not at all straightforward. But you do, in any case require a “competent person” to draw up your building plans, submit them to the local authority, and take responsibility for the project (including ensuring that construction is in accordance with the plans). A competent person should be well equipped to interpret fenestration and glazing guidelines as well as procedures and the calculations. If not, it is the responsibility of that person to include someone who is in the project.

Download (PDF, 250KB)

Mar 052013
 

Regulations for Lightning Conductors on Thatch Roofs

Thatch house lightning Thatch Roofs and Lightning

Thatch roofs are most susceptible to be set alight by lightning than any other roof type. For the protection of the public and property the South African National Standard 62305-3 was introduced in 2011.

SANS 62305-3: Protection against Lightning (published in 2011) is drawn from an international standard, IEC 62305. Part 3 deals with “physical damage to structures and life hazard”.

Remember that anything related to electrics must be dealt with by a qualified and registered electrician.

Introduction to the Regulations for Thatch Roofs and Lightning

This part of IEC 62305 deals with the protection, in and around a structure, against physical damage and injury to living beings due to touch and step voltages.

The main and most effective measure for protection of thatch structures against physical damage is considered to be the lightning protection system (LPS). This usually consists of both external and internal lightning protection systems.

An external LPS is intended to:

  1. intercept a lightning flash to the structure (with an air-termination system),
  2. conduct the lightning current safely towards earth (using a down-conductor system),
  3. disperse the lightning current into the earth (using an earth-termination system).

An internal LPS prevents dangerous sparking within the structure using either equipotential bonding or a separation distance (and electrical insulation) between the external LPS components and other electrically conducting elements internal to the structure.

The main protection measures against injury to living beings due to touch and step voltages are intended to reduce the:

  1. dangerous current flowing through bodies by insulating exposed conductive parts, and/or by increasing the surface soil resistivity,
  2. occurrence of dangerous touch and step voltages by physical restrictions and/or warning notices.

The type and location of an LPS should be carefully considered in the initial design of a new structure, thereby enabling maximum advantage to be taken of the electrically conductive parts of the structure. By doing so, design and construction of an integrated installation is made easier, the overall aesthetic aspects can be improved, and the effectiveness of the LPS can be increased at minimum cost and effort.

Once construction work on a site has started, access to the ground and the proper use of foundation steelwork for the purpose of forming an effective earth-termination, may well be impossible. Therefore, soil resistivity and the nature of the earth should be considered at the earliest possible stage of a project. This information is fundamental to the design of an earth-termination system and may influence the foundation design work for the structure.

Regular consultation between LPS designers and installers, architects and builders is essential in order to achieve the best result at minimum cost.

If lightning protection is to be added to an existing structure, every effort should be made to ensure that it conforms to the principles of SANS 62305-3. The design of the type and location of an LPS should take into account the features of the existing structure.

Notations

NOTE 1

Specific requirements for an LPS in structures dangerous to their surroundings due to the risk of explosion are under consideration. Additional information is provided in Annex D for use in the interim.

NOTE 2

This part of IEC 62305 is not intended to provide protection against failures of electrical and electronic systems due to overvoltages. Specific requirements for such cases are provided in IEC 62305-4.

NOTE 3

Specific requirements for protection against lightning of wind turbines are reported in IEC 61400-24 [2].

References

The following referenced documents are indispensable for the application of this national standard. These references are listed in the standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

IEC 60079-10-1:2008, Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres

IEC 60079-10-2:2009, Explosive atmospheres – Part 10-2: Classification of areas – Combustible dust atmospheres

IEC 60079-14:2007, Explosive atmospheres – Part 14: Electrical installations design, selection and erection

IEC 61557-4, Electrical safety in low-voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. – Equipment for testing, measuring or monitoring of protective measures – Part 4: Resistance of earth connection and equipotential bonding

IEC 61643-1, Low-voltage surge protective devices – Part 1: Surge protective devices connected to low-voltage power distribution systems – Requirements and tests

IEC 61643-21, Low-voltage surge protective devices – Part 21: Surge protective devices connected to telecommunications and signalling networks – Performance requirements and testing methods

IEC 62305-1, Protection against lightning – Part 1: General principles IEC 62305-2, Protection against lightning – Part 2: Risk management

Feb 152013
 

Proven Ways of Waterproofing Roofs

Waterproofing roofs is perhaps the most important factor when it comes to roof construction.

Flat Roof 435 Waterproofing Roofs

It is vital that roofs are correctly waterproofed, especially when they are “flat”.

The National Building Regulations and Building Standards Act states that roofs must be designed and constructed safely so that they are not damaged by wind or any other natural force. The law also states that they must be waterproof, specifically:

  • Roofs must be durable and must not allow the penetration of rainwater or any other surface water to its interior.
  • Roofs must not allow the accumulation of any water on its surface.

But the legislation simply lays down the basics. For additional guidance, anyone building needs to refer to the South African National Standard that explains how the law can be successfully applied.

In terms of waterproofing, the Standard, The application of the National Building Regulations Part L-Roofs specifically covers:

  • Roof coverings and waterproofing systems, and
  • Drainage and waterproofing of flat roofs.

Roof Coverings and Waterproofing Systems

The SANS elaborates on the legislation stating that roofs must be able to resist penetration of rain to the extent that water in category 1 buildings (see below) any water that penetrates the roof won’t run down the inside face of walls onto the floor, or form damp patches on the ceiling or the floor. In terms of all other buildings (i.e. those that are not category 1), if water penetrates the roof it won’t be intense enough to run down the inside surface of the roof or drip onto the floor or ceiling.

The SANS also state that roof coverings and waterproofing systems must be capable of being repaired if damaged, even if the materials are old.

In addition, roof coverings must be able to resist:

  • temperatures from -10 degrees C to +80 degrees C, as well as quick changes of temperature, without deteriorating
  • the effect UV radiation, without deterioration
  • effects of condensation under the surface
  • chemical attack from basic salt or gas in the atmosphere
  • growth of bacteria, fungi, lichens and so on
  • any penetration or puncturing while the roof is in use
  • movement of the roof structure

All products that are used for roof coverings and waterproofing systems must have a lifespan of at least 10 years. If the structure or system is particularly intricate, making it difficult to replace, then the expectation is that materials used should have a lifespan of at least 20 years.

So what is a category 1 building?

Part A: Administration of SANS 10400 classifies all buildings in terms of occupancy (which in terms of the law means “the particular use or the type of use to which a building or portion thereof is normally put or intended to be put”.

A category 1 building falls into various legislated “classes” namely:

  • A3 – places of instruction,
  • A4 – places of worship,
  • F2 – small shops,
  • G1 – offices,
  • H2 – dormitories where groups of people are accommodated in single rooms,
  • H3 – domestic residences that consist of two or more dwelling units on one single site,
  • H4 – dwelling houses where there is just one dwelling unit (or house) on the site, and possibly also a garage and domestic outbuildings. This is, of course, your most common “home”.

A category 1 building also has no basements, a floor area that is no larger than 80 square metres, and a maximum length of 6 m between intersecting walls or members that provide lateral support. So you will see that there are quite a lot of South African homes that don’t fall into the 1 category.

 Roof Coverings in Pitched Roofs

The SANS have useful standards that we have adapted for ease of reference. The three below specify:

  1. The minimum roof slopes of sheeted roofs
  2. The minimum roof slopes of non-sheeted roofs
  3. The minimum thickness of thatch layers

    Min slope sheeted Waterproofing Roofs

    Table 1: The minimum roof slopes of sheeted roofs

Min slope non sheeted Waterproofing Roofs

Table 2: The minimum roof slopes of non-sheeted roofs

If metal roof tiles are used on an existing roof of this category, the existing slope may be retained. But is is important to be aware that if there are strong gusts of wind, the suction force on the roof tiles might exceed the mass of the tiles. If the tiles are securely fixed it will usually prevent them from being lifted. But a much better option is to include an underlay membrane under the slates or tiles. This will reduce the risk of wind uplift because it can lower these pressures substantially.

Thatch Waterproofing Roofs

Table 3: The minimum thickness of thatch layers

NOTE: SANS 10062 contains fixing instructions for the fixing of different types of roofing. This national standard is available from the SABS.

The pitch indicated in Tables 1 and 2 are minimums. In addition to these, sheeted roofs in category 1 buildings that don’t have hips and valleys, may have a slope of 5 degrees, as long as all the end laps are sealed and have a lap of at least 250 mm. The slope of any valleys in the roof should then be no less than 11 degrees.

If tiles are laid at a pitch of 30 degrees they must (in terms of the SANS) be tested in a rig as specified in SANS 542. The relative humidity must be maintained at a minimum of 70% during the test, and droplets should not form on the underside of the roof. It is a little different for category 1 buildings – rather than droplets forming (or rather not forming) – the test must show that water doesn’t flow down the inside of the tiles. In other words the roof MUST be waterproof!

Sheeted roofs should be tested in accordance with ASTM E 1646 to check that they don’t leak. There must be no dripping of water onto the ceiling or floor of category 1 buildings.

All tiled and sheeted roof coverings must be installed in accordance with the manufacturer’s instructions and/or by workers with the correct skills.

Thatched roofs should be installed according to SANS 10407. The required thickness of the thatch is shown in Table 3 (above).

All roofs with a pitch less than 26 degrees or more than 45 degrees, and all roofs in coastal areas (to a distance of 30 km from the sea) should have an undertile membrane that is loose-laid so that water can drain between the rafters. If an undertile membrane is properly laid it will provide a very effective, impermeable barrier against wind-driven rain and dust. For this reason the SANS states that underlays should be provided for all slate and tiled roofs, no matter what the pitch (or slope), and where ceilings are not installed.

The manufacturers’ instructions must be followed carefully for tiles, slates and shingles.

Roof Lights

waterproof skylight205 Waterproofing Roofs

A well designed, waterproof roof light.

Roof lights have become increasingly popular over the past few decades; but if they are not properly designed and installed, they can leak.

The SANS for roofs state that any roof lights may have an opening of no more than 0.6 square metres. If it is the type that incorporates a translucent roof sheet, it may be 700 mm wide. In addition, roof lights must be able to resist UV degradation for at least 15 years, and hail (at any time) of 10 J (in accordance with SANS 10400-B).

Lastly, all roof lights must be designed and installed in a way that rain will not penetrate the roof.

Drainage and Waterproofing of Flat Roofs

Flat roofs can be extremely difficult to waterproof, which is why all so-called “flat” roofs should be built with a fall of 1:80. This might require a steeper design slope of 1:50 in concrete slabs where construction is not always 100% accurate. A 1:50 slope is also required where there is an interruption in the flow of water on the roof.

The slope should be towards external gutters, roof edges and outlets. Other factors that should be considered include:

  • an avoidance of “penetrations” through the roof, or they should be at least 200 mm away from vertical surfaces like walls and “upstand” beams
  • an avoidance of having clusters of plumbing pipes, air conditioning pipes, and electrical conduit 
  • formation of a suitable step between inside and outside areas to prevent water flowing or dripping into the interior of the house or other building

It is very important that precast panels and precast roof structures are designed in a way that if there is subsequent movement of the concrete elements, this will not damage the waterproofing system or compromise its performance.

The SANS has a really useful drawing that shows how construction drawings should clearly designate ridges and valleys, and indicate the relative fall – or slope.

Gutters and Downpipes

Gutters and downpipes are not mandatory. However, unless gutters are designed by a competent person, they may only be located on the “perimeter” of the building. They should also be designed to ensure that stormwater doesn’t penetrate the inside of any building if they become blocked at any stage.

Rain chains Waterproofing Roofs

Rain chains are a popular option to gutters

Outlets must be set flush with concrete. If there is timber decking they must be recessed so that there isn’t any ponding around the outlets. Any outlets should be at least 500 mm from upstand elements including parapet walls, and they should be at least one metre from any expansion joints.

Intallation requirements of manufacturers and suppliers of rainwater goods absolutely must be adhered to. 

Flat Concrete Roofs

Any concrete roof design should take the thermal properties of concrete into account. This will be determined by a concrete technologist or other competent person, who will determine the required thickness of the concrete and its density, and design the roof in such a way that a waterproofing layer is built up. Often the designer will incorporate a “thermally insulating layer” above the structural concrete deck. When this is done it is important that attention is given to ventilation so that any moist air that might accumulate below the waterproofing layer is vented to the outside.

The concrete used for flat roofs shouldn’t contain more than 7% moisture by weight; and sand-cement or lightweight screeds shouldn’t contain more than 10% moisture by weight.

If expansion joints to accommodate the flow of water are not custom-designed by a competent person, “twin kern upstand-type joints” should be installed over any expansion joints. These should be positioned away from any outlets, and should be built in accordance with the illustration given in SANS 10400-L. Upstand beams that are at least 170 mm high should be incorporated where masonry walls meet the concrete surface of the roof. At these “intersections” (i.e. where they join) corner fillets measuring at least 75 mm (vertically and horizontally) should be build in.  There is another drawing in the SANS that shows how this should be done. Another drawing shows how drips should be incorporated under all overhangs of concrete roofs.

In addition to these design elements, all concrete and screened roof surfaces must be waterproofed and constructed to the correct falls and cross falls (see first paragraph under Drainage and Waterproofing of Flat Roofs above and the relevant section in the SANS). It is vital that there are no undulations in the concrete surface, and nothing should be allowed to protrude into the concrete or contaminate it.

The recommended finish for concrete roofs is wood floating. While the final surface should be sound and smooth, concrete and screened surfaces should not be highly polished. So they should NOT be power floated.

Waterproofing Systems

Any waterproofing system that is installed on flat roofs MUST be done by a “competent person” who MUST follow the manufacturer’s instructions. SANS 10400-L states that for roofs to be up to standard, they must remain watertight for at least five years without the need for any form of maintenance other than the normal cleaning of downpipes, gutters and so on. It is also important that the person doing the installation is satisfied that the materials used are appropriate for that particular application, and should therefore take into account:

  • the degree of exposure the waterproofing system will be subjected to
  • how much protection the waterproofing material will have
  • and ultimately whatever affects the building because of where it is located

It is important to realize that waterproofing systems can delaminate if the substrates don’t allow any moisture vapor that has been trapped to escape. Generally a sand-cement screed that is not very dense will allow retained moisture vapor to dissipate and therefore protect against the possibility of delamination.

SANS 10400-L suggests that a 20 mm screed is laid over all “lightweight” screeds, because these are generally too friable and porous to provide good adhesion for waterproofing systems. The SANS also suggests that concrete and screened roof surfaces be allowed to dry thoroughly before any type of waterproofing system is applied.

Where waterproofing turnups are provided against brickwork and other masonry walls, they should be counter-flashed if they are not linked to the stepped damp-proof courses in cavity walls. The same membrane should be used, and the flashing should be cut into walls to a depth of at least 40 mm.

Generally sand-cement coves with a radius of at least 45 mm should be formed at all the inside corners of both vertical and horizontal surfaces – unless a particular waterproofing system has a different design and doesn’t require this. Where there is a timber deck, 38 mm timber fillets may be fixed at all the junctions of horizontal and vertical surfaces.

There is always a potential problem with outlets. The installer must therefore pay close attention to overflow pipes, flues and so on, and make sure that the waterproofing material used covers everything but the opening. Generally waterproofing membranes around any pipe work should be clamped with a hose clamp or something similar, before flashing is applied over the pipe.

If outlets aren’t the “full-bore”, coned type, pipes should be flanged so that waterproofing can be done correctly. Elements such as water storage tanks and solar absorbers should never be allowed to penetrate the waterproof layer.

All external corners and edges to be waterproofed should be rounded, and the height of all DPCs should be at the same level as the waterproofing turnups.

In addition to this part of SANS 10400, SANS 10021 also provides some information and guidance about the waterproofing of roofs. Remember, it’s important to do it correctly!

Nov 152011
 

RoofTrusses780 s Roofs

Building Regulations as They Apply to Roofs

When the South African National Building Regulations were updated by the Department of Trade and Industry in May 2008, the General Requirement relating to Roofs was changed to incorporate certain safety elements.

For example, instead of simply having to “resist any forces” to which the roof might be subjected to, the regulations now state that “The roof of any building shall be so designed and constructed that it safely sustains any actions which can reasonably be expected to occur and in such a manner that any local damage (including cracking) or deformation do not compromise its functioning”. In simple language, if there is a major wind or some other really horrible weather conditions (God forbid), the roofs of our homes are expected to be able to stay on the house and protect us from the elements without themselves being damaged.

Instead of simply being “durable and waterproof”, roofs are expected to be “durable” and should not allow “the penetration of rainwater or any other surface water to its interior”.

As previously, roofs must “not allow the accumulation of any water” (but not simply rainwater, which was the limit of the old building regulations) “upon its surface”. In addition, the roof should be “adequately anchored against wind uplift” which was not covered in the previous edition of the regs.

Lastly, the General Requirements specify (as they did previously), that the roof should be designed “as part of a roof and ceiling assembly” and should provide “adequate height in any room immediately below such assembly”. This last one, though, is open to interpretation as not all roofs incorporate ceilings as such.

The South African National Standard for Roofs

While the legislation changed in 2008, it was only in 2011 that Part L: Roofs was published by the SABS. And the changes are substantial. It’s not so much that they’ve changed, but rather that the guidelines are now much more comprehensive and useful.

General Rules for the Construction of Roofs

As with most of the National Building Regulations, those that apply to roofs relate to SANS other than the one specific to that particular element. For instance, where any roof is to be supported on the wall of a building as described in the relevant section of Part K: Walls, the roof MUST be constructed in accordance with the rules laid out by the relevant SANS (in this case 10400). In addition, the new SANS remind designers and builders that other sections are also vitally important when it comes to roof design, including Part A: General principles and requirements; Part B: Structural design; Part C: Dimensions; Part R: Stormwater disposal; Part T: Fire protection; and Part V: Space heating.

Of course they are. Any qualified designer knows that every one of the SANS that form part of 10400 needs to be considered as a whole. It’s just because the different new sections were published over a period of years that has made it more of a challenge for many.

Since anybody building a house MUST either BE a “competent person” in terms of the regulations, or must EMPLOY a “competent person” to put in plans and oversee the building operation, either you or the person you employ should purchase the updated section of SANS 10400 Part L Roofs from the SABS to double-check details and specifications. Also be acutely aware that circumstances vary from site to site.

There are several South African National Standards (SANS) that relate to roof timbers, all of which must be complied with when roof trusses and other roofing elements are constructed. In addition there are standards that relate to roof coverings and other elements. They include:

  • SANS 542, Concrete roofing tiles
  • SANS 1288, Preservative-treated timber
  • SANS 1460, Laminated timber (gluglam)
  • SANS 1701-1, Sawn eucalyptus timber – Part 1: Proof-graded structural timber
  • SANS 1701-2, Sawn eucalyptus timber – Part 2: Brandering and battens
  • SANS 1783-2, Sawn softwood timber – Part 2: Stress-graded structural timber and timber for frame wall construction
  • SANS 1783-4, Sawn softwood timber – Part 4: Brandering and battens
  • SANS 2001-CT2, Construction works Part CT2: Structural timberwork (roofing)
  • SANS 10407, Thatched roof construction

You’ll find the full list in Part L of SANS 10400 (or check with an SABS librarian for the relevant information).

Basic Requirements

Roof design depends on a number of factors including the type of covering you are going to use, and the span over which the roof structure is to be supported. More often than not, the roof structure is assembled from a series of roof trusses. These rest on wooden wall plates, and are designed to span the walls of the house. They will be either nailed or bolted together on site, or delivered to site on order by a specialist truss manufacturer.

TrussTypes1 s Roofs

Illustration courtesy The Complete Book of Owner Building in South Africa

The trusses themselves are made up of rafters, tie beams, posts and struts, all of which are assembled according to a specific design. The illustrations above shows some of the most usual configurations. The new regulations have simple line drawings for:

  • Four-bay Howe truss with a maximum clear span of 6 m (the same as centre right above)
  • Six-bay Howe truss with a maximum clear span of 8 m (called a King Post Truss above)
  • Two-bay mono pitched Howe truss with a maximum clear span of 3 m
  • Three-bay mono pitched Howe truss with a maximum clear span of 4 m

The regulations also state that no member of any truss should have a length that is greater than 60 times its smallest dimension.

The basic requirements shown in the table below, apply to Howe-type trusses as listed above. There are some additional tables mentioned below.

MAXIMUM TRUSS SPANS FOR RAFTER AND TIE-BEAMSHowe Trusses Roofs

*a  Heel joints should have 2 x M12 bolts per joint with 40 mm washers at each end

*b  All timber members should have a thickness of 38 mm or 36 mm if the timber is planed

*c  38 mm x 114 mm Grade 7 members may be substituted for 38 mm x 152 mm Grade 5 material, if required

*d  The maximum overhang of a 114 mm top chord or rafter is 600 mm. The top chord or rafter must be increased to 152 mm if the overhand is greater than 600 mm but less than or equal to 900 mm

[TC = top chord; BC = bottom chord; web = cross pieces that tie the structure together]

This table is considerably more useful that the one that was in the previous 1990 edition of the regulations, as not only maximum truss spans are indicated, but also the allowable and recommended pitch of the roof, and the member sizes and grades of timber that are specified in SANS 1783-2.

You will also see that the maximum centre-to-centre spacing of the trusses varies according to the type of roof covering you are going to be using.

Another element that is specified in this table is the type and number of bolts to be used at heel and splice joints (although it must be said that builders often use nails).

A heel joint (mentioned here) is simply an indentation that is cut into a rafter so that the timber can rest on the top plate. Normally this type of joint is about a third of thickness of the rafter.

The new regulations have a number of different tables that specify the maximum clear spans for rafter and/or purlin beams. Specifically for:

  1. Sawn softwood rafter beams that have a pitch of less than 26 degrees
  2. Laminated SA pine rafters that support tiled or slated roofs that have a pitch of less than 26 degrees
  3. Laminated SA pine rafters that support profiled metal or fibre-cement sheeting or metal tiles with a pitch of less than 26 degrees
  4. Sawn SA pine purlin rafters or purlin beams that support profiled metal or fibre-cement sheeting
  5. Laminated SA pine purlin rafters or purlin beams that support profiled metal or fibre-cement sheeting
  6. Gum pole rafters

The timber grades allowable for softwood and all SA pine rafter beams is Grade 5 and Grade 7. Laminated beams should be Grade 5 or higher and should comply with SANS 1460. Where relevant, specifics are shown in the tables for maximum clear spans for sawn softwood beams with a 26 degree pitch below.1. Clear spans softwood Roofs

Note that the type of roof covering in this table (maximum clear spans for laminated SA pine supporting a tile or slate roof with a 26 degree pitch)  is shown in the first column, and the rafter spacing in the other four columns. Also note that the maximum mass of tiles or slates, including battens or purlins, should not be more than 65 kg per square metre.2. Laminated Roofs

Note that * indicates the most commonly available sizes. Below is a table for maximum clear spans for laminated SA pine rafter supporting profiled metal or fibre-cement sheeting or metal tiles with a 26 degree pitch3. Laminated1 Roofs

Note that * indicates the most commonly available sizes. Below is a table for maximum clear spans for SA pine purlin rafters or purlin beams supporting profiled metal or fibre-cement sheeting (or metal tiles in the table below) with a 26 degree pitch.4. Pine purlin Roofs

5. Laminated Roofs

 

Below is a table for maximum clear spans for gum pole rafters with a pitch above 26 degrees and above 26 degrees.

6. Gum poles RoofsThe maximum mass of the tiles or slates, including battens or purlins, shall not exceed 65 kg per square metre.

In addition to maximum spans, there are also minimum requirements in terms of slope (or pitch) and minimum end laps. Min roof slopes Roofs

When it comes to thatch roofs, generally the slope should be 45 degrees, except at dormer windows where the slope should only be 35 degrees. The minimum thatch layers and thickness vary depending on the type of grass or reed used for thatching. Fine thatching grass or reed should have a 1.2-2.5 mm stem/butt diameter, and it should be 175 mm thick. Coarse thatching grass or reed should have a 2.5-4 mm mm stem/butt diameter, and it should be 200 mm thick. Water reeds should have a 1-7 mm stem/butt diameter, and a 300 mm layer thickness.

Some Important Factors Regarding Connections

It is vital that roof trusses and other roof framing elements have joints that are accurately cut, securely made and fitted so that the component parts are drawn tightly together. All trussed roofs MUST be provided with approved bracing that prevents any possible buckling of the rafters, tie-beams and long web members. The bracing also needs to keep the trusses in an upright position. Whoever is doing the maths need to be certain that no section of the truss has a length that is greater than 60 times its least (or smallest) dimension.

If rafter construction is used instead of roof trusses, and the roof covering is regular sheeting or tiles (as already mentioned), it is important to accurately assess the parameters for rafter spans and the size and grade of rafters. Please note that if the rafter spacing is not the same as that shown in the table below, intermediate values of maximum rafter spans may be interpolated within the range of values suggested for relevant timber grades.

When constructing a roof framework, the rule of thumb is that any purlin should have a minimum nominal depth and width of 76 mm or 50 mm, and max centre-to-centre spacing between the purlins ought to be 1,2 m. Joints between purlins next to one another should be staggered. But the tables that follow are a lot more specific.

All roof trusses, rafters and beams that are supported by a brick or concrete block (or even a stone) wall must be securely fastened to the wall using galvanized steel strapping or galvanized steel wire that complies with the National Building Regulations. It is also important that fasteners are resistant to corrosion.

If you order factory-manufactured trusses that are made with metal plate connectors, they may not comply directly with the requirements of the various tables in the SANS. But a “competent person” will be able to tell you whether they meet the requirements of the regulations. If you buy from a reputable company you can rest assured that they will be absolutely fine.

Remember that the National Building Regulations are not prescriptive. But because they were established as a guide to MINIMUM standards, you must never ignore them.

Pole Construction

You will notice that the last table above is for gum pole rafters. Pole construction is another new addition to the NBR SANS.

If this method of construction is used, softwood poles must comply with SANS 457-2 and hardwood poles must comply with SANS 457-3, and ALL poles must be treated in accordance with the requirements of SANS 10005. If they have cracked or the end are plot within a space that is equal to the diameter of the pole, they MUST NOT BE USED. This is simply a structural issue.

If poles are sawn or reshaped at the ends, any of the exposed ends must be treated with a Class W preservative. It is also necessary to cover at least 35% of the surface area of the end with a new nail plate to prevent or at least minimize cracking.

Thatched roof construction – which utilizes pole structures – is also mentioned, though there are additional standards that need to be referred to.

For thatched roofs, laths must have a minimum diameter of 25 mm and they must comply with the requirements of SANS 1288. Spacing must be done according to SANS 10407. If a thatched roof is constructed with gables, without hips, valleys or dormer windows, it must have a pitch of 45 degrees, and a clear span that is no more than 6 m. Construction must also be in accordance with SANS 10407 and with additional specification in SANS 10400-L that are shown in the form of drawings and a table. You will need to either buy the standard or visit an SABS library to access these. In the drawings, specifications for rafters state that if the poles are 100 mm to 125 mm in diameter, then the truss clear spans may not be greater than 4 m. If the poles are 125 mm to 150 mm in diameter, then the spans may be more than 4 m but not greater than 6 m.

Protection from the Elements

There are other factors that relate to fire resistance an combustibility, and waterproofing – which of course has to cover (excuse the pun) flashing and flat roofs!

  1. Fire resistance and combustibility relate to light fittings and any other components that penetrate the ceiling, as well as the non-combustibility of “such assemblies”. No part of any roof or ceiling that is made of wood or any other “combustible” material is permitted to pass through any separating element of a building.
  2. Waterproofing refers mainly to runoff water from the roof … and therefore relates directly to the slope of the roof. This, in turn, is totally reliant on the roof covering used. SANS 10400 has specs on minimum roof slopes and sheet end laps. The new regs include a number of invaluable drawings that show principal waterproofing details including parapet wall waterproofing on balconies; where it is required against a solid brick wall; where it is required against a concrete balustrade wall on a balcony or against an ordinary concrete wall; and various other balcony details. Additional waterproofing details include a stepped DPC in a cavity wall; tanking against a cavity wall; waterproofing under timber and aluminum door frames; and waterproofing at a shower base.
  3. Flashing, which is used to stop leaks coming in from around chimneys and other “projections”.
  4. Flat roofs are an issue all on their own! For instance, flat roofs are not actually flat, they MUST have a fall of about 1:50.

Part L of the updated national building regulations (published in 2011) also include new sections on roof coverings and waterproofing systems for pitched roofs, and drainage and waterproofing of flat roofs.