Roofs-Part L

Building Regulations as They Apply to Roofs-Part L

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.

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-BEAMS

*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.

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.

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 pitch

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.

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

The 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. 

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.

>

Stairways