Waterproofing Roofs

Proven Ways of Waterproofing Roofs

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

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 into their interior.
• Roofs must not allow the accumulation of any water on their 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 roofs, the Standard, The application of the National Building Regulations Part L-Roofs specifically covers:

• Roof coverings and waterproofing roofs 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

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.

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

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.

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!