Feb 152013

Proven Ways of Waterproofing Roofs

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

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

    waterproofing roofs

    Table 1: The minimum roof slopes of sheeted roofs

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.

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 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 chain

Rain chains are a popular alternative option to downpipes from 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!

  79 Responses to “Waterproofing Roofs”

Comments (79)
  1. We want to rebuild a house that burned down. The outer walls are still intact but we are thinking of building the roof on metal columns of 100 x 100mm, 2,4 meters apart. Is this allowed?
    Secondly what is the minimum angle for a flat roof? It is 8m to the drop side and 25 meters long. Is 7,5 degrees allowed?

  2. Hi,
    Are there regulations limiting the directing of storm water from the roof into the municipal waste water drains (where toilet and shower water goes)?
    The back of our plot is walled on three sides and water dams up after heavy rain. Could I allow the rain water to drain into the sewage system to solve this problem?

    • Albi the plumbing regulations are quite strict; and there are quite a few of them. SANS 10400 Part R deals with stormwater drainage in particular.
      First of all you may not channel stormwater into any type of drainage system. BUT all stormwater must be efficiently disposed of; and all ground levels, invert levels, sizes and grades of pipes and channels must be shown on the drawings submitted to council. Perhaps more importantly, the final disposal of stormwater from a site must be clearly defined and must be designed in consultation with the Local Authority where necessary.
      I hope this helps.

  3. My gutters drain into my neighbours yard as the angle of my roof is towards his hom. The gutter is about 20 m from his front door and drains into his garden. He is unhappy with the situation and threw bricks on my roof and blocking the gutters resulting in damage to my electrical equiment under my roof. Please advise on the proper procedures for gutters and the place where they may drain

    • Gerhard, first of all your neighbour may have a legitimate gripe (I would be very unhappy if I was in his position), but he is totally out of line throwing bricks onto your roof. What I would have done was take photographs and then get a professional to assess the damage – and bill him for it.
      In terms of the correct way to affix gutters and downpipes, you need a professional, registered plumber to do this for you.

  4. Water came through my roof into a room in my house. The builder says
    it is not a latent defect. I would like to discuss this issue to get a
    better understanding of what latent defects are and responsibility of
    the builder.

    • Lorraine, is this a house that you had built, or one that you bought? Essentially a latent defect is a defect in something that has been purchased that is not apparent after ordinary inspection by a “reasonable” person. A patent defect, on the other hand, is something that someone would notice. So if, for example, bad workmanship was the cause of a leak (perhaps no underlay in the roof), but there was no way you could have discovered this, then it IS a latent defect. Generally when purchasing a house, if the seller knew that there was something defective and didn’t disclose it to the seller (e.g. the roof leaked and the seller did not disclose this) then it is a latent defect.

  5. Hi,

    Can anyone please indicate the minimum crown coverage when using sheet roofing and Nutec barge boards or point me to a SANS code?


    • Gerhard there is some information on the general roofing pagecap on this site. However this Part of SANS 10400 does not mention the words “cap” or “crown”. Other standards discuss the materials and fittings used, but I don’t think will answer your question. I suggest you contact Everite for advice. I have given you the link. They briefly discuss design criteria on their website, and state that “Site service personnel are available on request and at no charge, to provide assistance on recommended storage, handling and erection of the Company’s products.”. Here’s a link to all their contact numbers country wide.

      • Hi Penny, much appreciated. I have spoken to Everite and have their letter confirming that using their product 260 x 260 barge board over a 0.762 Wide Deck profile and covering only one ridge/crown will suffice. However, the local authorities still insist on a coverage of at least two ridges/crowns. It is strange that the NHBRC has no detail on this.

        • Gerhard have you contacted the NHBRC? I have looked at their manuals and don’t see anything – but maybe someone there can advise you further. I would object based on the manufacturers guidelines. The legislation does give you the right to object. And in any case you are not non-compliant in terms of the NBR. I would ask them what they are basing their demands on.

  6. What tipe of material is allowed to seal ridging on an existing roof?
    Is “pap & lap” still allowed?

    • Hi Mariana,
      As far as I know “pap ‘n lap” has never been disallowed. The thing is that there have been technical advances in waterproofing products that will now last a lot longer. You do not want to redo the job every season. Just ask your local hardware store what they recommend.

  7. Great blog here! Also your website loads up very fast! What web host are you using?
    Can I get your affiliate link to your host? I wish my site loaded up as fast as
    yours lol

  8. I have seen gutters are no longer a requirement but seems still a needed….kindly advise on your findings on gutters and why it is perhaps a good thing

    • Gutters and downpipes are the most usual way we channel rainwater away from buildings, but outlets may also be used. Sometimes architects use chain (or something similar) instead of downpipes, to form an element that the rain can run down. It can be effective and looks quite quirky. I’ve added a picture to this page.
      According to SANS 10400: Part L Roofs, if outlets are used instead of gutters, these should be set flush with concrete or recessed into timber decking (depending on the structure) to prevent ponding. Part L of the Standard also states that “the position of all outlets shall be at least 500 mm away from upstands, parapet walls and 1 000 mm away from expansion joints”.

  9. WE purchased a house in a complex that was built by one of the nominated contractors. We took ownership in June 2008. There is a 5 year latent defect clause. The flat roof has been leaking every year despite cleaning the gutters. The contractor in question has been back several times patching up.
    As it will be 5 years up in June this year, do we have any claim to ask the contractor to repair the roof properly, so that there are no more leaks. OR do we pay for another contractor to come and sort it out? Many thanks

    • Ann, I am not sure where you live – but where I am we are into the rainy season and so leaks are going to be very evident in the next few months. You could put him to terms in writing detailing the history and stating that should the leaks recur he will be liable to pay for the job to be done correctly. But this would probably need to be done through a lawyer – otherwise you’re probably not going to get your money back even if the problem recurs before June. It might be better to call in two or three other contractors and ask for detailed quotations – so you can see what work they believe should be down – and then demand that the person who built the house fixes it. And/or contact the NHBRC because their warranty scheme covers leaks for five years – and let them fight with the contractor. Since this is a “nominated contractor” if he isn’t a member of the NHBRC there will be big trouble for the developers.

  10. how do you see the place that need water proofing on the corigated iron and how to solve the problem ,tell me step by step

  11. Currently I have Brosley tiles on the roof and as they are no longer avaiable and due to contractor applying TAL Bond , SBR latex to the roof as a primer, I now have to replace the whole roof.

    What are the requiremenst, do I need council approval – Guateng- and who can guide me to ensure that the correct selection of roof tile and fitting of roof tiles etc.Are there professionals that can ensure that the project is done according to the required regulations.


    • Charl you simply need to use SABS approved tiles. The manufacturer should be able to give you detailed information regarding how the tiles are fitted. They may also know of professional contractors who have experience working with their products. We are not able to recommend products specifically. Your local council might be able to recommend products, but be careful…

  12. Hi, kindly advise on the following major problem.
    A developer has not waterproofed the post-tension concrete slab which forms the roof of a basement parking, the telkom room, the electricity meter room, building supervisor’s offices and store room resulting in serious water leaks between the unbonded slabs and cracks that have formed due to the slab being exposed to (the elements) UV radiation and thermal expansion and contraction. Kindly advise on the legal requirements prescibed for developers regarding waterproofing the post-tension concrete slab which forms the roof of a basement as described above. Thank you. kind regards. Roy

    • Hi Roy,
      These seem to be problems that the local building inspector should have picked up before occupation/completion certificates were issued. You then have recourse to them to instruct the developer/builder to rectify the problem. Waterproofing is covered in SANS 10021(2012): The waterproofing of buildings (including damp-proofing and vapour barrier installation). There are several pages in the Standard that relate to waterproofing requirements for basements and semi-basements. I suggest you purchase a copy or go to your nearest SABS library and read it there. You might need to get an independent assessor involved to give you a full report. This will add to your case if this escalates to court.

  13. Hi. I purchased a house 2 years ago and now the roof is leaking
    terribly. I have collected the building plans from the municipality
    and alterations were made to the house prior to my purchase. How do I
    find the building inspector that verified that the alterations were
    done according to building regulations. The ceiling has caved in from
    what appears to be incorrect roof alterations. Gutter inside the roof.
    Can you please assist? Thanks Jaco

    • Hi Jaco,
      Sorry to hear about your problem. I am not sure what part of the country you are but I suggest that you contact the local municipality where you got the plans from and ask them to tell who the inspector was and ask them why the roof alterations were passed by them. The other choice, depending on the amount of damage and the cost to repair it, would be to get a qualified Independent Building Inspector to give you a report and he should be able to tell you if you have a claim and against whom. Is there a signature and an “approved” or “Passed” stamp on the copy of the plans? This should tell you who signed them off and when, this is also important when it comes to normal deterioration and wear and tear over time. Let us know what the outcome is.

  14. I’m building a granny flat for my gran on top of my garage. I live in
    a 3 bedroom house in Durban North. Somebody told me that its a law
    that i must provide parking for the granny flat regardless of whether
    or not the occupant is a granny who cant drive. I’m trying to avoid
    this as i will need to dig back a bank and create a huge expensive
    retaining wall. What are the rules for parking?

    • Trevor you will need to submit plans for a granny flat – and if parking is required, the local authority will tell you to add it. Parking rules are set by the local authority, so they are the folks you need to talk to. It isn’t something that is laid down in the national building regulations.

  15. hi i hope you can assist me . in 2002 we added a guest room to our
    house . recently we found out that leaking was not due to shoddy
    waterproofing work but the gutter was higher than the roof, so the
    water cannot drain properly. waiting to hear from you . thanks rike

    • Hi Rike,
      Eleven years is quite a long time to discover the cause of your leaking problem. If you are asking whether you have a claim against the contractor I think you will find that it will be way past any legal time limit (prescription) to take any legal action. I am sure you can get a decent “handyman” with references in your area that could handle the job of re-positioning the gutters.

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