I am constructing a building from reinforced concrete and AAC blocks in Indonesia.

The building will have a flat (2% sloping) roof of reinforced concrete. This will be exposed to tropical rain, as will a overhanging canopy. It will also have foot traffic, leaves, detritus, etc.

I understand that it is important that the roof not leak, in order to avoid water entering into the building, and the canopy also not leak, in order to prevent the rebar rusting and failing over time.

As the roof and canopy are yet to be constructed, I may have the choice of coatings after pouring, concrete additives/modification as part of the pour, or both.

I can find in terms of coatings:

  • 'waterpoofing slurry' based on resin modified cementious material (this is supplied as a bag of cement-like powder with a solvent) - this needs to be protected by cement screed as it is not UV stable
  • waterproofing mortar - (cement to which water is added)
  • ' a cold applied, one component waterborne liquid applied waterproofing membrane', a 'Polyurethane modified Acrylic Dispersion'. Apparently this may also enhance solar reflectivity. There is an optional 120g weight polyester fleece, which is a polyester fleece product, you can apply at the same time.
  • A combination of non-woven polyester membrane, and 'UV-resistant acrylic emulsion'
  • " one component, liquid elastic waterproofing coating, based on styrene-acrylics copolymers in aqueous dispersion reinforced with fiber to increase the tensile strength of the product. It forms a flexible, waterproofed and durable film layer"
  • self-adhesive, rubber modified, bituminous sealing tape - this is a roll of tape, so I guess you would not use it for a whole roof, but only for small areas
  • steel corrosion inhibitor - surface-applied (for the canopy only, which technically doesn't need to be waterproof)

Or concrete admixture:

  • Watertight/superplasticising (HRWR) concrete admixture (Combination Fatty Acid and Polycarboxylate )
  • Crystalline admixture

What's the best way to go? It seems like an admixture might save time and effort? But is it sufficient?

  • 1
    +1 for a well asked question! Do all these methods meet building code in Indonesia? Are they all (code) legal and commonly used there, and you're trying to determine which is the best for you? Are these simply coatings you've found online and want to pick one with no local code knowledge? I'm not trying to imply anything, just trying to get some additional info that someone will need to help you out. – FreeMan Jun 3 '16 at 20:06
  • Hi, there might be a building code in Indonesia, but none is followed in my area. People just do what they like (often, things leak), and there is no specific insight from my builders. – thelawnet Jun 4 '16 at 5:18

I would use the HRWR admixture to help seal the concrete. Alone, it might not be the best roofing solution, but it's well worth the minor cost for the extra protection.

The biggest problem that I heard was foot traffic. Normally I would recommend EPDM, but I'm not so sure about foot traffic. This sounds more like a terrace, in which case, silicone cement is most commonly used.

However, something like a layer of asphalt would be more durable. This could be layered over steel. I would consider using coal tar (sealer) on top of the asphalt (which works better than bitumen/petrol tar) for shedding water. However, using coal tar is against roofing codes (causes cancer) in most of the world.

But one alternative that was not mentioned would be a green or living roof. I don't really know much about the materials involved, but in a tropical climate, it might be best to just go with the flow (in a sense). Fighting the elements can sometimes be harder than working with them.

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I would not trust admixtures to make concrete waterproof. Concrete in inherently porous and hygroscopic. Adding things to concrete still leaves it concrete with these properties. Point is it is the wiser to cover the concrete with something water proof.

Waterproof materials typically will be a solid sheet of something like metal, plastic or "poly...", elastomerics, or EPDM (rubber), and finally bituminous (tar/asphalt).

Which waterproofing material is best is a matter of many variables and then many opinions based on those variable and the pros and cons of each option.

I personally like a well "lapped" steel solution, next is a EPDM or rubber solution in my book.

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