Replies

1. GBA Editor
   Martin Holladay | Nov 01, 2018 07:18am | #1

   K. Bing,
   Since you measure rainfall in millimeters, I'm guessing that your building is not located in the U.S. Under the circumstances, I'm going to recommend that you seek local advice. It hardly matters what roofing products I recommend, if those roofing products aren't available in your country. So you need to see what's available locally.

   In hot climates, low-slope concrete roofs can work well. In some cases, the concrete is left unprotected. In other cases, the concrete is covered by a paint or coating. (This would need to be regularly renewed.) And in other cases, the concrete is covered by membrane roofing.

   What factors affect the choice here?

   1. Concrete quality. Concrete will be more waterproof if the water content of the mix is reduced and if the grain size of the aggregate is increased.

   2. Roof slope. A roof with a slight slope drains water faster than a flat roof, of course. You don't want ponding.

   3. Climate. If annual rainfall amounts are high, you need to pay more attention to waterproofing than if annual rainfall amount are low.

   If you live somewhere with 1700 mm of annual rainfall, that's wet. (1700 mm = 67 inches, which is well over an inch a week.) You'll probably need roofing rather than a coating.

1. 
   KBing | Nov 01, 2018 09:51am | #2

   Thanks Martin!

   After some research on local product, possible type of roofing on market is A) single ply modified bitumen roofing OR B) Liquid Applied Type waterproofing.

   Say if we go for liquid applied or other single ply roofing, another fundamental question hit us is where the waterproofing layer should be on top or below the sloped screeding? (see attached) What is the principle or factors to decide whether to put the waterproofing layer on top of the screeding or below the screeding (while above structural slab)? Are there some rules of thumbs? We seems to see both usage in google...

   Open for suggestions and thoughts!

2. GBA Editor
   Martin Holladay | Nov 02, 2018 07:06am | #3

   K. Bing,
   If I understand your question correctly, it sounds like the practice in your country is to place the concrete in two pours: first, a level slab, followed by a second (non-structural) layer of concrete called "sloped screeding."

   I really don't know the answer to your question concerning the best location for the roofing layer. It seems to me that the answer depends on the product chosen (the roofing or waterproofing product), and that the answer should be provided by the product manufacturer. If you're not sure how to proceed, call up the product manufacturer and ask to speak to a technical help representative.

3. Expert Member
   Peter Engle | Nov 02, 2018 12:13pm | #4

   If the roof is to be used as an observation deck, the best practice is to install a paver system over the membrane, with the membrane over the sloped screeding. The membrane may (or may not) last longer if placed between the structural deck and the screeding, but it is much harder to maintain and to even find leaks when they happen. Also, the waterproofing layer would be level and would not drain easily if placed on the structural deck.

   Pavers can be placed on pedestal systems that hold them up off of the membrane and maintain the gaps between the pavers, allowing water to drain between the membrane and the bottom of the pavers.

   Either a modified bitumen system or a liquid-applied system would be entirely appropriate for this application. Liquid applied systems tend to be somewhat more expensive, but are far easier to detail around pipes, scuppers and other penetrations. For a small roof, the costs might be similar for either system.