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In the house I'm building, I will be using a flat roof with a parapet around it. The total area will be around 120 sq meter and the roof will be a concrete slab (at least 8 cm).

What is the best way to drain it? People in Brazil don't like to use those kind of roofs so I can't get good info on the subject.

I will be waterproofing it using some type of asphaltic membrane rools that are covered in alumminum foils (for protection).

I'm on the starting phase of the construction, so I'm quite flexible with what I can do, but here we don't have fancy siphonic drains, mostly PVC pipes are used (100 and 150 mm).

BTW: If possible I don't want anything inside or below the concrete slab (I'm partial to scuppers).

EDIT: What about this idea: make the whole roof sloped (2% inclination) towards the back, and there use a single gutter with a tube down each end? Is that good design? I know that one end of the roof will be almost 30 cm higher than the other (the lenght is 13,5 meter), but since I'm at construction phase I can make the roof concrete slab sloped from the start.

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  • I edit the question adding a new "option" to the the roof design.
    – Luiz Borges
    Commented Nov 7, 2012 at 20:36
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    this link: scieroofing.com/flatroofdesign.htm suggests a minimum of 1/80 1.25%, and that practice is 1/40 2.25%, so your 2% should be good.
    – Chris Cudmore
    Commented Nov 9, 2012 at 16:53

3 Answers 3

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You need to have a pitch somehow. Basically your options are either slope to a drain(s) in the interior or slope to the edge. If you don't want an interior drain, it sounds like you're going to be going to the edge—e.g. gutters or scuppers.

If you do use a drain or scuppers, you should also consider what would happen if they clog. You don't want to set yourself up for a situation where more standing water can accumulate than the structure can safely hold. If the parapet is solid, you could potentially get a lot of water up there over time. Every inch of standing water adds about 5 psf, which could add up quickly. Also, standing water has a tendency to "pond" — cause deflection in the middle of the roof, which draws more water to that spot, which causes more deflection... etc. etc.

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  • I know that it can't be really flat. In my EDIT in the question I asked about another option, using gutters in one side with the whole roof sloped down to it. Is that technique used?
    – Luiz Borges
    Commented Nov 8, 2012 at 2:00
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    @LuizBorges: I think that plan sounds fine as long as you appropriately size the gutters and downspouts. You need to make sure the water can leave the roof faster than it's accumulating. That depends on: 1) the size of the roof, 2) the maximum rainfall rate in your area (e.g. inches per hour), and 3) the number and size of downspouts. But gutter sizing is not specific to flat roofs, so you should have no trouble finding that info.
    – Hank
    Commented Nov 8, 2012 at 4:46
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In my country, people prefer scuppers and PVC pipes. Make the floor sloped to four corners. Make gutters at the four edges. That's to say, the center point is slightly higher than the four corners and edges.

Edit: Do you want exterior eaves? If you do, you can have scuppers there. If parapet is solid, you might have to make scuppers on the parapet. Exterior pipes are suggested as you can easily handle the clog. Not only make the roof sloped, but also the gutters inclined towards the scuppers.

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    What country are you from? It does matter if you have drastic 4 seasons or sunshine the whole year, you see.
    – Piotr Kula
    Commented Nov 7, 2012 at 11:08
  • How are those scuppers? Round like the PVC (made of pvc?). If so, how much of the opening is below the floor level?
    – Luiz Borges
    Commented Nov 7, 2012 at 14:03
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    @ppumkin I live in Guanghzou, China. There is a lot of rains, sometime storm and strong wind, during the summer. Thus, we have to think about the draining system.
    – jimmyk
    Commented Nov 7, 2012 at 16:06
  • @LuizBorges Yes, they are round, made of PVC, diameter is about 20cm. Make slot along the edges, and several holes connected to the PVC pipes. Don't forget add something like "filter" on the holes, as to avoid jamming
    – jimmyk
    Commented Nov 7, 2012 at 16:11
  • @jimmyk I didn't understand how it looks like. The holes you mention are the scuppers? If so, how much of the holes are below the surface of the floor? Isn't 200mm holes and pipes too much?
    – Luiz Borges
    Commented Nov 7, 2012 at 20:32
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Ensure that you are using corrugated metal decking, and pour high quality concrete within a form involving that decking, to at least 6" in depth. Also, form a grid of epoxy-coated 1/2" rebar consisting of 1' x 1', 1" off the bottom of a top rib of the decking. Ensure that the concrete is air-entrained and fibre-reinforced. 25 mpa. Pour it flat, and don't forget to mechanically vibrate. The span of the ribs may be no longer than 8'. The seams of the decking must be crimped, and welded. The rebar must be tied with epoxy-wire. After all that, install Blueskin and then insulation and then gravel base!

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