I have a flat cement roof that is leaking into the ceiling of the top room. How would I seal it?

It is also a patio. What would work? Should I tile it or seal it with rubber paint (can you still walk on it)?

  • Can you clarify "flat roof cement roof"? Is your building's roof made out of poured concrete? Does it have some sort of "cement-like" coating over a wooden structure? What type of sealing coating does it currently have on it? If you're not sure, a couple of pictures (especially of the leaking area) would be very helpful. Please edit your post and add the answer in there to ensure that the majority of people read it (not everyone reads all the comments).
    – FreeMan
    Nov 20, 2020 at 18:10

1 Answer 1


A typical approach (there are many options, some more prone to damage than others) to a concrete slab roof that you want to have foot traffic on without damaging the waterproofing membrane is to have something like an EDPM rubber sheet (continuous, or joined in an approved manner to be effectively continuous) a layer of insulating foam board (XPS) and a layer of cementboard or pavers to take the foot traffic and spread that load onto the foam board, which further spreads the load onto the membrane and helps to protect it from damage. Cementboard is sometimes bonded to foamboard to make a panel for this purpose.

If your roof has developed leaks, it's rarely the case that a simple patch will hold, as low-slope ("flat") roofs are relatively more difficult to waterproof effectively than roofs where the water quickly runs off due to slope.

What sort of patch you might expect to work would depend greatly on what the actual waterproofing membrane is at present, and the nature of the damage causing the leak. If the membrane has deteriorated to the point that's it's failed in one spot, it may fail in other spots soon. If there is a clear damage site that is localized to one spot, repair may make more sense, but overall deterioration often means the most sensible repair is a whole new roof. If repair is viable, the repair method should be one compatible with the roofing membrane - hot tar is different from EDPM rubber is different from Acrylic/fabric composite membrane. Using incompatible patch methods may temporarily work, and then fail again shortly.

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