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I have a "roof deck" above my condo. It's a flat surface that doubles as the roof of the living area below.

Roof Deck Overview

I noticed an approximately 2-inch diameter bubble had formed on the surface and decided to try to fix it.

I was easily able to cut the bubble and expected to find a piece of dry rotted plywood, but the material appears to be some sort of cement board. See example below.

The hole is approximately an 1/8 inch deep.

Hole where bubble was

What is the proper way to repair such a hole? Obviously the repair needs to be waterproof.

A guy at homedepot suggested something like Behr DeckOver but reviews on this online seem pretty mixed. Also, I'm not sure if that's an appropriate material for a patch job like this.

Result

Using Bondo All-Purpose Putty as suggested

enter image description here

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    Next time, don't cut a hole in your roof (which was presumably still watertight at that point) before you sort out the right way to patch it. "flat roofs that get walked on" are a small and highly specialized portion of the roofing market. Leaks are very likely if you don't get something compatible with what you already have.
    – Ecnerwal
    Commented Mar 29, 2022 at 23:31
  • Didn't think I was cutting into the roof itself, but the paint. Productive suggestions would be very welcome. What type of material is usually used in this application? I'm not sure what that surface is.
    – agf1997
    Commented Mar 30, 2022 at 0:08
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    If your condo has maintenance staff or at least management that might have records, try to find out what, exactly that is, or at least who put it there so you can ask them what it is. I'd put solid odds on it being the waterproof membrane in that installation. I don't see the typical "pavers over membrane over structural roof" arrangement, and I do see what appears to be a roof drain in the corner.
    – Ecnerwal
    Commented Mar 30, 2022 at 0:37
  • it is quite possible that you do not own the roof of the condo ... check with your condo association for directions about what to do next
    – jsotola
    Commented Mar 30, 2022 at 0:45
  • I do own that surface. I’m on the board. Records from original construction are scant.
    – agf1997
    Commented Mar 30, 2022 at 0:48

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Looks like it might be a fiberglass-based multi-layered deck waterproofing system. Professionals will tell you that the deck should be redone, entirely.

I think I would patch with bondo (auto-body filler) and paint it with deck paint, knowing that it may crack or fail but watching for it and addressing immediately if a problem arises. Eventually the deck will need recoating...

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  • Any particular bondo product? 3m.com/3M/en_US/p/c/b/bondo
    – agf1997
    Commented Mar 30, 2022 at 1:23
  • I would use something to abrade the perimeter of the patch area, like a "cheese-grater" planer (aka "Surform planer) and/or sandpaper, then blow it clean with my air compressor and use rags with alcohol (or paint thinner) to clean it. Then I would probably actually use 2-part wood filler, which is just like bondo but (in my experience) sets a bit slower, allowing you to shape/work it a bit. Then plane it flat to match surround (it will be noticeable, but whaddaya gonna do...) prior to prime and paint.
    – Jimmy Fix-it
    Commented Mar 30, 2022 at 1:33
  • Do you think this would be reasonably waterproof?
    – agf1997
    Commented Mar 30, 2022 at 18:09
  • I do. If the filler material (the 2-part resin-based wood filler/bondo) adheres well to the surrounding material (the abrading will help ensure this) and you use a deck paint, it should be waterprooof. However, patches of any sort are always a watch point, so monitor frequently and address at first suspicion of cracks. Anyway, the alternative is an expensive recoat (or intrusive and expensive demo and recoat).
    – Jimmy Fix-it
    Commented Mar 31, 2022 at 0:35
  • Is this the kind of stuff you have in mind? homedepot.com/p/…
    – agf1997
    Commented Mar 31, 2022 at 5:18

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