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Our building has decided to paint a 13-ft x 13-ft section of the roof that's leaking (just a few drops with lots of rain). This roof has a fence since it's an "outdoor" area that we all share, so it's secure to work there. We live in the Caribbean, so this roof receives lots of sun. The roof is not covered.

Someone went over there to quote the job, and he said that he would apply a primer and then an "ultra-white" roof coating. I was checking in home depot, and most (if not all) roof coatings are very white.

So my questions:

Since we don't need the roof coating to reflect the sun or UV rays, is it possible to just apply a very good/thick primer and that's it? Many years ago they applied a white roof coating, and it was so bright that we couldn't sit up there.

We only need a product that will seal the concrete and stop leaks. I don't know much about this, but I was thinking of something like a Satin Clear Seal Concrete Protective Sealer or a Water-Proofing Membrane. Whatever we apply, it can't be a bright white.

The ideal product would have the following characteristics:

  • Seals the concrete and stops the leaking
  • Since the concrete is very rough, it'd be great if the coating's thick enough so that the concrete doesn't fill so rough if someone falls.
  • If the product has some color, that'd also be great. But it's OK if it's clear. It cannot be ultra-bright white.
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  • White is chosen to reflect light and therefore heat. Are you ok with the surface getting warmer?
    – JPhi1618
    Jan 24, 2020 at 18:48
  • I'm OK with surface getting warmer. It can also be a not so bright white that the eyes can tolerate.
    – rbhat
    Jan 24, 2020 at 19:02
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    Primer by itself is not sufficient. You should be able to get a product in colors other then white ( or have it tinted grey ). I would not chose black as the heat build up would then be transferred in to the living space below it, increasing your costs to cool it in a tropical environment. Also once it is sealed you could prevent future damage by building a floating cedar deck over it. I would be happy to trade labor to build it next winter in exchange for room and board and round trip flight from Alaska :)
    – Alaska Man
    Jan 24, 2020 at 19:15

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You really need a sealer. White is chosen to reflect the heat AND this will last longer than black. When we used hot tar on roofs we then painted them silver or white. If we did not do this the tar would end up in puddles.

Primers are made to seal wood and wall coverings, not a concrete roof. You need a coating with some “body” that maintains flexibility and don’t go cheap, get it sealed before it gets worse. Multiple coats may be needed but a quality product will probably work with one coat based on your description.

So get the right product for the job and don’t go cheap.

Edited: I noticed the link for the Elasto lock , read the instructions this product suggests power washing, they mention acid etching to clean newer pours and do not mention primer at all. DO NOT USE PRIMER WITH THIS PRODUCT!. It needs the porosity of the concrete to bond as I have mentioned in several comments. But I believe it would be a good product to use to seal your leak. As I have not used the satin sealer in the original question I looked up some info. A question was asked about a concrete roof leaking here is rustolium’s response:

Rust-OleumSupport March 04, 2019 Thanks for reaching out, Richard. Rust-Oleum Sealkrete Clear Seal Concrete Protective Sealer will offer waterproofing qualities, but you may need to consider two coats for adequate coverage. You will get 150-300 sq ft of coverage and you can recoat the product in 2 hours. - Rust-Oleum Product Support 1

So so as I said I would not use that sealer it only had sealing properties, but the ElastO lock you linked to looks like a good flexible product made to seal cracks. And no priming is required but cleaning is.

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    I found this on the intertubes sanitred.com/waterproof-roofing-repair/# It comes in tan, grey, black and off white.
    – Alaska Man
    Jan 24, 2020 at 19:38
  • Thanks. One of the products I posted was a sealer (the Satin Clear Seal Concrete Protective Sealer). Do I need to apply a primer and then the sealer?
    – rbhat
    Jan 24, 2020 at 20:28
  • I have not used that particular brand the one I use is clear and doesn’t require a primer but it is like 50$ per gallon I can’t remember the name right now.
    – Ed Beal
    Jan 24, 2020 at 21:21
  • From what I've read, since a rubber membrane coating is stronger than a sealer, I was considering using a primer and then the rubber membrane. What do you think? Thanks.
    – rbhat
    Jan 25, 2020 at 4:43
  • Membrane coatings are better if they are made for the structure type. In this case I would follow the instructions for the membrane coating, primer might sound good but be exactly the wrong thing to do. My example is The epoxy paint I use requires etching the concrete, if the concrete was primed the epoxy paint would peel up because it could not grip the concrete properly. So read the instructions on the coating if it recommends cleaning like the metal roof coating I use then I clean. use the type of primer or cleaning your coating recommends if it recommends cleaning I would etch concrete.
    – Ed Beal
    Jan 25, 2020 at 15:57
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For a flat roof I wouldn't just apply a primer and paint. That might work with something with a pitch but I am getting from your question it is just a big flat deck area.

Meaning there should be some sort of membrane (silicone, rubber, epoxy, there are lots of choices) then a sealant. Sealant in most cases is not made to be walked on so just putting that down in a flat area is a recipe to have to do it again.

(But I agree with Ed's sentiments in that there is no way you should just use a primer - that would be scuffed up and useless in a few months if there is traffic)

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  • So you're saying that I should use something like this rubber membrane coating instead of a sealer?
    – rbhat
    Jan 25, 2020 at 4:32
  • Very few people actually walk on the rooftop... 2-3 people/week tops.
    – rbhat
    Jan 25, 2020 at 4:35
  • It doesn't matter how many people walk on it you need a membrane. Paint isn't good for flat roofs period, ones with any sort of traffic is a basic no-no. The link you have is definitely suitable for your use, but still needs a sealer on top of it - or it just won't last. The membrane installation should be just a hair more than the paint. It takes a bit more time to paint on the membrane and it costs a little more. It will feel better on the feet and perform way longer (get one that is graded for foot traffic).
    – DMoore
    Jan 25, 2020 at 19:34
  • So right now, I have bare concrete in the roof. I guess my question is: besides the membrane, should I apply something before and/or after that?
    – rbhat
    Jan 26, 2020 at 0:25
  • For example, should it be primer and then the membrane? Or primer, then membrane, then sealer? Or just membrane and then a sealer?
    – rbhat
    Jan 26, 2020 at 0:26

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