Hot Topics: Low Spots on a Flat Roof

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Original Post: Low spots on a flat roof

MiamiCuse Member

I have some areas on my flat roof that are low. When it rains it puddles there. This new roof was installed (2 ply hot mopped) in 2015.

Recently, I noticed the low spots have began to "blister" (not sure this is the correct term).

A flat roof.

When I took a blower to it, it kicked up a lot of sand and black stuff and now I see some bald spots.

I called the roofing company out and they said the only way to repair this is to put more asphalt paper over the low areas, another smaller sheet over that, and then another smaller sheet over that, so that you're basically building a pyramid to create a high spot. They will make a rectangular pyramid composed of a slightly smaller sheet for each layer. Is this the right strategy?

The low spot shape is irregular. So if you lay a rectangular sheet down, you just raised the outside again, and so the inside spot will be lower by the same amount. I guess I don't see how this would solve the problem. I am wondering if there is some sort of self-leveling liquid that can be poured over this to level the whole thing, and then another ply can be put over the whole thing to seal it off better?

Tolyn Ironhand Group Moderator

I can tell you on commercial buildings it's very common to have standing water on the flat roof. Normally, it just evaporates if it doesn't drain off. I am not sure if this is really an issue. How long does the water sit?

MiamiCuse Member

It sits for a few days. The problem is we are in South Florida so the sun is hot, and for a good part of the year it rains every day. The sun bakes the standing water. Over time, it eats into the roof and leaks.

GregH Super Moderator

Likely the best approach is to lay down felt or another material with tar between the layers. If you try to use some type of self-leveling material, there is potential for it to crack, making it useless. Layering would be a stronger and longer lasting solution much like how plywood gets it strength.

Also, it may help if after the repair cures to overlay a white or silver coating over the whole roof to minimize the effect of absorbed heat on the dark surface.

MiamiCuse Member

What about this? I am not a roofer so maybe it's a crazy idea.

Since the ponding of water has kind of traced the outline of the low spot already, what if I get a few bags of playground sand and spread that over the area, using a piece of straight 2x4 to gently screed across the sand?The sand will fill in the low spot.

Then, I cut a piece or two of rectangular felt covering the affected area, attaching it to the existing roofing around the edges with tar. Maybe I add another layer over it. I use fiberglass mesh around the edges, applying tar or bulling over the patched area, addding granules. Would that work?

GregH Super Moderator

You're right. It's a crazy idea. The roofer had the right idea by layering roofing felt. Go with that and you will not have to deal with this problem for a long time.

MiamiCuse Member

I have a hard time seeing how the roofer's solution will solve the problem.

He said he will cut a rectangular piece of felt and apply it over the low spot. Around the lips of the low spot, he will be building it up by, say, 1/16." The low spot will also raise by the same amount. He will just end up creating another ponding depression. He could add, say, five layers and with each layer the edge and the middle will raise by the same amount. Nothing has changed. Something has to fill in the low spot so water can run off. If he stretches the felt so it doesn't dip at the low spot, it will dip as soon as someone walks on the roof.

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