water on flat roof?

[srob]

Is standing water on a flat roof bad? I don't know anything about flat roofs but I saw it on a property I was checking out...it covered most of the roof. Shouldn't it drain off, not relying solely on evaporation? There were no roof drains but there was a gutter on the side.

[Macrolide]

I passed up a cool mid century modern for this very reason. I didn't seek a professional opinion, It just didn't sit well with me. I'd be worried about roof deterioration, leaks, and mold above the ceiling.

[Spork]

Flat roofs are their own specialty voodoo.  I don't pretend to know anything about them.  But yes, that would scare me.

Every large commercial building around here has one.... but they also have some amount of drainage.  We are in the south where we don't get a huge number of snow/ice days.  I worked in an office that was a small portion of a flat roofed ex-Walmart.  Every time there was snow/ice blocking the roof drains, we had about 10 ceiling leaks.

[GuitarStv]

A flat roof is a stupid idea.  Regardless of the materials you have on the roof . . . they'll eventually fail.  Gravity will be waiting for that moment.  All roofs should be built with a slope, even if it's only slight.

[BudgetSlasher]

A quick google says that a flat roof should not be truly flat (~.25" per foot slope) and that water should not stay for more than 48 hours (a few pages state this).

So if it is truly ponding there yes that is bad.

[Green_Thing]

I'm an architect... I was motivated to register and make my first post just to respond to your question.

The thing about flat roofs that don't drain well. It's not that they can't be corrected. You can reroof and add tapered insulation and crickets and end up with a well behaved "flat" roof. Think $30K.
The thing is that ponding water on a flat roof is a compounding problem that can have serious consequences for the structure. A puddle forms. It's weight makes the roof sag eventually. The sag allows a larger puddle to form. The larger sag stretches and  tears the roofing and little leaks begin. Rinse and repeat. It doesn't take long to have a serious structural problem develop under that weight. Untended, the roof will cave in. Are you looking at buying a building in that condition?

I wouldn't do it. If nothing else, a dead flat roof is an indication of poor design or construction.

My home has a flat roof that was having drainage issues. It had the right slope but it was always leaves clogging the drains. We made a major investment into a new roof, scupper drains, crickets, and insulation. It's fabulous now but it was serious bucks to deal with.

[srob]

Thanks for the replies guys. I was considering making an offer on this investment property but I will let it go. I have never had a flat roof and don't want to deal with it. Thanks for the good explanation green thing! I think it is sagging a little bit because it is dry around the edges and has about an inch of standing water everywhere else. I bet it would be pricey to fix, since it is 3 stories up and the roof is about 3000 sq ft, plus there are hvac units on top of it.

[Well Respected Man]

I can confirm that even with a new roof, if you don't go with the more expensive solution, sloping to the drain or gutters, it will quickly fail. We recently had one of our roofs redone with a bit of a slope, new gutters, and lots of insulation. The other roof was replaced about 8 years ago, and because of the water, failed very quickly. We had to have another specialist company come in to make repairs, but this reminds me to have them back before it starts leaking again. When we insulate this second roof, maybe we'll do a new roof at the same time.

Also, make sure you hire a contractor who specializes in flat/membrane roofs, and is highly recommended by friends and neighbors.

[Fishindude]

Standing water is no good, something is wrong.
Any competent commercial roofer could check it out for you and give you an opinion of what is wrong.  Might be a pretty simple, cheap fix ..... or it might have some serious issues and require an expensive fix?