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nando11211 08-07-2009 10:48 AM

Ponding issue on flat roof
Hello all, Im a new first time homeowner and new to this board so Im sure I'll be around here often.

Now onto my problem... Im in South Florida, where we get lovely afternoon long rains almost everyday. About a third of the entire roof is a flat roof in my new house. In the middle section, approx. a 20' x 15' section, it ponds, and remains there for a while. Also, on the east end, there is a 10' x 4' section that also ponds. It is visible that these areas are lower than the other portions of the roof. Also, there is discoloration on the material (asphalt roll & tar/cement?) in these areas and it also is not as smooth as in the other areas where there is no ponding.

I plan on posting pictures soon.

My question is, is this something that I can repair with a cold press? Or should I do a hot press on the entire flat roof? Approximate cost of hot press? And how soon, or better yet, is the ponding a serious issue?

Thanks in advance.

Ed the Roofer 08-07-2009 10:58 PM

Their is a compound available at Roofing Supply houses that can build up those deflected areas, but the real question is, what is causing the deflections under the roof in the first place?

The compound is sort of like a drive-way patch asphalt material, but I believe it is intended to be roofed over.

Sorry, I have never used the stuff, so I am a bit scarce on details and specs.


Michael Thomas 08-08-2009 01:07 PM

Roof ponding, and what to do about it, is a controversial topic.

At one extreme the majority of flat roofs I inspect experience at least some ponding, and the manufacturers installation specifications for modified bitumen roofing (the most common type of flat roof membrane in my area) allow for ponding water to stand on the roof provided it evaporates within 24 to 48 hours.

At the other extreme I sometimes see ponds or 3, 4 inches deep - or even deeper - these ponds can be imposing loads of thousands or in really extreme cases tens of thousands of pounds on the roof structure and the problem is only going to get worse: the high loads compress the roofing material and deflect the structure below it, deepening the pond, imposing a heavier load, deepening the pond .... until roof collapse provides foolproof drainage.

So it really depends on the type of roof membrane (some commercial PVC and related membranes are rated for continuous water exposure), the extent of the ponding, and the nature of the roof structure.


If you are in a non-freezing climate, one option if you have substantial ponding is a pond pump, for example:

Yoyizit 08-08-2009 02:03 PM

"How flat is a flat roof? For many years, NHBC Standards have recommended a 1:40 fall to flat roofs.
This figure has been amended to take account of British Standards and many BBA Certificates which refer to a finished fall of 1:80. Particularly with flat roofs using timber joists, the design fall should be 1:40 to be reasonably certain that the finished fall will be at least 1:80. (NHBC Standards Extra, Issue 29, April 2004)."

David Bailey 11-23-2010 03:38 PM

Ponding issue on a flat roof
A seriously ponded roof needs to be stripped back, re-decked and re-felted (and some joints may need replacing), although it may be possible for a professional to apply foam.
If the ponding is only local, try using liquid bitumen from the DIY store.
Apply Thomson's Roofseal ("with added fibres") or similar, using a trowel to produce a layer about 1/8" thick. It dries in a few hours, but leave it a fortnight to harden properly. Then apply additional layers at fortnightly intervals until the roof is levelled off.
You can virtually halve the cost of this operation by using a cheaper water-based bitumen (Thomson's or Wickes), though it takes longer to harden. Finish off with the better stuff.
You could also lay a coat of polystyrene beads (available in 10cu metre bags), using brushed-on liquid bitumen as an adhesive. Don't try it on a windy day!! Then trowel in liquid bitumen. This gives lots of volume without adding much weight (a bit like foam).

bob22 11-24-2010 10:56 AM

In case you, like me, can never remember:

The fortnight is a unit of time equal to fourteen days. The word derives from the Old English fēowertyne niht, meaning "fourteen nights".[1][2]
Fortnight and fortnightly are commonly used words in Britain and many Commonwealth countries such as Pakistan, India, New Zealand, and Australia, where many wages and salaries and most social security benefits are paid on a fortnightly basis.[3] The word is rarely used in North America, except regionally in Canada and in insular traditional communities (e.g. Amish) in the United States. American payroll systems may use the term biweekly in reference to pay periods every two weeks. Neither term should be confused with semimonthly (in one year there are 26 fortnightly or biweekly versus 24 semimonthly pay periods).

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