Flat roof deflection causing ponding
Flat roof deflection causing ponding
(OP)
Hi,
I have to do an analysis on a flat roof that is deflecting and has a pond at the lower end full of water rusting through the roof sheet.
The purlins are 200x47 oregon spanning up to 7.5m (so they are undersized already). The roof sheet is deep profile (Kliplok here in Australia) with 41mm ribs. The manufacturer recommends 1 degree min, preferable 2 degrees min slope. I'm not sure what the slope of the roof actually is, but looks very flat.
I've been told that a build-up of hail has caused the ponding in the first place. I haven't been to the site so can't tell you if the gutters and downpipes are undersized and a blockage has caused a build-up of hail.
My question is: if there is now a permanent pond in the roof holding water, my assumption is that it is caused by general deflection of the undersized rafters under the amount of water that it is currently holding and not by a specific hail event.. is that correct or do you think an overloading case for max a couple of hours may have induced a permanent deflection greater than it already had?
Maybe people who live in snow prone areas can tell me more about this..
I guess I'll need to work out the depth of water that would build up during a storm event over the length of roof also..
Thanks
I have to do an analysis on a flat roof that is deflecting and has a pond at the lower end full of water rusting through the roof sheet.
The purlins are 200x47 oregon spanning up to 7.5m (so they are undersized already). The roof sheet is deep profile (Kliplok here in Australia) with 41mm ribs. The manufacturer recommends 1 degree min, preferable 2 degrees min slope. I'm not sure what the slope of the roof actually is, but looks very flat.
I've been told that a build-up of hail has caused the ponding in the first place. I haven't been to the site so can't tell you if the gutters and downpipes are undersized and a blockage has caused a build-up of hail.
My question is: if there is now a permanent pond in the roof holding water, my assumption is that it is caused by general deflection of the undersized rafters under the amount of water that it is currently holding and not by a specific hail event.. is that correct or do you think an overloading case for max a couple of hours may have induced a permanent deflection greater than it already had?
Maybe people who live in snow prone areas can tell me more about this..
I guess I'll need to work out the depth of water that would build up during a storm event over the length of roof also..
Thanks






RE: Flat roof deflection causing ponding
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Flat roof deflection causing ponding
What is the purlin spacing? Hail buildup can certainly cause yielding of steel roofing, so permanent deflections between the purlins. Hail is a load case not dealt with well by the Australian loading standard, but after every major hail event, there are reports of leaking roofs or roof collapse of flat roofs and/or roofs with box gutters.
RE: Flat roof deflection causing ponding
I've seen something like this on a steel framed roof, a heavy tree branch fell on the roof during a storm, causing enough deflection for a pond to form. The pond was still there when the tree was removed until the water evaporated, allowing the roof to go back to normal. The members were still in the elastic range.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flat roof deflection causing ponding
It may have permantently deflected or it may have been built wrong. The problem is how to fix it. More info would be required to even give a wild guess.
Good luck.
RE: Flat roof deflection causing ponding
Flat roofs work very well in places like The Gobi, The Sahara, The Mojavi....I kid, of course.
RE: Flat roof deflection causing ponding
Sounds like a good location for a roof drain!
RE: Flat roof deflection causing ponding
There is your best solution, temporarily. Still should look at the rest of the roof structure though and possibly beef up the framing.
If there is any capacity in the structure, you could also consider installing tapered foam insulation over the structure with a 1/4" per foot minimum slope, and roofing over. Cost may be prohibitive though.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Flat roof deflection causing ponding
You can't figure this problem out sitting at your desk or chatting on a forum, you must get out to the building to determine what's actually going on, and the owner should have no qualms about paying for this investigation work. Stretch a string (laser) on some of the various roof framing members to assess their current deflections, over the areas of concern. What is the actual roof slope, if any? This should be a meaningful average of conditions for your own edification. Then do an analysis of the various framing members to determine how much of the total deflection each contributes. Maybe a purlin needs to be added every other purlin space or some such. Stiffening which system gives you the most bang for the buck, and brings the entire roof back into code complience? You say the roof sheathing is already being damaged by this condition, and that would seem to indicate a new roof, which would all be part of your argument for doing it right this time. Look more closely at the original design loads (both DL & LL) maybe you can find a few lbs./sq.ft. to help compensate for the new sloped insulation.
RE: Flat roof deflection causing ponding
Here's a photo attached for your viewing pleasure
I think a pond on a roof is unlikely to not dry out long enough for the roof to rust, so this indicates that there is permanent dead load deflection causing the pond to refill every time it rains.
The roof sheet is rusted, so no problems asking for that to be replaced with an appropriate profile for the slope. . The purlins (at 1m centres) are undersized for dead load deflection regardless, so they are going to be replaced or supplemented with new ones. I will recommend they go to 2 degrees minimum, regardless of the manufacturer's recommendation, as part of the replacement process.
Noone is in doubt that the roof needs replacing. It's a cheap roof on a shed with no internal lining, just roof sheet and purlins. I'm doing the calcs remotely, so can't visit the site
I believe calcs showing that the purlins are undersized for dead load and an explanation of the dangers of flat roofs will be sufficient for the owners of this roof.
I like rowingengineers suggestion of upping the deflection criteria to deal with the flatness aspect.
RE: Flat roof deflection causing ponding
If they are only supporting the cladding, even halving the spacing won't reduce the dead load significantly.
At least turn them around if reusing so that the camber is up.
RE: Flat roof deflection causing ponding