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Question DetailsAsked on 3/22/2018

What would cause ceiling to leak in rain after roof was repaird?

Patched my flat roof after rain, repaired rain damaged ceiling. It's raining hard now and ceiling is set again. Why

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1 Answer


Assuming the leak is a similar place (where it is appearing as a leak into the attic or roof rafter/truss bays) either the actual leak was not adequately patched, or you had multiple leaks in the roofing and not all of them were found - or a seam in the roofing is coming loose and leaking multiple places along the seam. Water orstaining showing up the same place in the ceiling would NOT necessarily be an indication it is leaking the same place in the roof - could be another leak in same general area dripping down onto ceiling drywall and running to its low spot before it shows up as a ceiling stain or drip - can sometimes (especially if there is a vapor barrier above the ceiling drywall) travel dozens of feet before showing up as an interior leak.

Commonly, roofing leaks - at least ones larger than nail drips - can be fixed the first time around because the roofing is visibly missing or deteriorated or bubbled, though finding leakage points on flat roofs is tougher than on sloping roofs.

A very common leak point which can defy early detection is at flashing (around chimneys, skylights, raised A/C platforms, dormers, next story walls, valleys, etc) and roof penetration boots/seals. Of course, if leaking around a roof penetration or flashing and dripping down directly from that penetration that generally ties down the source - though not always, especially if those happen to lie at a low point in the roofing (generally mid-span of trusses or rafters or along an edge on flat roofs) where the leakage through roofing will naturally accumulate anyway and drain down. I have chased a fair number of roof leaks which were expressing themselves as drips down the pipe or vent penetrating the roof, but actually originated elsewhere and just happened to find that place to come through the water barrier and sheathing.

Plugged/iced up or corroded roof drains are also a common source for leaks, as are curb drains or overflows around the edge of the roof.

With flat roofs, is much tougher because you cannot easily follow the wetness uphill to the leak - it can spread out from a roofing leak or crack or damaged flashing many feet in any direction before it finds a way into the attic.

For this sort of situation, and especially useful on flat roofs, while it sometimes is not real definitive during actual rain (totally wet surface tends to mask the image), a thermal infrared camera scan can usually detect the extent of the wet area well. Though again, on flat roofs finding the source is much tougher than on sloping roofs where the highest point in the wetness in the roofing layers will be at or very near to the leak.

Thermal IR shows up wet spots best when there is a significant temperature difference between the attic and outside air temp - so may require having contractor (or yourself, using a rented thermal IR camera for typically $40-50 / $100 rental for 1/2 and full day respectively from Home Depot or tool rental places and also some auto parts stores) during the early morning (pre-sun exposure on roof) or evening at least several hours after the sun has gone down, when there is a significant temperature difference between attic and roof surface. (Works because the wet spot, being wet, retains heat longer than the surrounding material, and shows up as a warm spot from inside heating due to higher conductivity of the wet area at night).

Depending on how much water is in the roofing, and especially if the water has travelled in the roofing materials or fully saturated the sheathing, it may take multiple trial repairs to find the source. I have seen cases where after multiple tries to repair possible leak points, they either waited till the wet roofing drained/dried out (meaning several weeks of dry weather) then used a thermal scanner to find the leak point by spraying the roof with a hose, using the camera/scanner to locate the wet spot in the roofing/sheathing right after the leak started wetting the underlying materials - catching the wet spot as it gets started and is still small, thereby tying down the source to within a foot or two.

In other cases, they give up and do a total reroof or overlay - especiallyo if trying to chase several distinct wet areas. Sometimes flat roofs, especially built-up "tar and tarpaper" roofs, can look almost like a dalmation in a thermal scan - with several to even dozens of wet spots showing up at multiple small leaks, and sometimes older roofs which have started letting water through the built up roofing as it ages and gets permeable can be essentially totally saturated from the multiple leaks.

Finding the leak is obviously critical - a tough question for you is whether the roofer you already used was competent and just did not find the leak (or all of possibly several leaks), or if he was not competent and you should be looking at getting a new roofer to check it out. With a second attemtp to find a leak and especially with a flat roof, I would definitely be going with a roofer who can bring and is experienced with a thermal camera.

Answered 2 years ago by LCD

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