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Question DetailsAsked on 10/28/2014

Mysterious roof leak: is a water test a good idea?

It's a six-year old roof, and leaked for the first time in a recent heavy rain storm. It's impossible to tell inside where it's coming from, and the roofers can't find any damage. They recommended a water test. Is this a good idea, or is there another way?

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2 Answers

Voted Best Answer

I completely agree with your roofer. Leaks can travel down rafters and find their way in. Sometimes it is the best way to find the leak. The advantage is that it eliminates the communication gap between you and the roofer. Similar to going to the doctors office and when you get there the pain goes away. Have the pain show up when the professional is there to catch it, find it and fix it. Otherwise it may be a long drawn out process. You don't want a mystery leak to fester. That is how mold problems occur. You need repeated moisture on a surface that the mold can grow on and feed on. Eliminate it fast and get rid of the headache. Then go have a margarita and enjoy your home.

Answered 5 years ago by ExteriorUpgrader


Thanks so much, ExteriorUpgrader! That's very helpful--especially the margarita part.

Answered 5 years ago by Guest_90904005


I am a little surprised the roofer could not find it because while finding flat roof leaks from the top can be tough I don't think I have ever seen a sloping roof one that eluded detection, assuming you have an accessible attic - because a leak big enough to run down into the house should have left staining and a wet spot on the underside of the sheathing or on one or more rafters, which could be traced back to where it came through the sheathing. Then, working on top of the roof, you trace from that point upslope to likely entry points - or at worst, start tearing off shingles and underlayment to track the wetness on top of the sheathing up to the leak, though usually the likely entry point is pretty obvious once you tie down where it came through the sheathing. Exception - where you have a lot of penetrations lined up downslope/upslope line on the roof, in which case you sometimes end up just reflashing and resealing all of them to be on the safe side.

Also, checking the outside AND inside of vent ducting can nromally tell you if it was leakage around a roof jack or into a vent hood or cap.

Another alternative, especially if the leak could cause significant damage if you water test it, is to use a thermal infrared camera to locate the wetness in the roof - and of course the leak will be at (or in case of windblown rain entry, very close to) the high point of the wet zone, assuming you have a sloping roof. Some roofers have these (especially commercial roofers who do flat roofs), or you can rent at tool rental places and some Home Depot stores (they are just starting a program to rent them) and a few auto parts stores - for about $75/day, some places will rent for a half-day for less. Many energy audit and insulation firms also have them.

By scanning from the underside in the attic and also from the top of the roof you can usually find significant leaks (ones that wet the underlayment and sheathing) pretty easily, especially in the morning before the roof fully heats up due to sunlight, and late in the evening when the heat contrast is greater - of course, the leak area has to still be wet for this to work, so either after a significant rain or after a water test.

Google this search phrase for examples of what the imaging looks like - images for thermal IR roof leak . The images link should be the first one that comes up (after any paid ad links).

The problem with water tests are, like in your case, if you can't find the leak right off by inspection after the leak first occurs, it is likely to be travelling ovaer and/or under the underlayment a ways before breaking through into the house, so a water test just replicates the leak - but you can end up tearing a fair amount of roofing off to try to locate the entry point through the shingles, whereas the thermal IR camera shows the wet zone non-destructively so generally you tie down the actual leak location (at or very near the head of the wet zone) within a foot or so.

if you want some more advice or educated guesses about the cause, you can post photos or info using the Answer This Question link right under your questions - photos post using the leftmost yellow icon right above the Answer box. If you post photos, and describe where the leak appeared in the house - particularly with respect to how high on wall (and if interior or exterior wall), which story, how far from corner of house, how far from kitchen or bathroom or furnace vent penetrations in the roof, how far from valleys in the roof, etc, perhaps we can give you some ideas of the likely cause. Assuming a sloping roof, if the roofer found no problems visible up on the roof then it evidently is not punctures or missing shingles or metal roof panels or torn off or rusted through flashing or missing vent/duct hood or cap, and if a significant leak (dripping water into the house rather than just a damp spot) it is likely more than a leaking nail or screw hole, so that normally narrows it down to the roof jack (seal) around a pipe or vent penetration, water blowing into a vent hood whose flapper stuck open, or a leaking valley flashing - assuming of course you did not have a water backup on the roof due to debris accumulation, which the roofer should have noticed if that was the case.

Or source is not roof at all, and is actually coming from a siding or window problem. Reply back if you wish for more input, or rent a thermal IR camera, or get a different roofer who has one and can also do the repair when he finds the leak.

Answered 5 years ago by LCD

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