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My Dad has a flat roof over his garage. It's covered with tar and felt.
We had an unusually hard and cold winter last year (Republic of Ireland) and since then the roof has developed a leak.
We've made several attempts to patch it, with no success. We know where the water is exiting the roof but we can't tell where it's getting in.
Is there some device that will trace the water from the leaks exit back to where it's getting in under the felt?
I'm thinking more tricorder, and less dowsing rod :)
Knowing the type of roofing system on the home would greatly assist in giving advice on how to track the source of the leak. A moisture meter is what you're thinking of, but in general, a moisture meter will not help you track a leak any more than simply doing so with your eyes. It is used more generally as a way to prove whether water exists in the roof system or if, let's say, a plumbing issue is to blame for the interior leak.
Most flat roofs are never truly flat and have slight slopes and ponding areas. My recommendation would be to pinpoint the location that the water is entering inside of the building while on the roof, and then tracking it back uphill in a "V" shaped pattern. Check around flashings and seams for discrepencies if a hole is unable to be located.
An Infrared camera can be used to look for water infiltration and damage. I would not recommended they for DIY use. They are expensive (low end models $1,000 US) and it takes a lot of training and experience to interpret the results. There may be a local IR company in your area that can assist.
I don't have the money for that so I actually found it like you would find a leak in a car tire.I got a spay bottle of water and dish soap and spayed where I thought it might be then went inside and blew air at that point while my daughter looked for air bubbles. She marked where the soap bubble was . I patched it. If you have a sheetrock ceiling like I do you have make small holes for the air hose but they are not hard to patch.
The closest thing would be a moisture meter (here are a few...). This is not a silver bullet or magic solution to your problem. It will not tell you exactly where the leak is but you can use it to measure different areas of the structure to help you find areas of high moisture.
Even if you do find a certain spot that has higher moisture, this tool will not tell you where it is coming in. For example, you may have a small leak in your flashing somewhere that causes the water to run along a certain area and pool. The moisture meter will help you find the "pool" of water and maybe even help you trace it back along its path, but it will not pin point the actual leak for you.
