How can I locate and repair a leak on a flat roof?

Clearly this has been a problem for a while and also the standing water and patch job on the flat section of the roof is to blame.

There is no real shortcut here, as you can see, so the solution is to remove the roofing, repair the water damage, and then have it re-roofed properly. There are ways to apply a flat roof so that it will not leak but slathering it with roof patch is not one of them.

Your problem is going to be finding where the leak is coming from. I have found leaks that came in more than 10’ from the entry point. The water can travel under the rolled roofing and drip down at a seam then run on the rafters especially double plates prior to finally coming down.

Today I use a FLIR camera I can see the water when it is traveling between boards because of the temp difference.

The only way to patch in wet weather is a patch like Henery’s asphalt or other brands of similar type. I would stick with asphalt as that is the best material and the base of the rolled roofing.

I used to do hot tar as a kid and for patches we used Henery’s you might have the leak up in the valley or to the side that is running down so identifying the entrance point will be the most critical part and sealing that point. Asphalt patch can even handle standing water and most brands I have used can be put on in standing water but you have to work it a bit to get it to seal and it works better on dry surfaces.

I used to use a leaf blower to dry roofs off before sealing it helps.

One option which I used long ago when I did not want the cost of a new roof ; I got a gallon of liquid roofing tar and brushed it on every suspicious spot . I had a few damp spots in the roof decking plywood . It stopped the leaks for a couple years until I sold the house. It was ugly to see when on the roof but not so noticeable on the dark shingles from the street level.

If your sure of the leaks location or general vicinity here's a couple of alternative suggestions:

If the leak is coming from the red shingles on the pitched section of roof there's a roof product called "Kool-Roof" which is an elastometric rubber coating. It is applied with a brush or roller. I goes on as a thick liquid and dries to a flexible skin that fills holes and stops leaks. It can be applied to damp surfaces, but not in standing water.

My last suggestion to locate the leak would be to closely examine the shingled roof. Look for any signs of failure (torn or worn shingles, nail heads pushing up through bottom, shingle joints that line up over an exposed nail head). If the leak is reoccurring look for the plywood sheathing to be swollen from water and bouncy when walked on. Zero in on those areas and you will have the general vicinity of the leak. From there you can remove rows of shingles to expose the leak source.

I would ask a few more questions, like, is this a home you plan on living in for years or just a long term Airbnb that you want to patch ?

Maybe I'm misunderstanding your comment about the hole, but couldn't you just insert a stiff wire at that point and have someone up on the roof look for it?. If not measure the distance from a known exterior wall from inside and add 6-8" outside and you have a closer measure of finding the spot of the leak.

Since you have the end point of the leak... work backwards and start removing the interior framing and that will likely lead you to the wetspot ie the hole in your roof which TBH looks like a drunken toddler was your roofer. You can slowly work your way up, since water really only goes down, you will find many wet boards along the way and you won't have to tear a hole in your roof looking for it. Then patch that hole, reroof that job and if you can salvage the framing otherwise put in new boards.

I recently had to fix a bad dry rot problem that was patched so poorly by the previous owner that they tore off the rotted siding, sheeting and installed a new piece of plywood as a nailer strip. Then they reinstalled new plywood over dry rotted framing, installed siding and sold the house. $22,000 later we found/solved the issue and am suing the previous owner for lack of disclosure of the repair, fraudulently sold a house claiming there was no repair as we now know he knew there was an issue but covered it up in addition I have testimony from neighbors they witnessed the work being done the prior summer.

If you are skittish about swinging a hammer or doing repairs blindly, I have a few options to fix this once you find the leak, you can get someone with 15+ years experience in roofs who will happily supply you five different references of old jobs where the customer is happy. Ask that roofer to come over and give you advice, I've had so much luck making friends with contractors, beer and pizza go a long way to get advice. Its likely that this is a common occurence and the contractor has a way to deal with this that you can benefit from.

Next option you can repeat the mistake of the previous person and try to patch this job by draping a tarp over an area or doing other half measures, this is not undoable but its hit or miss and will waste time. Then you become the same type of person who does a half way job and there is a special place in the underworld for people who half ass things.

I guess a nerdy option would be to draw squares on your roof and use different colored water to pour onto that area, when you get that color water into your wood framing you have at least a vertical start point.