Leak in valley: how to fix?

[Yankeesouth]

I have a leak somewhere in one of the valleys on my roof.  I plan to get a new metal roof next year but need a quick fix to hold me over.  The roof has asphalt shingles that are about 15-20 years old. Not in bad shape but also not in great shape.  I've seen some roof patch stuff, 1 gal, in the box stores.  Does it work?  Someone told me to use blacktop tar. (like one uses on their driveway)  Will that work?

[glenn kangiser]

Leaks can be elusive and travel quite a distance unseen between shingle and felt layers.

I would look under the roof for clues then get on top and look for more clues.  Check especially around roof penetrations.  There are special roof patch products made by Henry and others.  I like the kind that can be applied to a wet or dry roof with reinforcing fibers to keep it from running.

[Yankeesouth]

Glenn
Finding it is the problem.  Unfortunately I have done the things you suggested. I cannot find the slightest trace of even a possible leak.  I traced it from the end of the structure to the peak....and nothin!  Oh I did it inside and out.  Checked for sticks, punctures, crumpled shingles, etc....  I am looking for a cover-all type of patch to get my through until next year.

[Redoverfarm]

If you have narrowed it down to a particular valley I would be tempted to either replace those shingles adjacent to the valley and apply a layer of ice or storm guard on the sheeting underneath.  Any patch would just be that a patch or temporary and it could fail shortly or longer.  Thats a gamble.  There should be tell-tell signs on the sheeting in the attic  (water stains) to give you the general direction. 

[MountainDon]

From PEte

As Glenn suggests, "Leaks can be elusive and travel quite a distance unseen between shingle and felt layers." They can also travel on the top of sheathing, and that might be the full width of a 4x8 sht. of plywd. or OSB; or they can travel down the top surface of a framing member until they find a joint or some such, and finally appear on your ceiling, or whatever.

You have to look at the underside of the sheathing and framing, in the attic, and up-slope in both directions from the valley, all the way up the valley. If you can track it upward any distance from the inside, it will be above there on the exterior surface of the roof, per Glenn's comments. The obvious leak points are at a shingle crack over an open joint or space btwn. tabs in the next shingle row down; or a crack in a shingle below, right under a tab space above, and these are less easy to see but more likely, once the outer surface of the shingles looks O.K. Look for a slightly lifted tab which might indicate a popped roofing nail below. Look at up-slope roof penetrations where water can follow the top of the sheathing or framing to the valley or toward your leak point.

If all else fails, take a 6' + piece of 15# felt, and at a suspicious area, tuck it up under a row of shingles and weight it down with a few planks. Water the roof and if the leak shows up you guessed wrong, but if it doesn't your leak origination point is under the felt. And, the felt will probably prevent the leak for a year until you replace the roof.