The first step to locating where the roof leak may be coming from starts in the attic. Get a good flashlight even if you have lighting in the attic already. You will also need a tape measure that is at least 25′ in length.
Now go up to the attic and take a general look around the area above where you noticed the leak coming through the ceiling. The best time to diagnose a roof leak is when it is raining or snow is melting on the roof. However, there will still be some signs of where the roof leak is even if it is dried up already.
If it is raining then look for any water coming in and trace the water trail until it stops. Use the flashlight to reflect off the water. If it is not raining the water should have left a darker stain on the rafters or roof plywood. Trace any water stain as far back as you can to pinpoint the general area it may be coming from. You can also look for daylight coming through as well. If you don’t see a water trail try looking around each vent pipe or anything that goes through the roof to the outside. Another way to figure out where the water is dripping from is to look at the insulation to see if it is wet or packed down from being wet. This will only tell you where the water dripped down and not necessarily where the roof leak is. Water can travel a good ways down the rafters before it drips down to your ceiling.
Now that you have located the general area or found the actual spot where the roof is leaking; take out your tape measure. Measure from one end of the attic to the area you think the roof is leaking. Now take another measurement from one of the sides of the attic to the roof leak area. Wright down these measurements so that you will remember them when you go up on the roof to repair the leak.
Now that you have determined where the roof leak may be coming from; it’s time to go up on the roof. Just make sure you wait for good weather before you go up on the roof. It’s simply dangerous and difficult to repair a leak when the roof is wet. Before you go up on the roof you will need a couple of simple items. The first thing you need is a caulk gun with a tube of roofing tar. You can find roofing tar in the roofing section of most home improvement stores. You will also need a hammer and your tape measure.
First use the tape measure to locate the same area you measured out from the inside. Take a look around this area for anything that might look like it may be the cause of the leak. Some causes could be a torn roofing shingle, a missing shingle, popped out nail, caulking missing around a stack pipe or anything else that is caulked to seal out water. If you think the leak was coming from the peak of the roof, you may have a ridge vent that has become separated, caulking leaking or nails popping out.
It may take a bit to find where the actual leak is coming from but if you take your time and continue to investigate upwards of the area you think the leak was coming from; eventually you will find it.
When you do find it, use the roofing caulk to seal up any holes or missing caulk. Use the hammer to drive down any nails that may be sticking up. While you’re up on the roof it’s a good idea to check the gutters for blockage. Blocked gutters can cause the roof to leak when they back up and drip in through the eaves.
If you find missing or broken roofing shingles you should replace them as soon as possible. Once one shingle goes, it is easier for the wind to catch the surrounding roofing shingles and rip those off as well.
The last step is to check the attic again when it is raining or you could have someone use a garden hose on the roof while you investigate the roof from the attic to check for leaks.