Roof Maintenance: How to Replace a Missing Shingle
Replacing a shingle is pretty easy to do, but first let's review the concept of how shingles work. The idea behind shingles is that you can keep water out if you can keep water moving down the roof slope. Never give it a place to go but down and the home will stay dry.
When roofers install shingles, they don't start at the top. If they did, the second row of shingles would create a little lip for water to catch on and soak under. Roofers start at the bottom of the roof and overlap each subsequent row over the top of the previous row, overlapping more than half the width of the shingle. Water flows down the roof, and even though it might soak in between the rows of shingles, because they overlap so much, the water can't travel back up far enough to soak into the roof.
When replacing a shingle, you need to maintain the integrity of the shield originally created by the rows of overlapped shingles. If you install a shingle, but nail it in place and leave the nail exposed, water will soak under that nail head, through the hole in the shingle and into the roof. You must install the shingle so that water has no where to go but down.
When replacing a shingle, start be removing any remnants of the old shingle and any debris that may have accumulated between the shingles. If there is a nailing standing proud under the shingles, it will eventually damage the shingle above it, so it must be hammered down. however, if you hammer on the shingle covering the hidden nail, you will damage the shingle. Use a flat pry bar or large sturdy screwdriver and place it under the shingle and lay the flat edge over the nail head. Then strike the exposed portion of the pry bar with a hammer to drive the nail down.
Insert the replacement shingle under the upper row of shingles and position it so that the front edge lines up with the other shingles. Lift up the shingle immediately above the replacement and position a roofing nail an inch or two up from the edge. Insert the pry bar over the nail and strike the pry bar to drive the nail.
Asphalt shingles also have a sticky tar strip on the under side to adhere it the row beneath it. Prewarming the shingle for a couple hours in the sun will make the strip tacky and help to seal the shingle to the other shingles.
If you are replacing a wooden "shake" shingle, you can help it blend better by "aging" it. Mix together water and baking soda and paint the mixture onto the new wood shingle. After a few days in the sun, the new shingle will age in appearance much more quickly than an untreated shingle.