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Roof 5: Re-Roofing a Roof

When it is time to replace the shingles on the roof, what are the correct procedures? Do you cover the old shingles, tear them off and start again or a combination of the two?

According to the building code, you are allowed only 3 layers of shingles on a roof. Asphalt shingles are heavy, starting at 210 pounds per square (100 square feet) for the old butt shingles, to 250 pounds, and more, per square for the laminated shingles. This is the reason there is a limit on the number of layers, allowed on a roof.

Now, the shingle manufacturers are saying they won't warranty their shingles unless they are on a roof deck with a layer of 15 pound roofing felt under them. Most of the re-roofing jobs you see done now always remove the first layer of shingles. The new shingles coming out today are warranted longer than the 10 or 15 years in the past. We can get fiberglass based laminated shingles lasting 40 years. Getting warranties like this tells us to do the job that the manufacturers want, that is start right back down at the roof deck. I've done it both ways, roofing over existing shingles and tearing off the old and starting back at the sheathing over the rafters or trusses. The tear off is not that big a deal, but does a much better job. Get yourself a tool like a garden spade with a built up back and dive into it. Start by removing the ridge caps and go down the roof, opposite to the way the shingles were laid. If these roofs are going to last 30 or 40 years, most of us won't have to worry about re-doing it, anyway. So let's do it right the first time.

The advantages of tearing off the old shingles is being able to see the roof deck; re-nail it; replace boards if needed; replace old flashings, plumbing jacks and roof vents. The job is going to be expensive, so we may as well do it right. The only added expense, really, is the labor and cost of getting rid of the old shingles, a small proportion of the total. The only thing I would be leery of disturbing is the flashing on the chimney that goes over the roof flashing. If this stuff is rotten, of course replace it, but it is embedded, or should be, in the mortar originally by the bricklayer building the chimney. The roofer then does his part later. Installing flashing around a chimney, skylight or dormer is not too complicated if you follow procedures. As you go up the roof with your shingles, go around the chimney or dormer, as well. The front apron is easy since it goes over the top of the shingles below the chimney or dormer. Ask for a front apron which has the edge rolled over on itself to keep it stiff on the exposed edge. On the sides of the chimney or dormer we use step flashing, every row of shingles has a step flashing. Half goes under the next row of shingles, above it, and the bottom half goes over the shingle, below it. When coming to the back of the chimney or dormer, we install a back pan which is about 12" wide at the back.

Now the tricky part: The front apron extends out each side of the width of the amount the step flashing extends out from the wall. This step flashing goes over the front apron on the sides and flush with the front of the flashing. The building code requires 3" of flashing over the shingles.

The back pan extends out from the sides, as well, but goes over the step flashing. Hope this drawing helps explain:

roof flashing

In the Plan View of the drawing, notice the heavier lines extending out the side of the curb of the skylight or the wall of the chimney or dormer. The step flashings extends along the sides, flush with the front width of the apron flashing and the back pan extends out the sides, flush with the width of the step flashings.

Follow the steps in the drawing:

  1. Install the front apron flashing, extending out each side to the width of the step flashing. Cut the bottom of the front apron from the end to the point of the width of the skylight, chimney or dormer. Bend it over, as shown, up the curb or wall. Nail along the front apron high up on the curb or wall, along the front. No nails should go into the horizontal part of the flashing into the roof deck, just into the top of the vertical part of the flashings. Nails should be galvanized roofing nails 1 1/4" long.
  2. Install the first step flashing, along the side of the curb or wall, covering the front apron and flush with its front edge. Nail the flashing in position, high up on the side, through the folded over part of the front apron. This holds both flashings in position.
  3. Since the skylight needs to cover over and down the curb, the flashing needs to be cut down about 1" to allow for this. Fold the remaining extended part of the step flashing over, towards the inside corner, to form an angle, as shown. This stiffens the flashing, as well as, hides the sharp edge. A bit of caulking is squeezed out to fill in this inside corner.
  4. The remaining step flashings are installed, as the shingles are laid up the roof. The final step flashing is cut, on the bottom, and folded over the back curb or wall, as shown. The back pan is then cut to length to include the width of the step flashings on both sides. The part of the back pan that extends out past the curb or wall is dropped down, for a skylight and bent over as was the front step flashing. Caulking is again added to the inside corner. The inside corners are the only places that caulking is used if this procedure is done correctly.

To start off the roofing, place a strip of 30 pound non-perforated roofing felt or other choice of ice protection, along the bottom edge of the roof, over the eaves, extending about 1 1/2" past the roof edge. This edge will eventually fold over into the gutter. The first row of shingles is doubled to alternate the position of joints, positioned flush with the roofing felt, at the front. The manufacturer puts extensive instructions on his packaging pertaining to the particular shingle you are installing. Follow these instructions, referring to spacing of joints and nail patterns.

For ridge and hip caps, I prefer to purchase butt joint shingles in the same color as the roof shingles and for the same warranty period.

Clean out the gutters, after a roofing job and use a magnetic pickup tool for finding any loose nails that may have ended up on the driveway.

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