Installing your new roof, roofing over an old roof

Roofing over an existing roof eliminates the need for underlayment and saves the time and effort of tearing off the old roof. However, if the old one is too irregular for a smooth finish, tear it off. Otherwise, the new one wont look right and wont protect as well.
Roofs are heavy; local codes may permit as many as three layers or may limit you to two. Check before you decide to simply layer over the existing roof.

ROOFING OVER AN OLD ROOF

Roofing over an existing roof eliminates the need for underlayment and saves you the time and trouble of tearing off the old roof. However, if the old one is too irregular for a smooth finish, tear it off. It won’t look right and it won’t protect as well.

Remember, roofs are heavy. Some local codes permit as many as three layers, others may limit you to two.

COVERING COMPOSITION SHINGLES WITH MORE SHINGLES:

Any irregularities in the existing roof must be repaired in order to produce a smooth roof. Warped or bent shingles should be split and nailed flat. Missing shingles must be replaced so there won’t be a sag in that spot.

When covering an existing roof of composition shingles, it’s best to match the shingling pattern already used on the old roof.
The first step in roofing over an existing asphalt roof is to apply a starter strip along the eaves. Measure the shingle exposure on the existing roof (usually 5 or 6 inches). If the existing shingle does not extend far enough out to spill water into the gutter, add enough to your measurement so the new ones will.

Using whole new shingles, cut the tabs off. Then measure up from the newly trimmed edge to match the exposure in the old roof, plus any small addition to spill water into the gutter. Cut them again at this point. They should now fill the exposure and be level with the old second course. Nail them in place with the adhesive strip adjacent to the eaves. Cut enough starter shingles to go along all the eaves. Discard the tab ends.

On a 5-inch exposure roof, remove 2 inches from the top of shingles used for the next course and butt them up against the bottom of the old third course. This 10-inch shingle covers the starter course and the second course on the old roof. Cut enough of these 10-inch-wide shingles to run the length of the eaves.

For a 6-inch exposure, apply a full shingle to cover the 6-inch-wide starter strip and the second course. Now cut 5 or 6 inches—depending on the old roof’s shingling pattern—off the rake side of the next shingle and butt it up against the bottom of the fourth course of old shingles. This second course will leave a 3-inch exposure on the first course. This will probably not be noticeable. If you have a steep roof, where this course can easily be seen, trim 1 inch from the top of the second and third courses. This will give the first and second courses a 4-inch exposure, which will be less noticeable. On either of these with a shorter exposure, put a dab of cement under each shingle that sits on top of it. The second method requires more cutting and more cementing.

For all remaining courses, cut and apply just as you would for a new roof. The exposure follows automatically, because all new shingles butt up against existing courses. You’ll probably use longer nails—1-inch nails with a 1/2-inch plywood deck or 1,25 -inch nails with full 0,75-inch decking.

COVERING TAR AND GRAVEL WITH COMPOSITION SHINGLES:

You should remove a built-up roof rather than shingling over it. If you must shingle over one, however, prepare the roof by removing the gravel and nailing down any bubbles.
Another option is to lay fanfold insulation — typically applied under siding—over the gravel. This will smooth the surface so the new roof won’t look lumpy. Reroofing will also require a change in drip edge, from one designed to keep gravel in place to one made for shedding water. Once prepared, shingle the roof as you would a stripped roof.

COVERING WOOD SHINGLES WITH WOOD SHINGLES OR SHAKES:

It’s best to install shakes over old wood shingles because there is less likelihood of leaks and rot. If you use wood shingles, make sure they’ve been treated with a preservative.

If the old wood shingles on a roof are in good condition, you can lay new wood shingles over them. Some preliminary steps are still necessary. First, nail down any curled or warped shingles to provide an even surface. If one won’t stay down, split it, pull out the pieces, and slip in a new shingle.

Remove shingles along the eaves, rakes, and ridges, and replace them with 1×6 boards. To do this, measure 5,5 inches back from the edges of the rakes and eaves, then snap a chalk line as a cutting guide.

ROOFING OVER AN OLD ROOF

Set the blade on a circular saw just slightly beyond the shingle depth, and cut. Use an old saw blade or one specially made for remodeling, because you will hit some nails. Follow the same procedure on both sides of the ridge.
Sweep the roof, then nail down the 1×6 boards along the eaves and rakes. At the ridge, use a length of cedar bevel siding, with the thin edge on the down side. Apply new flashing in the valleys.

With the surface prepared, follow standard shingling procedures for the new roof.

WOOD SHINGLES OVER OTHER ROOFS:

Applying wood shingles over composition shingles, roll roofing, and tar and gravel roofs is quite similar to new shingling. Shingles must be laid over spaced sheathing nailed directly to the existing roof.

For an asphalt roof, remove the shingles along the ridge and hips. Removal isn’t needed with roll roofing or tar and gravel.
Trim the ends and edges of existing composition shingles where they overhang the rakes and eaves. You can cut them with tin snips or a utility knife.
Next, nail 1×6 boards along the rakes, from ridge to eaves. This provides a finished edge when the shingles are in place. Nail 1x6s along the eaves and on both sides of the ridge, with the edges of the boards meeting at the ridge. Finally, nail 1×4 boards down each side of the valleys to provide support for the new valley flashing that must be installed. Place spaced 1x4s over the roof, spaced the same as your shingle exposure.
Before shingling, lay the valley flashing in place. Now shingle the roof in a standard fashion, as described earlier.

PANEL ROOFS OVER OTHER ROOFS:

Panel roofs can go on top of a variety of roofs, except other panel roofs. Remove the existing panels before installing new ones.

GUIDELINES FOR COVERING OLD ROOFING

OLD COMPOSITION SHINGLES
You can install the following new roofs on an old composition shingle roof that is worn but relatively flat (nail down warped shingles and replace missing ones):
– Composition shingles
– Shakes
– Metal roofing

OLD TAR AND GRAVEL
This roof probably has a pitch of 4 in 12 or less, which limits your choices. Check the edge to see how many layers of gravel exist, with each one representing a new roof. If you find two or less, you can probably apply another roof. However, if the gravel is left in place, most roofing products won’t lie flat.

IF YOU REMOVE MOST OF THE GRAVEL, YOU CAN INSTALL:
– Composition shingles (a pitch of 2 in 12 or greater)
– Shakes (a pitch of 3 in 12 or greater)
– Metal roofing
– Roll roofing

WOOD SHINGLES
If the roof is in good condition but uneven, you can improve it by nailing beveled strips of wood, commonly called horsefeathers or feathering strips.

YOU COULD THEN COVER THE ROOF WITH:
– Composition shingles
– Shakes
– Metal panels
Before you nail on a new cedar shingle roof over the old, it’s best to nail on additional 1x4s to provide good air circulation. That’s why shakes are popular as a second layer for shingle roofs.

SHAKES
A shake roof is too irregular to cover with anything new. You’ll have to tear it off.

ROLL ROOFING
Assuming an adequate slope and a relatively even surface, you can install:
– Roll roofing
– Composition shingles
– Shakes
– Metal roofing
Wood shingles usually require spaced 1×4 sheathing boards, and new roll roofing should be put over an old roof only if it is still even.

TILE OR SLATE
These materials last so long that it’s unlikely you need to replace them unless the roof deck is showing signs of wear. If you have to replace them, have a professional do it.

METAL OR VINYL PANELS
Always remove panels before applying a new roof.

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