Page 148 - 2018 National Home Improvement

2018 National Home Improvement Estimator, All Rights Reserved Page 123

Before signing a contract for reroofing over an old roof, consider:

• Building codes - Some building codes prohibit installing a new roof over old. Others allow either two
or three layers of roofing. A few allow a one-layer tear-off – remove and replace just the top layer of a
two- or three-layer roof. If in doubt, place a call to your local building department.

• Bad decking - Walk the deck. If you see sags between the rafters or trusses, or if the deck feels
weak under foot, the sheathing may be deteriorating or damaged. You may find that only a small
section of the deck needs to be replaced. This is often caused by a long-term leak. If possible, go into
the attic space and inspect the deck from underneath. This may show you the extent of the problem. Do
a tear-off and plan on repairing the deck. The deck under a wood shingle roof may use skip sheathing:
1" x 2" to 6" boards laid with a 1" to 3" gap between boards. Skip sheathing is fine for wood shakes and
shingles, but asphalt shingles need a solid board or plywood deck.

• Ice dams - A full tear-off is best if there’s a history of damage from ice dams.

• Incompatible shingles – Installing heavyweight architectural shingles over lightweight 3-tabs is
nearly always successful. A lightweight shingle installed over a heavyweight shingle may leave obvious
bumps and ridges.

• Poor condition – If the existing roof is in really bad shape, such as with curled tabs and crooked
rows, a tear-off is in order.

• Shorter life span – Most roofing professionals agree that the life expectancy of a new roof is
reduced by 10 to 20 percent when it’s installed as an overlay on an old roof.

When installing new shingles over old wood or asphalt shingles, manufacturers may recommend
removing a 6" wide strip of the old shingles along eaves and gables. Then nail down a nominal l" x 6"
board where the shingles were cut away. If the old shingles were asphalt 3-tabs, use thinner boards.
Remove the old covering from ridges and hips and place a strip of lumber over each valley to separate
old metal flashing from new.

Wood Shingles
If you decide on wood shingles as your new roofing material, use Number 1 grade shingles. Number 1
shingles are all heartwood, all edge grain and tapered. Western red cedar shingles and redwood
shingles offer the best decay resistance and shrink very little as they age. Wood shingle widths vary.
Narrower shingles are classified as either Number 2 or Number 3. Figure 7-2 shows common shingle
sizes and recommended exposures.