If conditions are right, new shingles laid over old shingles can be just as attractive and durable as shingles laid on bare sheathing. Installation is easier than for a tear-off job because you can use the existing shingles as guides for laying the new ones. But you must take care to install the shingles correctly so they lie flat. And a good job involves installing new flashings rather than relying on the existing ones.
Reroof jobs are sometimes done without replacing flashings. However installing new flashings—including special drip-edge flashings made for reroofing—will ensure a tight seal and a long life. Some roofers install a layer of waterproof shingle underlayment (WSU) over the existing shingles at the eave end, as would be done for a roof laid on bare sheathing. This provides added protection against ice dams.
Asphalt shingles can be laid over cedar shakes or shingles, a job best left to pros. Beveled wood pieces called "horse feathers" must be laid along the thick edges of the shakes to make a fairly flat surface.
Remove the ridge caps. Doing it now will make it easier to keep the job clean. Use a roofing shovel or flat pry bar to pry out and remove the ridge shingles. Remove all nails.
Remove air vents and pipe flashings by prying out or unscrewing the fasteners holding the fixture. If you damage shingles while doing this, repair the shingles (see next step). Reuse a vent or flashing only if it is like new; otherwise replace it with a new one that will fit the hole or pipe.
If a shingle is torn or cracked, glue the broken piece back in place using roofing cement. If the broken piece is lost, cut a piece to fit. The goal is to provide a reasonably flat surface for the new shingles to lie over, with no gaps greater than 1/2 inch.
Using a large broom, brush away all broken shingle pieces, twigs, and any other debris that could become trapped under the new shingles that you install. Keep the roof clean as you work.
At the eave and the rake, install U-shape drip-edge flashing made for reroof jobs. Install the eave piece first, then the rake piece over it. The two pieces should meet neatly at the corners. Drive nails at high points on the underlying roof—the bottoms of the shingles.
If you have an open valley, install new W-shape metal or vinyl valley flashing to fit directly over the old flashing. Attach it by driving nails into the outside edges only. Do not drive nails less than 6 inches from the center of the flashing.
Use a starter strip or cut pieces for a starter course. Rip-cut the starter strip or starter pieces so that they butt up against the second course of existing shingles and are even with the front edge of the existing roofing. Attach with nails along the top edge of the strip.
Rip-cut the shingles for the first course so they butt up against the third course of existing shingles and are even with the front edge of the starter course. Their tab slots should not line up with the tab slots of the starter course, if there are any. Nail the shingles just above the tab slots.
Butt the succeeding courses of shingles against the bottoms of existing shingles and apply them by driving nails above the tab slots. Snap vertical control lines; there is no need to snap horizontal lines. Be sure the new tab slots do not align with the old ones.
Once you have installed shingles just past a plumbing vent, install the flashing piece so it will lie on top of roofing at its bottom but be covered with roofing at its top. Depending on the width of the pipe, you may need to tear away a segment or two of the rubber boot. Apply roofing cement, slide the flashing over the pipe, and push so it lies flat on the roof.
Install air vents in a similar manner. For both plumbing-vent flashing and air vents, you'll need to cut the shingle above to go around the flashing and then install the shingle.
Where you meet a chimney or side wall, install step flashing. Apply a shingle, then a piece of flashing, then a shingle, and so on, so that each piece of flashing rests on top of the lower course and is covered by the upper course. You'll need to pry the siding outward to slip in the flashing; in some cases you'll have to remove the siding.
Protect the step flashing with counterflashing. On a chimney use a grinder to cut an indentation into the mortar. Cut and bend the counterflashing to fit snugly into the cut mortar and to cover at least 3 inches of the step flashing. Apply mortar with a caulking gun (inset) and set the flashing into the mortar.
Install roofing on both sides of a ridge and cut them so they butt closely together but do not overlap. Cut shingles into ridgecaps. Snap lines on either side and install the caps.
At the rake snap a line directly above the edge of the existing shingles. Cut with a utility knife; you may want to use a straightedge. You may find it easier if you first cut from below using the existing shingles as a cutting guide.
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