Expert Tips for Roofing Over Existing Shingles

Is your roof in disrepair? It's possible to shingle over shingles, to lay new shingles over existing ones and avoid the cost of a tear-off.

white home front exterior
Photo: Anthony Masterson Photography
Project Overview
  • Working Time: 1 day
  • Total Time: 1 day
  • Skill Level: Advanced

If conditions are right, new shingles laid over old ones can be just as attractive and durable as shingles laid on bare sheathing. Installation is easier than for a tear-off job, since you can use the existing shingles as guides for laying the new ones. However, you must take care to install shingles correctly so they lie flat. And a good job involves installing new flashings rather than relying on the existing ones.

Reroofing 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, but this is a roofing project best left to the pros. Beveled wood pieces called "horsefeathers" must be laid along the thick edges of the shakes to make a fairly flat surface. Before you reshingle a roof, read our step-by-step guide for adding new shingles over old ones.

What You'll Need

Equipment / Tools

  • 1 Power nailer or roofing hatchet
  • 1 Tape measure
  • 1 Roofing shovel or flat pry bar
  • 1 Hammer
  • 1 Drill
  • 1 Carpenter's square
  • 1 Straight-cut tin snips
  • 1 Utility knife
  • 1 Chalkline
  • 1 Broom

Materials

  • 1 Roof cement
  • 1 Flashing
  • 1 Nails for flashing
  • 1 WSU sheet
  • 1 Shingles
  • 1 Nails long enough to penetrate sheathing

Instructions

man removing ridge caps
Dave Toht
  1. Remove Caps

    Start by removing the ridge caps. Doing it now will make it easier to keep the reroofing job clean. Use a flat pry bar or roofing shovel to pry out and remove the ridge shingles. Remove all the nails.

    removing air vent pipe flashing
    Dave Toht
  2. Remove Obstructions

    Remove air vents and pipe flashings by prying out or unscrewing the fasteners holding the fixtures. If you damage shingles while doing this, repair them (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.

    attach replacement shingle
    Dave Toht
  3. Repair Broken Shingles

    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 new piece to fit the existing shingle. The goal is to provide a reasonably flat surface for the new shingles to lie over, with no gaps greater than 1/2 inch.

    sweeping roof top
    Dave Toht
  4. Sweep the Roof

    Using a large broom, brush away all the broken shingle pieces, twigs, and any other debris that could become trapped under the new shingles that you'll install. Keep the roof clean as you work.

    installing drip edge flashing
    Dave Toht
  5. Install Flashing

    At the eave and the rake, install U-shaped drip-edge flashing made for reroofing 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.

    installing new metal valley
    Dave Toht
  6. Install Valley Flashing

    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.

    starter strip course
    Dave Toht
  7. Install Starter Strip

    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.

    Editor's tip: If the existing roof has wavy horizontal lines, you can take out the waves by snapping a horizontal line a 1/2 inch below the shingle bottoms of one course. Install a course along the snapped line, then install succeeding courses using a guide or a snapped line.

    install first course shingles
    Dave Toht
  8. Cut and Install Shingles

    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.

    Editor's Tip: The thicker the new shingle, the better it will hide any imperfections in the roofing underneath. It's a good idea to use at least 40-year shingles; architectural shingles are an even better choice.

    roofing over existing shingles
    Dave Toht
  9. Continue Installing Shingles

    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.

    Editor's tip: In a reroof job, the starter course (or strip) is rip-cut so it butts up against the third course of the existing shingles. Its front edge is even with the front edge of the drip edge. The rest of the shingles are full width.

    cutting flashing piece
    Dave Toht
  10. Work Around Vent

    Once you've 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.

    cut shingle above air vent
    Dave Toht
  11. Install Air Vents

    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.

    installing step flashing
    Dave Toht
  12. Work Around Walls

    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.

  13. Install Counterflashing

    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 and set the flashing into the mortar.

    attaching ridge caps
    Dave Toht
  14. Cover Ridge

    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.

    cutting existing shingles
    Dave Toht
  15. Cut Shingles

    At the rake, snap a line directly above the edge of the existing shingles. Cut with a utility knife; you might want to use a straightedge. You might find it easier if you first cut from below, using the existing shingles as a cutting guide.

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