Tips for maintaining roof flashing

July 28, 2015

When you do your annual roof inspection, make sure to take a good look at all the flashing, too. Damaged flashing on the roof can lead to household water damage.

Tips for maintaining roof flashing

Spot repairs

  • First scrub the damaged spot with a wire brush to clean and roughen the metal surface. Then fill the hole with gutter sealant and smooth the patch with a putty knife.
  • Wetting the knife with paint thinner (mineral spirits) will help you put a smooth finish on the patch.

Chimney flashing

  • Chimney flashing is especially complex. To let the chimney move independently from the house, it has two-part flashing: a base flashing on the roof overlapped by a second flashing on the chimney.
  • Scrub away the corrosion with a wire brush; then clean the surface thoroughly with soap and water and rinse. Cut a piece of fibreglass mesh large enough to cover the rusted area and extend beyond it a little on all sides. Apply a coat of plastic roof cement to the flashing area and press the fibreglass over it, then top with a second coat of the sealant.

Valley joint flashing

  • Valley joints — those troughs where two roof sections meet — are especially vulnerable to leaks.
  • Usually the problem is not the flashing itself, but rather that the edges of the overlapping shingles have curled up slightly, letting water seep in. It's easy to fix this problem using a cartridge of plastic roof cement in a caulking gun.
  • Starting at the lower end of the flashing, one by one, lift the edge of each shingle that overlaps the flashing, and apply a heavy bead of roof cement to the edge of the flashing and to the top of the shingle below. Then gently press the shingle into the cement.

Avoid stepping on flashing

  • Whenever you or your contractor is on the roof, avoid stepping or kneeling on flashing or exerting any pressure to the area within 30 centimetres (12 inches) of it. The weight may damage the joints beneath the roofing.

Replacing the flashing

  • Get a telescoping two-piece flashing unit, which has a base that goes on the roof and a separate sleeve that fits snugly over the pipe.
  • Carefully lift the edges of the shingles that overlap the vent flashing with a pry bar. Use a putty knife to scrape away any old adhesive from the undersides of the shingles. Pull out the nails holding the old flashing and remove it, being careful not to damage the surrounding shingles.
  • Fit the new base flashing over the vent pipe and slide the sleeve over the pipe. Lift the shingles above and on both sides of the vent pipe and slide the new flashing underneath, rotating it as you do to ease the installation.
  • Nail the flashing to the roof at the top and sides, making sure that the surrounding shingles will cover the nails. Seal the nail heads with roof cement. Press the surrounding shingles over the new flashing. The bottom edge of the flashing should be over the shingles below it so that it will shed water, not trap it.
  • Flashing can be made of copper, galvanized steel, or most commonly, aluminum. It doesn't create a waterproof seal but rather it overlaps or is overlapped by the roofing, diverting rainfall so that it runs harmlessly off the roof.
  • Every joint is vulnerable, especially if it joins materials that shrink or expand at different rates, like vent pipes and shingles. Joints may also pull apart as the house settles.
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