Roof Flashing and Your Historic House | Wisconsin Historical Society

General Information

Roof Flashing and Your Historic House

Roof Flashing and Your Historic House | Wisconsin Historical Society

If you hire a roofing professional to repair or replace the roof on your historic house you should be aware that flashing mistakes are common. Understanding how the flashing functions can help you ask your roofing professional the right questions. If your flashing is installed properly and maintained it should outlive your shingles or other roofing materials.

Roof Flashing Systems

Every element that sticks up from or through your roof is a hole that has the potential to leak. Roof flashing diverts water away from the joints where these elements connect to your roof. You should install roof flashing to anything that sticks up from or out of your roof, such as the following elements:

  • Chimneys
  • Pitched roofs abutting vertical walls
  • Plumbing vent pipes
  • Dormers
  • Skylights

The most effective roof flashing systems consist of metal flashings with rubber pipe boots. Under no circumstances should you use a bucket of tar or a caulk gun as a flashing system. You should ensure that the roofer does not propose to tar or caulk the metal flashing to a wood or masonry face because this practice will cause leaks. Tar and caulking will not last a year.

Flashing for Pipes

Your roof will have round pipes sticking up through its shingles. These pipes ventilate the plumbing waste lines throughout your house. They can range from 1-1/2 inches in diameter up to a large cast iron vent stack around 5 inches in diameter.

Historically, roofers and plumbers worked together to create custom-formed lead boots that fit around these pipes, up under the shingles and behind and on the sides of the pipe. Modern flashing systems for pipes have rubber boots that are designed to fit the exact diameter of the pipe. They are slipped over the pipe and the flat base is tucked under the shingles. Most roofers can install this flashing system.

Flashing for Vertical Walls

Your roof will need metal flashing anywhere that shingles butt up against a vertical surface. These surfaces include masonry walls, clapboard walls, wood-shingled walls, dormers and parapet flat-roof walls. A flashing system for a pitched or angled roof that butts against a vertical wall must include a step flashing and a counter flashing. This two-step metal flashing system consists of flashings the size of a row of shingles installed down the vertical wall and then bent under the shingles.

The counter flashing is installed over the step flashing to prevent water from entering the top joint of the step flashing. The counter flashing is either tucked under the vertical wall siding or set into a mortar joint for brick structures.

Flashing for Chimneys

Chimneys are flashed just like a vertical wall with a two-step system of metal step flashing and counter flashing. A counter flashing should be stepped or cut into an existing mortar joint. The flashing should never be installed at an angle, which would require slicing a groove through the bricks. This cut groove could cause brick failure.

If your chimney penetrates through your roof in an angled or pitched area, your roofer should install a saddle, sometimes called a cricket. Saddles are like small gable roofs installed behind the chimney on the high side. As water hits the back of the chimney area, the saddle diverts the water around the chimney.

You should also have your roofer install a chimney cap. A chimney cap looks like a little metal roof installed over a chimney with screening. The metal roof keeps water out and the heavy screening prevents bats and other animals from getting in.

Flashing for Roof Vents and Skylights

Roof vents and skylights have their own built-in flashing systems. Quality and cost go hand in hand with flashing systems for roof vents and skylights.