Chimney Flashing

What is Roof Flashing?


Flashing is an essential component of any roofing system. It’s unfortunate that flashing is sometimes forgotten about or installed improperly, because it keeps your roof both airtight and water proof, and is placed around sections where a chimney, skylight, wall, or another roof intersects with the roof being worked on. If areas like these are not properly sealed, you’ll eventually be subjected to some serious leaks. Intersections on a roof do contract and expand depending on outside temperatures and humidity levels, so flashing should allow for variable movement while also keeping moisture out of the roof. There are different types of flashing, though, and we’re here to explain them to you.


There are two types of flashing used when a pitched roof comes up against a wall, and it really depends on the steepness of the pitch and the type of roofing material used. The first type of flashing is called double-lap flashings, which are made in two parts, and suitable for roofs made of plain tiles or slates that are pitched over 30 degrees. “Soakers” are sheets of metal made from lead or zinc that are folded at right angles along their length so their face is about 3 inches tall. This is also known as the “upstand”, which is put to the wall while the other face is placed flat to cover 4 inches away from the wall. The higher edge of the sheet is then turned down over the upper edge to help retain position. A stepped flashing is then placed over it, with the top edges secured into bed joints with wedges and mortar pointing.

There are also single-lap flashings, which are used on roofs with contoured tiles. This type of flashing is placed to the wall and then over the tile. The required width of flashing might be higher if the roof is shallow, however.

Apron Flashings

Apron flashings are the simplest of all flashings, and are used when the top of a lean-to roof meets a wall. The top edge of flashing is placed into the joint two courses above the head of the roof, and then dressed down to overlap the roof covering by about six inches or so.

Chimney Flashings

These are a bit trickier than other types of flashing. The front portion of the chimney gets an apron flashing that extends to the sides of the chimney stack, with the ends folded around the sides of the stack. The sides of the chimney are then covered with either single-lap or double-lap flashings, depending on which kind is more appropriate. The back then needs a gutter, which is created by turning lead up the back wall of the chimney to create an upstand. The lead upstand is then covered with separate flashing placed into the back wall. The back portion of the gutter then follows the roof pitch and is overlapped by the tiles above it.

Valley Flashings

These types of flashings are used when two roofs meet at an angle, but they are less common than the other types listed above. Boarding is built to run down the pitch of the roof from the ridge to the eaves, with fillets secured along both long edges. Sheeting is then dressed over the fillets, and roof tiles or slates are cut to overhang the valley.

While often overlooked, roof flashing is quite important. REI Roofing will make sure your flashing is installed properly the first time. Call us at 781-848-1999, or fill out our online contact form!