When people think of flashings they tend to think of roofs, but there are many external areas of the house which need flashing. Porches, walls, doors, windows and even foundations all use flashing. In addition to these, anything that joins to the roof will have a flashing such as a chimney, dormer windows, skylights and vents will all have a flashing where they join, or disappear through, the roof.
What is roof flashing for?
The primary function of roof flashing is to ensure that the roof is watertight. Flashing covers the weak and vulnerable areas of roof (such as joins where chimneys join the main roof) and to guide water down and away from the roof. The importance of a properly fitted roof flashing cannot be overstated. Without it, it hardly matters what kind of tiles you’re using, how nice the roof looks or how much it costs. Rainwater WILL penetrate the roof cause massive internal problems with damp and cause a huge string of problems with the building.
On a house with a pitched roof where the two roofs join on both sides are called the valleys. Without a flashing built into these valleys, rain water running down both roofs would have little option but to pour through into the house. Flashing ensures the joints remain watertight.
Applying a cement fillet to the joint is not acceptable. Even with a bonding agent added, water will still find its way behind the fillet. Within a short space of time the cement will crack and begin to fall away, allowing a major leak into the house interior.
What is roof flashing made from?
Up until fairly recently, flashings were made from lead sheet painted with waterproof paint. Lead is a soft, easily worked metal, which can be easily moulded to various shapes without cracking or splitting.
Now, due to cost, and fears surrounding the toxic nature of lead, materials such as zinc sheet, rubber, plastic, and even waterproofed impregnated paper, are also used.
How do you fit roof flashing?
If the house is a new build, or is having a new roof fitted, the flashings are often fitted before the roof gets tiled. If the flashings are being replaced due to damage or old age, then – in the case of valleys – a few lines of tiles need to be removed on either side. This will allow you to easily remove the old valley flashing and fitting of the new one.
As a rough rule, chimney stack flashings are two-piece per side. The lower piece is fitted under the tiles and brought a couple of centimetres up the vertical chimney wall. The top piece is fitted into a groove chased out of the mortar between the brickwork, about 15cm high. The top edge of flashing is fitted into the groove, which is re-sealed with mortar. The flashing is then carefully pushed down to cover the bottom flashing, to end 2cm or so above the roof tiles.