What is Roof Flashing and why do you need it?

Flashings can be found in many external areas of a house. Porches, walls, doors and windows, even the foundations, but the most common area is the roof. Anything which rises through or joins to the roof, will have a flashing. Chimney, dormer window, skylight, vents and roof valleys will all have a flashing where they join, or disappear through, the roof.

The Purpose of Roof Flashing

Roof flashings are fitted primarily to maintain a watertight roof. Without a flashing fitted, rain water would run down the chimney, or the higher area of roof tiles, and just disappear into the house interior where the chimney rises through the roof.

Likewise, on a house with a pitched roof that might contain a dormer window with 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 pour through into the house. The 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.

Materials used for Roof Flashings

Years ago, almost all flashings were made from lead sheet. Lead is a soft, easily worked metal, which can be easily moulded to various shapes without cracking or splitting.

Now, due to cost, and because of concern about lead contamination, materials such as zinc sheet, rubber, plastic, and even waterproofed impregnated paper, are also used. Where lead is used, it is generally painted with a waterproof paint.

Fitting a Roof Flashing

Firstly, the importance of a properly fitted roof flashing cannot be overstated. Without it, it matters little what tiles are used, how nice the roof looks, or how much it costs. Rainwater will penetrate the roof, will cause massive internal problems with damp, and will bring down ceilings if the fault is not corrected quickly.

If the house is a new build, or is having a new roof fitted, the flashings are normally fitted before the roof is tiled. If the flashings are being replaced due to damage or age related leaking, then, in the case of valleys, two or three lines of tiles need to be removed on either side. This allows easy removal, if required, of the old valley flashing and fitting of the new one.

Chimney stack flashings are generally two-piece per side. The lower piece is fitted under the tiles (shingles) and brought a couple of inches up the vertical chimney wall. The top piece is fitted into a groove chased out of the mortar between the brickwork, about 6 inches high. The top edge of flashing is fitted into the groove, which is resealed with mortar. The flashing is then carefully pushed down to cover the bottom flashing, to end 1 inch above the roof tiles.

Written by Morgan Asphalte, the leading providers of roofing services in Wimbledon, London and the wider South East. Call 0800 998 1378 to obtain a quotation or email enquiries@morganasphalte.co.uk.

Posted on 09 April 2015 in Roofing.

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