Preparing a New Roof Deck – Flashings

Last updated: 2026-10-28 by Ted Sellers, Owner

Flashings

There are many different parts of a roof and flashings are what protects the roof at all its vulnerable points. These points can be found along the eaves, in the valleys and at the edges of the roof chimneys, roof vents and skylights. It is basically there to protect the roof from water leaks at the joints and to protect the sheathing underneath. Flashings are normally made out of galvanised and rust-resistant sheet metal. When you are doing work on your roof, you might not have to change the flashings as long they are in a good condition. Check it over and reseal any flashing joints, if need be, with a bit of plastic cement. Remember to use protective gloves if you are working with metal as any sharp edges can cause bad cuts which might lead to your project being delayed.

New Valley Flashings

It is important that the flashings in the valleys are particularly sturdy as they transport more water to the gutters than any other parts of the roof planes. Despite most valley flashings being made of galvanised steel, you can use mineral surface rolls for the valleys on asphalt roofs. There are different finishes such as open and woven valleys. The first are where shingles, or tiles for that matter, are cut in order to expose the valley. The latter one is where asphalt shingles overlap and covers the valley. If you have checked over the existing flashing and come to the conclusion that it will have to be replaced, this is how you install it.

Installing New Valley Flashings

If you are roofing over a new or a completely stripped deck, it is advisable to use galvanised metal of 28-gauge for sloping roofs. This is regardless of whatever roofing material you are planning to use. You do not have to make your own valleys, unless you want to, as you can buy them from most roofing suppliers. Just make sure that they match the slope of your roof. If you purchase them, you will notice that they often come with a splash guard at the centre. The outer edges will have to be crimped in order to direct any water to the center. The metal should reach about 8 – 11 inches up along each side of the valley. It you are planning to roof over asphalt shingles, a roll roofing of 36 inches wide can be used. Just make sure to match it to the new shingles – this is most important at the valleys.