A roof valley is basically a gutter set between two meeting pitched roofs. Depending on the roof area it serves, the valley is the exit point for a large volume of water so extreme care should be taken with installation. If the roof has been leaking for a while or if there are any signs of rot, you will need to start by replacing the valley boards. Lead sheet is not self supporting and should be placed on treated roofing boards of sufficient strength to hold a large person. (Most roofing contractors are big guys!) Fit boards of sufficient width to accommodate the lead plus 100mm either side. This will give you something to nail the roofing batons to.
The top of the valley boards should be at the same level as the top of the roof rafters. If you lay the boards directly on top of the rafters it may cause the roofing tiles to kick up and restrict water run off. You will need to cut the valley boards to fit in between the rafters. Support the valley boards with studs or noggins. The valley should finish on an even plane at the eaves. It should not kick up higher than the bottom rafters. If it does, you will need to cut the fascia board or adjust the gutter to suit. It is a good idea to fit a tilting fillet each side of the valley. This angled strip of wood runs along the valley length and should be a minimum of 150mm from the centre of the valley. It should sit no higher than the roofing batons with the thinnest end closest to the centre of the valley.
It is common practice to fit a single sheet of roofing underlay the entire length of the valley. The adjacent roofing underlay will rest on top of this sheet. I recommend you use one of the new advanced synthetic underlay materials. The older bitumen based felts are fine for normal roofing situations but are not suitable for valleys. Over time the bitumen will bond the lead to the boards and restrict thermal movement. You should ensure you buy lead of a sufficient grade/code for valley applications. This should be between 1.80mm and 2.24mm thickness. If you are unsure ask your roofing merchant of the correct grade. The lead should be cut into sections no larger than 1.5 meters in length to allow sufficient thermal movement. Bend a welt into the lead 25mm each side. This acts as a last line of defence for water penetration. It also has the added benefit of stiffening the lead, which makes carrying it up the roof a lot easier.
Starting at the bottom of the valley, dress the lead neatly onto the valley boards and over the tilting fillets. The bottom of the lead should allow correct drainage into the gutter. Fix two rows of nails at the very top of the flashing. Use copper or stainless steel nails. Never use galvanised or aluminum nails which will just react with the lead and corrode. I recommend you use the minimum amount of fixing possible to hold the lead in place. If you over fix lead sheeting it will eventually split due to thermal movement. So don’t nail the sides. When you have successfully dressed the first sheet you can move up the roof laying subsequent sheets. Overlap each sheet a minimum of 150mm. On lower pitched roof valleys you will need to increase the lap. Where the valley ends at the ridge, you will need to dress the lead so it can sit neatly under the ridge tiles. You are now ready to start fixing the batons and laying the roofing tiles. The key points to remember are to keep the sheet lengths down to 1.5 meters and don’t over fix. If you follow the procedure outlined and take care with the dressing you will produce a durable maintenance free valley.