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Install a roof valley



How to Install a roof valley

Lead valleys are one of the oldest and most durable forms of roof valley construction. A well fitted roof valley constructed from lead sheet will outlast all other roofing materials. If your lead valley needs replacing, chances are it is either over 70 years old or it was installed incorrectly in the first place. Poor installation is the number one cause most roof valleys fail. For information on roof valley replacement and repairs, please view our website. 




Valley boards

The valley boards should be strong enough to support the valley and the weight of a large person to allow future maintenance. (minimum 18mm ply) The boards should be treated against rot and insect manifestation. If you are replacing a roof valley and you intend to re-use the origional boards, make sure the boards are free from old nails.



Tilting fillet

A wood fillet sits either side of the valley to prevent leaks and provide a second line of defence. Each fillet sits 150mm from the centre of the valley and runs the entire length of the roof valley.




A length of roofing underlay is laid down the entire length of the valley. Bitumen underlays are not suitable for lead valley instalations. The bitumen will heat up over time and bond the lead to the valley boards. This restricts thermal movement and causes splits and leaks. Modern polyester geotextile roofing felt should be used.



Lead sheet

The minimum thickness of lead sheet for pitched valleys is 2.24mm, (code 5). The maximum length for each section of lead is 1.5 metres. Each section of lead should be doubled nailed at the top with stainless steel or copper nails. The first row of nails sits 50mm down from the top of the lead.  The edges of the lead valley are clipped to secure the lead while still allowing for thermal movement. From the bottom up, each section of lead is dressed into the valley and over the tilting fillet. Use a lead dresser to get a neat finish. Work up the valley placing each 1.5 mtr section of lead over the preceding section. The minimum lap is 150mm which should be increased if the pitch of the roof valley is low.



Tile or slate slip plane.

If the roof tiles or slates are to be cemented to the valley, a barrier should be placed between the lead and the roofing cement. The barrier helps prevent cracking of the mortar as the lead expands and contracts. Small fillets of roofing slate are ideal and can be temporarily fixed in place with a small dab of silicone. Ping a chalk line up the valley as a guide for the slate fillets. This will provide a neat finish for the edge of the roofing cement.



Roofing Cement

Roofing cement for the valley should be mixed at a ratio of 3:1 sand:cement. The mortar is laid on top of the slate and the roof tiles are layed on top. The roofing cement should be well compacted. To help adehsion the roof tiles should be gently tapped down onto the cement fillet. Work up both sides of the valley until all the roof tiles are in place. When all the tiles are in place, the excess cement is cut back with a trowel and pointed in to give a neat appearance.    


Common mistakes made with roof valley construction.

Below you will find some of the more common mistakes made when using lead sheet to construct a roofing valley.
  • Incorrect grade/thickness of lead used on roof valley

    Lead sheet is expensive and sometimes a lighter or thinner grade is used in an attempt to save money. This is a false economy as the valley will fail prematurely. This is also related to sheet size below. 

    Use the correct grade of lead for roof valley construction.


  • Roof valley lead Sheet length to long

    Lead changes in size according to the temperature. (Coefficient of thermal expansion) Temperature changes on a roof cause the lead to constantly expand and contract. If the length of lead sheet is too long the expansion and contraction will be greater which will eventually cause the roof valley to split and leak. Each section of the valley should be no longer than 1.5 meters in length. The 1.5 meter sections are then lapped over the adjacent valley section with sufficent lap coverage for the valley pitch. (minimum 150mm, the lower the pitch of the roof valley, the greater the lap).


  • Lead Sheet overfixed

    If the lead sheet is over nailed or overfixed it will restrict the metals movement. As the lead expands and contracts overfixing will cause the lead valley to split. Lead clips along the edge of the lead are preferable to nails. They allow for a degree of thermal movement. Stainless steel or copper nails may be used 50mm down at the top of each lead section. ( The nails will sit underneath the lapp of the next lead section)


  • If you would like any advice about repairing your roof, or help from one of our wellington roofing contractors, visit this page