Peter Hemmings wrote
I have a problem with an angled roof valley. It has concrete gully tiles
which leak at the joints resulting in the felt rotting under the joints.
This has been mainly caused by strong south-westerly winds which tend to
"funnel" straight up the valley.
I intend to lead the valley over gravel boards and have a pamphlet
(Lead Association) but it does not show precisely the position of
the felt. At present I have a (damaged) felt running down the valley
with the normal felts on top of it.
Can someone give advice on the correct/best method:

1. Do the felts go under the whole valley (with
boards and lead on top?
2. Do the felts go between the lead and boards?
3. Do the felts go just up to the gravel boards with the lead overlapping
it by, say 80 mm?
I have also been advised to lay an extra triangular piece of board
along the length of the valley to give it a flat bottom rather than leave
it as a "V" shape. Any comments welcome.

Valley tiles are vulnerable to leaks because they are actually at a lower pitch
than the main roof slope. The lead sheet should be on the bottom, directly on
the valley boards, and anything else, such as the sarking felt under the tiles,
should be dressed over it, not under as you suggest. The lead should be about
500mm wide in min Code 4 thickness and in lengths not exceeding 1.5m with
150-200mm overlaps. It should be fixed only at the top of each length and 500mm
down each side from the top, otherwise expansion and contraction will cause
splits. The two long edges of the lead should be welted and turned up over
preserved softwood battens or ideally triangular fillets about 200mm apart, to
prevent water run-off and to support the edge of the tiles. There is no need for
the valley to have a flat bottom board. The cut tiles should overhang the
fillets by about 38mm so there is about 125mm between the cut edges, and the
sarking felt should be trimmed along the bottom edge of the tiling. All the cut
tiles should be nailed and bedded in mortar, but be sure to keep the mortar off
the lead as this will cause corrosion.

Have you looked at using a preformed GRP valley liner? A lot cheaper and
easier.

Peter