Home Contractor's Corner Navigating the valleys: A focus on pitched roof valley construction

Navigating the valleys: A focus on pitched roof valley construction

by Jennie Ward
John Mercer, Pitched Roofing Consultant.

In his latest Consultant Case Study, Pitched Roofing Consultant John Mercer focuses on tiled valleys, and dispels some myths around mortar and water tightness, plus offers some key guidance on an area that, in his experience, presents “some of the most common causes of roof leakage.”

In this next case study, I turn my attention to tiled valleys. In my long experience, some of the most common causes of roof leakage I have come across on tiled roofs are related to valleys. Many people think that the mortar is there to make the junction watertight, which could not be further from the truth. If you stop and think about it, slate and many plain tile valleys are constructed without mortar and in Scotland, even clay and concrete single lap tiled valleys are constructed without mortar.

The photo above shows an extreme example of a poorly constructed valley I came across recently; the lead has been laid flat over the valley boards, with no upstand or welt. The contractor was hoping that expanded foam would keep the water out, with just a thin layer of mortar pointing at the edges. The tile battens have been extended over the lead to within 10mm of the outer edge of the pointing. To make matters worse, the lead has been cut by what I presume was the disc cutter as tiles were cut in-situ.

Above: Lead-lined valley diagram.

Lead-lined valleys
The main elements that keep the water out are the upstands and welts at each side of the valley. The most common open width for a traditional valley is 125mm and this is generally satisfactory for normal domestic roofs. There are exceptions, such as low roof pitches and/or long rafter lengths and further guidance for these is available in the Lead Sheet Training Academy (LSTA) manual.

A lead valley lining must be supported on timber boards which can sit on top of the rafters if counter battens are being used, or be fitted flush with the tops of the rafters, supported on timber noggins, with a 4mm plywood lining over the top.

25mm deep tile battens can be used to construct the upstands as these will finish flush with the tops of the tile battens.

The valley boards should be at least 225mm wide each side of the valley to allow for a 125mm wide open valley, with timber upstands set 150mm from the centre line of the valley and enough width left to support the ends of the tile battens. If the open valley needs to be wider than 125mm, then the valley boards must be widened accordingly.

Code 4 lead is suitable for most domestic buildings, with Code 5 sometimes used on larger buildings, laid in maximum lengths of 1.5 metres.

The width of the lead sheet will depend upon the open width of the valley, plus 200mm on each side to allow for the width of the mortar bedding, 25mm gap, the tilt fillet, a 25mm gap, and the welt on the outside edge.
The lead should be dressed into position with the correct amount of lap with the sheets above and below it. The headlap will depend upon the true pitch of the valley. At 30 degrees and steeper the lap should be 150mm. At lower pitches, the lap will increase to, for example, 220mm at 20 degrees and 290mm at 15 degrees. The welt should be formed away from the tilting fillet, not on top or the side of the tilting fillet, as it would be ineffective and may cause the tiles to kick up.

The general roofing underlay must finish over the edges of the valley boards so that any water in the batten cavity will drain onto the valley; the underlay must not extend under the valley, otherwise water will run underneath the valley construction and may leak into the structure.

The tiling battens should be cut in line with the valley, approximately 10mm short of the lead welt to allow any moisture on the underlay to drain down the eaves uninterrupted. The 10mm gap also helps to prevent the ends of the battens puncturing the underlay where it rises onto the welt. The ends of the battens should be supported and nailed on the edge of the valley support boards.

Mortar must be laid onto an undercloak of slate or fibre-cement board rather than be bedded directly onto the lead, otherwise the mortar will crack and dislodge as the lead expands and contracts. There should be a clear gap of at least 25mm between the mortar bed and valley upstand to prevent water tracking over the upstand. The mortar must not block tile interlocks and should be bedded and pointed up in one operation or be pointed up as soon as possible afterwards to ensure the bedding and pointing mortar set as one piece.

Above: GRP valley trough diagram.

GRP valley troughs
GRP valley troughs are a great alternative to lead lined valleys and are suitable for most situations where the open valley is 100 to 150mm wide. They are flexible enough to cope with valleys where the roof slopes are within 15 degrees of each other. For greater differences, check with the manufacturer of the valley trough.
Generally, GRP valley troughs should be supported on valley boards as described above for lead valleys, though some manufacturers say their valleys are sufficiently robust to enable them to be installed directly over the rafters. These valleys have pre-formed upstands to prevent the ingress of water and, for mortar bedding, some have integral sand strips to ensure a good adhesion between the mortar and valley lining.

Above: Dry fix valley trough.

Preformed GRP valleys with central upstands
An increasingly common alternative to traditional open valleys is preformed types with a central upstand. The roof construction is the same as that for open GRP valleys, but instead of having an open channel like traditional valleys, these valleys have a central upstand and the tiling is finished close to the upstand on each side. These valleys have channels at each side of the upstand to direct water towards the gutter without discharging over its edges.

Valleys with central upstands are beneficial where two roof slopes of unequal pitches meet at a valley. With open valleys, there is a risk that water from the steeper roof slope will run across and run over the edges of the valley on the lesser pitch slope. A central upstand will prevent this from happening.

Tile cutting
Tiles cut to the valley rake should be cut in a designated safe area on the scaffold, not in position in the valley. Where possible, tile-and-half tiles, or double tiles should be used – if available – to ensure the cut pieces are as large as possible. All perimeter tiles, including cut valley tiles, should be twice fixed. Some tile manufacturers provide special clips to secure the valley cuts. If possible, nail the head and clip the tail of each tile. For small cut pieces, secure using a cut tile clip and a suitable adhesive.

In summary:
• Always ensure the valley lining is adequately supported on valley boards which are set at the correct position in relation to the rafters and tile battens.
• Remember that it is the upstands and welts in a lead valley that prevent water ingress – never rely on the mortar for water tightness.
• Make sure that the open valley width is appropriate for the size and pitch of roof.
• Twice fix all cut tiles adjacent to the valley – use special cut tile clips and adhesive where standard nailing and clipping cannot be used.


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