How is a Cambrian valley with unequal pitches constructed?

Special attention must be given to unequal pitches discharging into a Cambrian valley. The pressure of water running down the steeper slope can drive in under the shallower side of the valley. To counteract this an additional raking batten must be fixed on the shallow side of the valley to act as a barrier to this greater water flow.


What is the correct eaves detail for a GRP valley?

GRP valleys should be recessed into the fascia board at the eaves to allow the bottom end to discharge into the gutter without the corrugations of the valley raising the tiles in the eaves course which causes tile gapping and hence the possibility of rain ingress. Cambrian Slates require the eaves course to be clipped using verge clips. This is not possible unless the top of the valley is flush with the top of the fascia board. Where a valley discharges onto another roof as with a dormer, the GRP valley should finish at the head of the tile course onto which it discharges and a lead saddle installed under the valley and over the tiles.


Should plain tiles be mortar bedded at lead valleys?

Many roofing contractors do not mortar bed the cut tiles at the sides of a plain tile open lead valley. Whilst not against the British Standard, we have always recommended that plain tile lead valleys be mortar bedded. BS 8000-6: 2013, Code of practice for slating and tiling of roofs and walls – Workmanship on building sites, states where bedding is used that “The tiles should be bedded only and not pointed between the tiles to allow the flow of water between tiles and into the valley.” This means that mortar should only be placed between the lead valley and the underside of the lowest tile and not between tiles. The reason for this is the need for water to drain freely between overlapping tiles. As plain tiles are typically 13mm thick, gaps of up to this figure can occur through which wind, rain and even large insects can gain access to the batten cavity. The mortar bedding prevents this as well as acting as the first of three lines of defence, the others being the tilting fillet and the lead welt. Without mortar bedding the risk of the valley leaking is much higher.


What is the plan angle of a building?

A plan angle is the angle of intersection between the eaves of one slope and the eaves of the adjacent slope when viewed on plan. The most common plan angle is 90º since most buildings have square corners. However some buildings have walls that do not meet at 90º such as where a wall follows the line of a boundary which is not parallel with the orientation of the building or bay windows that are formed in a series of facets to produce a panoramic view.