We’ve already looked at the materials commonly used for roof flashings – lead, zinc, mortar and roofing felt. Now let’s look at the more typical designs of flashing: apron flashings, abutments, chimney flashings and valley flashings.
These are probably the simplest of all flashings, used where the top of a lean-to roof meets the wall. The top edge of the flashing is pointed into the joint two courses above the head of the roof, and the flashing is then dressed down to overlap the tiles (or other covering) by at least 150mm (6″).
This is where a pitched roof comes up against a wall. There are two basic types of flashing used here, depending on the steepness of the roof pitch and the material used:
Double-lap flashings are made in two parts. Soakers are straightforward sheets of metal (lead or zinc) folded at right angles along their length so that a face 75mm high (3″) – known as the upstand – can be presented to the wall, with the other face placed flat on the tile, covering at least 100mm (4″) away from the wall. The higher edge on the sheet is turned down over the upper edge of the tile to help retain it in position. The sheet should therefore be long enough to cover the overlap from the tile above, plus 15-25mm (½-1″) for the turndown. A stepped flashing is then dressed over it, with the top edges secured into the bed joints with lead wedges and mortar pointing.
Double-lap flashings are suitable for roofs pitched at over 30° and with plain tiles or slates.
Single-lap flashings are used for roofs with contoured tiles (eg pantiles). The flashing is stepped into the wall and then dressed over the tile. The width of flashing required may be higher: 150mm or more in the case of a shallow roof, and in any case at least to the first raised contour.
As you’d expect, these are a little bit more complex, but not too much. The front of the chimney gets an apron flashing that extends beyond the sides of the chimney stack, with the ends of the upstand folded round the sides of the stack. The sides of the chimney are covered with single or double-lap flashings as appropriate.
The back needs a gutter, which is made by turning the front part of the lead up the back wall of the chimney to create an upstand and covering it with a separate flashing pointed into the back wall. The back part of the gutter follows the pitch of the roof and, as you might expect, is overlapped by the tiles above it in the usual way.
These are less common than the other types and are found where two roofs meet at an angle. Boarding is constructed to run down the pitch of the roof from the ridge to the eaves, with fillets nailed along both long edges. The sheeting is then dressed over the fillets. The roof slates or tiles are cut so that they overhang the valley but are separated by a gap of at least 100mm (4″).