Verges

Is there a dry verge system for Stonewold I?

Ambi-Dry verge units can be used with Stonewold I slates. Additional battens are fixed to a minimum of two supports 2 mm below the tiling battens to which the Ambi-Dry Verge Units are fixed. This is an ideal solution in situations where the original mortar bedded verge has failed and a new dry verge is required.

What is the correct width for mortar at verges?

BS 8000: Part 6: 1990: Workmanship on building sites states 'Provide about 100 mm of mortar width and bed tiles solidly and finish edge neatly'. The more surface contact of mortar the less likely the verge tiles are to fail. However it is essential that the bedding mortar does not come in contact with the ends of the tiling battens.
The width of the undercloak is normally 150 mm of which 50 mm laps under the batten ends. If 100 mm width of mortar is placed onto the undercloak it will touch the batten ends. Once the tiles are bedded into the mortar it will spread by a further 25 mm around the end of the battens. Therefore a 50 mm width of mortar should be applied which will spread to 75 mm when bedded.

What should the tile overhang be for a verge with a gable ladder?

BS 5534: 2003, Code of practice for slating and tiling states that a verge overhang can be between 38 and 50 mm. However on a verge formed with a gable ladder, 35 mm wide rafters, 19 mm bargeboard and 38 mm overhang, the distance from the verge edge to the ends of the tiling battens is 75 mm. The finished mortar bedding width at a verge should be 75 mm meaning the ends of the battens would touch the mortar, which is not good practice.
If the overhang is increased to 50 mm, the distance from the verge edge to the ends of the battens becomes 87 mm giving a gap of 12 mm between the mortar and ends of the battens. It is for this reason that the overhang should always be 50 mm on gable ladder verges.
On a brick gable, the ends of the battens rest halfway across a 102 mm wide brick. With a mortar width of 75 mm there is no risk of the mortar touching the ends of the battens with either a 38 or 50 mm overhang as there will always be a gap of at least 14 mm.

What could be the cause of water ingress at a verge?

Placing the undercloak under the underlay at the verge can be the cause of water ingress to the roof because the bedding mortar is not truly waterproof. Under driving rain conditions water can run onto the undercloak behind the mortar. The quantity of water will increase if a hairline crack between the mortar and the undercloak or roof tile exists. Once the water is on top of the undercloak it may run down the roof slope. If there is a fall in towards the roof water can run off the inner edge of the undercloak. It may run under the underlay into the roof or the brick cavity.
If the undercloak is placed over the underlay water will run onto the top of the underlay and down to the eaves. If the underlay is placed over the undercloak and is allowed to come into contact with the mortar, water can be sucked in through the mortar and create a slip plane between the mortar and the undercloak. By keeping the underlay under the undercloak this will not happen.
It is essential to finish the underlay half way across the outer skin of the brickwork with fair faced verges or on the bargeboard with gable ladders. Water will only travel uphill by capillary action so finishing the edge of the underlay slightly lower than the top of the rafters will discourage water from running into the roof. The undercloak should fall away from the building. The ends of the tiling battens should not come into contact with the mortar as this will increase the suction of water through the mortar and cause the ends of the battens to rot.