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Tips 13

When looking at the gable end wall of a building the two verges that meet at the ridge are described the opposite way around. The one on the right is a left-hand verge and the one on the left is a right-hand verge. This is due to roof slopes being described whilst looking at the roof slope, not the gable end.

Overhang
Verge tiles should overhang the building fabric by between 38 and 50mm. Less than 38mm and watermarks will show down the face of the wall. Greater than 50mm and end tiles in each row will become unstable due to the overhang and buffeting on the underside and the suction on the upper side as wind blows over it.

Batten end
To support the bedding mortar a 150mm wide fibre cement, or slate, undercloak needs to lap under the end of the tile batten by no more than 50mm, leaving the outer 50-75mm for mortar and a 25-50mm gap between the end of the batten and the mortar. The end of the tile batten should not come into contact with the mortar as moisture from the mortar will wick into the timber, causing it to swell. This will cause the mortar to crack at the batten position, and eventually the timber batten will rot. Plain tile undercloaks should not be used with interlocking tiles with a gauge greater than 265mm due to the need to lap the undercloak under the ends of the tile battens. Also, below 30 rafter pitch, as the camber of the tile can encourage water draining over the verge to run back to the wall face, rather than dripping clear of the wall.

Underlay
Underlay at the verge should finish under the undercloak, never above it, and never come into contact with the bedding mortar. By putting the underlay under the undercloak, any water on the undercloak will drain onto (not under) the underlay.

Fixings
With the end of the tile batten located approximately 100mm from the verge edge, it is just possible to hang a half tile for a flat interlocking slate on the end of the batten. British Standard 5534, Code of practice for slating and tiling: Design: 1997 states that all perimeter tiles must be mechanically fixed. The correct choice of a nail or a clip will depend upon the calculation for the individual roof at that location. Whilst the calculation may say that the verge tiles only need to be nailed, it is not safe to nail the half tile to the batten, as in most cases the nail will either miss the end of the batten or penetrate the batten close enough to the end to cause it to slit, giving no grip to the nail. It is for this reason that all interlocking tiles that finish

Poor detail  the end of the batten is embedded in the mortar, causing the mortar to crack at each batten position. It is possible to clearly see the end of the batten through the crack in the mortar bedding, an ideal entry point for water.

with a half tile at a mortar bedded verge should be clipped on both the right and lefthand verges.

The clipping of verge tiles should be undertaken with the verge clip recommended by the manufacturer, as each tile design will have a different profile and interlock height (two products may look alike but the strength of the fixing may be very different). When clipping the tiles it is essential that the clips are nailed to the batten prior to the mortar being laid (not pushed into the mortar after the tiles have been laid). Every tile on the verge should be held down by a clip at its tail, and that includes the right and left hand eaves tiles that are most vulnerable to wind uplift. In this situation the verge clip should be nailed to the top of the fascia board and the hook section bent to accommodate only one tile thickness.

Mortar bedding
Mortar should always be bedded as work proceeds rather than pointed after the tiles have been laid. It is the compaction of the tile on the mortar that has a beneficial effect. Pointing will not produce the same degree of compaction.
      With some tiles that have a high corrugation, the height of mortar at the verge may cause the mortar to slump. Traditionally in this situation dentil slips laid lengthwise were used to thin out the mortar. A safe precaution would be to clip the verge tiles such that the mortar is only acting as a filler, not as an adhesive.
      Mortar bedding if used correctly can provide a very neat, economic and long-lasting verge detail. However, if not done correctly it can result in an ongoing maintenance problem for the building owner.

Compiled by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove, Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774
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