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The verge construction that I am about to discuss will be a traditional
mortar-bedded verge, that is at right angles to the eaves and ridge,
using tiles or slates.
Let’s start by getting the fundamentals right on the majority
of tiled and slated verges. Part 1 will deal with the generic types, the
environmental influences, and the first part of the construction up to
fixing the battens. Part 2 will finish the verge construction, and Part
3 will deal with raking verges.
While proprietary dry verge systems exist and will be
mentioned, they do not form part of this discussion.
There are two general types of verge, those that are directly
above a gable wall, and those that are spaced away from the gable wall
on a gable rafter with a barge board, commonly known as a gable ladder.
Generally speaking, the constructions are similar, but not the same, and
need to be appreciated.Rain
Deluge rain occurs when there is no wind, and water just falls out
of the sky. As the wind increases, the volume of water decreases, until
there is all wind and no rain. Depending upon the force and direction of
the wind, rain will either be blown against the edge of the verge, or
over the verge.
No amount of slope on the edge tiles can stop the wind
blowing rain over the edge of the roof when it is in the right
direction. Kicking up the edge tiles on the verge may direct deluge rain
away from a verge, but at the same time it will generate gaps between
the tiles, especially with tiles or slates that are laid broken bond,
and therefore should be avoided.
While it is impossible to stop rain being blown over a verge,
it can be directed by forming a 38mm-50mm overhang to encourage the
water to drip clear of the gable wall, or run down the edge of the verge
to the gutter at the eaves.
Wind
Wind blowing against a gable wall will cause air to rise up over the
gable end and vortex over the verge. This will create positive pressure
on the underside of the verge, and suction on the top surface of the
roof covering, a short distance in from the verge. The greater the
overhang of the verge the greater the vortexing that will occur.
Roofs in windy locations are rarely designed with wide
overhangs at the eaves or verge. Maximum suction occurs in from the
verge edge, the distance will vary with the wind force and the size of
the building. Wind hitting a narrow gable will spill around the corners
and go down the sides of the building rather than rise over the verge,
while with a wide gable most of the wind will go over the verge and only
the wind close to the corners will spill around the sides. It is for
these reasons fixing all verge tiles and slates is essential.
Underlay
The underlay should extend as far as possible into the verge. With a
verge directly above the gable wall the underlay should extend to within
50mm of the outer wall edge. This is to allow the under-cloak to be
bedded into position on the edge of the brick or block wall. |
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With a gable ladder construction the
underlay should extend to the outer edge of the barge board to give
maximum protection. In both situations the underlay should always remain
under the under-cloak, never above. The reason being that some water
will get through the mortar bedding and can run off the inner edge of
the undercloak. If this happens it will be above the underlay and safe.
If the underlay is above the under-cloak this water will be
below the underlay and can run along the underside of the underlay
inside the roof void; something that should be avoided.
Where the underlay meets the bottom of an open valley
the underlay should lap onto the edge of the lead sheet or trough. With
mitred valleys or valley tiles it should remain under all the tiles and
slates.Battens
Tile or slate battens should finish as close to the verge without
being exposed or embedded in mortar as possible.
Generally with a verge directly above the gable wall the
battens should be cut square approx 50mm from the face of the wall. This
will ensure that a 150mm wide under-cloak can lap under the ends of the
battens by 50mm and overhang the wall face up to 50mm.The end of the
batten should not be further than 300mm from the last rafter to which
the batten is nailed. If the position of the last rafter is further
away, perhaps due to the thickness of the external wall and cavity
construction, a timber wall plate may need to be installed above the
inner wall skin. This will allow the battens to be nailed to it, keeping
within the 300mm requirement (BS5534 clause 6.3.1.3).
With a gable ladder construction the battens should finish on
the outer face of the gable ladder frame, in line with the inner face of
the barge board. They should be nailed to the gable rafter once the
undercloak has been installed to the correct line. If the battens extend
to the outer face of the barge board there is a risk that the battens
will come into contact with the mortar.
Tips
- Keep the ends of the battens clear of
the mortar bedding.
- Never let the batten finish further
than 300mm from the last batten fixing.
- Always keep the underlay under the
under-cloak.
- All verge tiles and slates must be
mechanically fixed.
- The edge of the verge should be
38mm-50mm clear of the wall or barge board.
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| Compiled
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774 |
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