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The fact that all tiles and slates in the
middle of a roof slope have their top end resting directly on a batten
and the bottom end on another tile or slate means that the true angle of
the top surface of the tile or slate is shallower than the pitch of the
rafters on which the battens are fixed. The difference between the
rafter pitch and the true tile/slate pitch depends upon the batten gauge
and the thickness of the tile or slates at the batten position.
At the eaves, where there is no tile below, interlocking tiles normally
rest on a fascia board or tilting fillet to provide a support for the
bottom edge. But the height of the leading edge of this support is
critical ¨C and often incorrect. The height of the board or fillet
above the top of the rafter should equal the combined height of the
batten and one thickness of interlocking tiles. This should ensure that
the eaves tiles lay in the same plane as all the tiles above them.
Eaves tiles that rest on a fascia board that is too high will
sprocket, or lay at a shallower angle than the tiles above. Whilst some
architects like the look of sprocketed eaves, it can be detrimental to
the performance of the roof, especially if the tiles are close to their
minimum rafter pitch limit. A tile that is not parallel with the tile
above will result in the upper tile resting on the eaves tile along the
knife edge. The effect of this will be to make the weather bars designed
into the underside of the bottom edge of the tile ineffectual in keeping
rain out. Combine this with a tile that may be below its minimum true
pitch and rain water will be easily blown in onto the underlay. For
every 10¡ã of sprocket on one tile the tile above will be affected by
approximately one degree. This figure will vary slightly from one design
of tile to another and the actual length of head lap used.
Eaves tiles that rest on a fascia board that is too low will dip or
lay at a steeper angle than the tiles above. This can look unsightly as
a gap will open up between the eaves tiles and the tiles above (unless
they are cambered). The gap will allow wind driven rain to gain access
directly to the top of the tile where the upper tile rests on a knife
edge. Again the weather bars on the underside of the bottom edge will be
ineffectual in keeping out the rain.
However, things are not as simple as may first appear. The height of
the fascia board may need to be lowered to accommodate the height of an
over fascia ventilation grill. Each manufacturer's product will have a
different overall height and some are designed to span forward over the
gutter so altering the point of contact of the eaves tile with the top
of the grill. The further forward the fascia grill spans, the lower the
top of the fascia board needs to be.
The amount the bottom of the eaves tile spans
over the gutter to form a drip will also have an affect, along with the
thickness of the fascia board. With some interlocking tiles the
thickness of the bottom edge of the tile is less than the top edge and
this can change things.
The knock on effect of sprocketing or dipping eaves tiles are that
with some designs of tile clips, holding down the second row of tiles
may be difficult and reduce its performance. The nails holding the head
of the eaves tiles in the batten may not penetrate the batten fully,
reducing the performance of the fixing. At junctions with other
perimeter features such as where a verge rising from a lower eaves meets
a fascia, the level of the tiles will not match, leaving an open side
interlock or excess pressure on a closed side interlock, which will
cause it to break. Unless the eaves tiles lay in the same plane as the
tiles above, the risk of wind driven rain blowing in, and tile breakage,
is also greater.
Some manufacturers provide tables of fascia board heights,
especially with over fascia ventilation grills. Where no table exists
for the combination of rafter pitch, tiles, eaves vent grill and fascia
board thickness that is to be used, you must either draw the detail
accurately to full size and measure off the height, or on site set out
the first three battens and lay three tiles, one above each other. Cut a
small section of fascia with a section of over fascia grill attached and
by sliding the fascia board up the face of the vertically cut face of
the rafter, lift the eaves tile until it is in line with the two tiles
above. This is best done with a metre long straight edge resting on the
three tiles. The straight edge will rest on the leading edge of the
tiles, if it touches only the first and third tile the eaves tile is too
high. If the straight edge touches only the first and second or the
second and third tiles the eaves tile is to low. But when the straight
edge touches all three tiles at the same time the eaves tile is in the
right position. By repeating this exercise in several locations, the
carpenter can be given the correct height dimension for fixing the
fascia board. Too often the carpenter has fixed the fascia board to suit
the position of the soffit board long before the roofer arrives on site.
The co-ordination of trades is the responsibility of the main contractor
unless otherwise stated in the contract and will always be a problem
unless co-operation between trades is improved.
- Interlocking tiles at an eaves should
never be laid with a sprocket or a dip.
- An eaves tile not laid in the same
plane as the tiles above will not perform correctly.
- The size and shape of an over fascia
ventilation grill will affect the fascia board height.
|Compiled by Chris Thomas, The Tiled
Roofing Consultancy, 2 Ridlands Grove, Limpsfield Chart, Oxted, Surrey,
RH8 0ST, tel 01883 724774