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The vast majority of pitched roofs with more
than one roof slope will have the same rafter pitch on each roof slope.
This is done for the good sound reason that it makes all of the roof
construction and roof covering as simple as possible. But once roof
slopes of different angles meet, then all sorts of complications arise
that generate problems for the installation of the roof covering. What
looks logical on an architect’s drawing, is often a nightmare on site.
There may be extenuating circumstances that prevent
adjacent roof slopes from having the same rafter pitch, but the reasons
for such actions are mostly forced on the construction by a planning
constraint that requires a visual break between the original and the new
roof, without exceeding the ridge height, or some such excuse. To
overcome such demands, compromises have to be made to make the resulting
differences in the rafter pitches work.
Starting at the eaves, unless the top floor windows are at different
levels the bottom of the fascia boards should be level to allow the
casement windows to open without hitting the fascia board. The height of
the fascia board should be fairly consistent to allow the guttering to
link around the building without back-falls or steps. This means that
the eaves course of tiles or slates should all start within approx 25mm
of the slates or tiles on an adjacent roof slope. If the adjacent roof
slopes are vastly different in angle, the soffit overhang for the
shallower pitch will have to be reduced.
There is a theoretical point at which the
top line of both the shallow and steep pitches will coincide, and this
should dictate the eaves overhang, however this point may not be far
enough out to suit the design of the building. The alternative is for
the level of the wall plate on the steeper rafter pitch to be raised, or
for a sprocket, or reduction of rafter pitch, approaching the eaves. A
sprocket on the steeper rafter pitch at an eaves can produce other
problems, and only works well with a plain tile roof, with interlocking
tiles and slates the sprocket may look architectural, but will
compromise the performance of the tiles and slates.
Where adjacent roof slopes have different rafter pitches the number of
tile or slate course on each slope will be different. Therefore the cuts
will be different and a lead roll, or a hip/ridge tile will be needed to
weather the junction. If there is a sprocket on one, or other, roof
slope then the line of the hip will change where the sprocket starts,
with the result that the hip will have a kink in it. If the two rafter
pitches meet at a plan angle of more than 90 degrees then the hip can
change to a position where water runs into the side of the hip on one
side, and away from the hip on the other. This results in a hybrid hip
and top edge abutment detail.
Where different pitch roof slopes meet at a valley at more than one or
two degrees different, it will not be possible to use plain valley tiles
or a mitered valley with plain tiles, or a mitered valley with double
lap slates, therefore an open lead valley should be used. As with a hip,
where one roof slope has a sprocket and the other does not, the line of
the valley will not be straight. If the building is not square and the
roof slopes meet at more than 90° on plan then the valley can change to
a position where water runs into it on one side, and away from it on the
other. This results in a hybrid valley and raking top edge abutment
Where different pitch roof slopes meet at
a valley the water run off from the steep side will be faster than from
the shallower side, and therefore the water flow will not equalise in
the centre of the valley. In extreme cases the water will drive in under
the tiles, or slates, on the shallow slope and can cause leakage at that
Hip and valley
Where hips and valleys are in close proximity to each other there is
always the risk that with differing rafter pitches the two will converge
and meet in the middle of a roof slope, forming a very difficult
junction to weather.
Where the roof slopes either side of a ridge are different, the ridge
tile will not sit square but will tilt towards the steeper roof slope.
If this is corrected it will result in one slope having insufficient lap
under the ridge tile and difficulty fixing into the ridge board or
batten. Whilst it may look cockeyed, the ridge tile should sit equally
relative to the two roof slopes, and not square to the horizon, which is
most noticeable with ornamental ridge tiles. To visually overcome most
of these problems half-round ridge tiles should be used, as angular
ridge tiles accentuate the problem.
With all projects, always check the rafter pitches of each slope to see
if there are any roof slopes with a different rafter pitch to the rest,
as the drawings may not be correct or there may not be any drawings.
Check to see on the roof plan if all hips and valleys are straight and
equally bisect the angle between the adjacent eaves lines; if they do
not then expect differing rafter pitches. Having understood the
variations in rafter pitch and the difficulties that it will create,
think twice about each perimeter detail. It will not affect the tiles or
slates in the middle of a roof slope, but will affect the perimeter
detailing. Wherever possible keep all rafter pitches the same, and at
most have two rafter pitches over the complete project – more than that
will become a nightmare.
- Check each rafter pitch angle before
- Set out the shallower rafter pitch
first and adjust the steeper pitch to suit.
- Avoid using hip and ridge tiles that
have angle or sharp profile lines that can accentuate the
misalignment caused by the different rafter pitches.
- Wherever possible avoid the use of a
sprocket at the eaves to overcome the differences between different
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774