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In 2003, BS5534: the code of practice for slating and tiling was
revised, with a new method for determining the minimum rafter pitch and
head-laps for the various sizes of double-lap slate. Some slate
suppliers were slow to take up the new method, and still use the figures
from the old ‘angle of creep’ method for their recommendations.
The new method included in BS5534:2003 is in the
form of five Formulas and two Tables of factors, which make it less user
friendly than the old table of pitches and laps used in the previous
editions of BS5534, but the results are far more accurate.
Water can migrate by capillary action up between two slates that lay
together. The surface area of the slate that the water will spread over
will depend on the true angle of the slates and the size of the gap
between them. Smooth slates that are in close contact are more
vulnerable than rough slates.
The shape of the area of the water contact will be in
the form of a compound curve and not a straight line, as in the old
‘angle of creep’ method. Water will seep in sideways from the side-lap
and up from the leading edge of the slates above. This will produce a
fairly defined area within which nail holes and the edges of the slate
should not occur.
The steeper the pitch, the smaller the area of water
between the slates will be. The wider the slates, the less risk of water
reaching the nail holes. Therefore, a slate that is wider and shorter
can perform better than a longer and thinner one. The ratio of 2:1
(500mm x 250mm) does not perform as well as a slate with a ratio of
1.5:1 (460mm x 305mm) at low pitches.
The further the nail holes are from the side-lap of the
slates above, the better. However, the nail holes have to be 25mm- 30mm
in from the edge of the slate to ensure the slate around the nail has
sufficient strength. If the side-lap of the slate above is off centre,
or if the slates are reholed closer to the centre of the slate, or if
the side-lap gap is more than 5mm, the performance of the slate will be
affected. In theory, slates that are hook-fixed with no nail holes
should perform better, but there is a risk of water tracking up the
slate hook. The notes to Table 5 state that the minimum pitch for hooked
slates is 25° (and below 30° the hooks should be crimped).
The calculation method in BS5534 begins by determining the head-lap for
a given rafter pitch between 20° and 45° relative to the two driving
rain exposure categories above or below 56.5l/m2 per spell, with a
minimum lap at any pitch above 45° of 54mm below, and 69mm above, the
56.5l/m2 per spell requirement, by using Formula 2 and Table 4 for the
value of C.
The figure of 56.5l/m2 per spell is taken from the
‘moderate and severe exposure’ diagram in BS8104:1992 and the BRE
report: thermal insulation – avoiding the risks. This would infer that
the anticipated driving rain will be the same in the Outer Hebrides as
in Sussex. The band above 56.5l/m2 per spell is very wide and could be
sub-divided for more accurate results. Also, the driving rain diagram
only applies to ridge heights of up to 12m, therefore buildings more
than 12m high should refer to BS6399-2: the code of practice for wind
Where the slates are short or thick, and the true slate
pitch is reduced by more than 3°, the new true slate pitch should be
determined using Formula 6. The result of the calculation from Formula 2
is not defined as the minimum head-lap, as it is presumed that there is
no need for a greater or lesser head-lap than the figure calculated.
The headlaps for the higher driving rain
conditions are greater than for below, so there would appear to be
justification for allowing greater head-laps, but there is no easy way
of determining what the increased head-lap should be.
Having determined the headlap, the effective side-lap
is calculated using Formula 3 and Table 5 for the value of El, and uses
the results from Formula 2. The results are then used with the distance
of the nail hole from the edge of the slate in Formula 4 to determine
the minimum width of slate. This assumes that the sidelap gap is not
greater than 5mm – which, with some slates, it will be.
Having done all this, the results only apply to slate
roofs that are below 30°, where the rafter length is greater than 9m at
below – and greater than 6m for above – the 56.5l/m2 per spell driving
rain figure. Finally, the nail hole gauge is determined by using the
results of Formula 2 and Formula 5.
These calculations produce some unusual results. For 255mm x 510mm
slates, the calculations allow them to be laid to a minimum pitch of 20°
with a 143mm head-lap when the rain exposure is in excess of 56.5l/m2
per spell, but should only be used at a minimum head-lap of 25°, with a
head-lap of 91mm at less than 56.5l/m2 per spell. Meanwhile, 250mm x
500mm slates can only be laid to a minimum rafter pitch of 27.5° where
the rainfall is in excess of 56l/m2 per spell with a 106mm head-lap.
Logically, if a greater head-lap were used up to a maximum of one third
of the length of the slate, the minimum rafter pitch should come down.
The other result of the calculation is that slates
150mm wide should never be used under 45° and slates 180mm wide should
only be used in locations where the driving rain exposure figure is less
than 56.5l/m2 per spell; when traditionally in some locations they have
been used at 30° rafter pitch, fixed with slate hooks, with no problems.
- Pitches and laps recommended by the
slate suppliers should be adhered to, as they often form the basis
of any guarantee.
- Try to keep the side-lap as big as
possible by keeping the sidelap gap as small as possible, and
centred on the slate below.
- Do not attempt to guess the head-lap
for any given pitch, especially below 30° rafter pitch.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774