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The term ‘centre nail slates’ is misleading,
as the nail fixing is not located in the dead centre of the slate or in
the centre of the length of the slate. So where should the nail holes
be? There are some fairly simple rules and complicated calculations to
ensure that the slates are fixed in the right place.
With double lap slates the length of the slate is equal to two gauge
lengths plus one headlap length. Each slate must lap over the slate
below by one gauge length, plus one headlap length, with the second
gauge length extending up under the next slate to the centre of the next
The horizontal line along which the nails can be fitted
has to be at the lowest point where the nail will not penetrate the
slate below. If it is higher up the slate it will miss the batten,
unless the slates are being nailed directly to rigid sarking boards. The
higher the nail fixing is up the slate the less effective to wind uplift
forces they will be. The best location for the nail fixings is 3- 5mm
above head of the lower slate, making the nail hole position one gauge +
one headlap + 4mm from the leading edge of the slate. But as the head
lap and gauge dimensions can vary proportionally the headlap needs to be
determined to allow the gauge to be calculated.
Take the length of the slate, deduct the headlap and
divide the answer in half for the gauge dimension. For a 500mm long
slate with a 100mm headlap the gauge is 500-100=400x0.5= 200mm. In this
case the nail holes should be 200+100+4=304mm from the leading edge of
The maximum headlap dimension is defined as one third
of the length of the slate, which for a 500mm long slate will be 167mm.
The more critical minimum headlap is defined by the action of water held
in the small gap between slates, rising up by capillarity. The minimum
headlap will vary with pitch but should never be less than 54mm. Water
can rise up to 75mm by capillarity under the most ideal conditions,
which are two smooth parallel surfaces approx 1mm apart.
Gaps between natural slates will vary with the texture
of the slate but are likely to be worse with smooth surfaces like glass.
Therefore when slates are laid on shallow rafter pitches, water will
creep further up between the slates than when laid on steeper rafter
pitches. This means longer slates can generally be used on shallower
rafter pitches as the headlap will be bigger without exceeding the one
third of the length of the slate maximum headlap rule. The calculation
to find the exact minimum headlap figure can be found in clause 220.127.116.11
of BS 5534 The code of practice for slating and tiling: 2003. Where the
driving rain exposure is higher, the capillarity can be assisted by
wind-driven rain, so different values within the calculation are used
for above and below the 56.5l per m2 rainfall rates.
Water running down a slate will run into the side lap joint between two
slates, and soak in sideways under the adjacent slates. This water will
meet the water rising from the headlap and combine in a curve between
the two. All nail fixing holes in the slate should be above this curved
line of water, otherwise water may run down the nail hole onto the
batten or underlay below. The distance that water will seep in sideways
is partially determined by the rafter pitch, the gauge, the water flow
and the texture of the slate.
Normally the nail holes should be
located close to the outer edges of the slate, as far away from the
side lap joint of the slates above as possible. But if the side lap
gap between the slate increases from the recommended 1-5mm to a
maximum of 9 for fibre cement slates, sometimes more with some
natural slating, the distance from the edge of the upper slate to
the nail hole reduces, making the nail hole more vulnerable. If the
side lap drifts off the centre of the slates below, or if a narrow
cut is introduced where two areas of slates are stitched together
above a dormer, the normal nail hole position may fall within the
wet area of the headlap and sidelap. This must be avoided.
common instance where water can reach a nail hole is when salvaged
slates are reholed, either for a new headlap size, or where the
original nail hole is too large and the nail head pulls through.
Generally to maintain some strength in the slate around the nail hole,
the new hole is punched 25-30mm away from the previous hole, normally
closer to the centre of the slate, reducing the side lap distance. If
the slates are very wide relative to their length, the relative
performance of the slate improves as the distance to the nail hole
increases, but at the expense of cost and weight. Generally the widest
slate proportion is a proportion of 2:1.5 compared with the narrowest
slates which are 2:1.
The distance a nail hole can be formed in a slate relative to the edges
of a slate is determined by the strength and thickness of the slate
material, and the method of forming the hole. With some slate material
that is very brittle, it may be impossible to punch holes, as this would
break the total slate. These slates have to be drilled, or fixed with
Slates that are thinner and capable of being punched
cleanly can normally have nail holes formed between 25mm and 30mm from
the edge; any closer and the slate will break between the hole and the
edge under wind suction loads. The steeper the rafter pitch the greater
the distance the nail hole can be placed in from the edge. In BS5534,
formula 3 is used to calculate the minimum side lap distance between the
edge of the upper slate and the nail hole position. Whilst this gives
you the minimum total slate width, it can also determine the maximum
edge distance for the nail hole. For example a 500 x 250mm slate laid on
a rafter pitch of 25° in a moderate driving rain exposure area installed
with a 100mm headlap, would require the maximum edge to nail hole
distance to be 25mm. While at 60° rafter pitch the maximum edge distance
rises to 42mm with the same 100mm headlap. But at 60° with a 54mm
headlap the edge to nail hole distance falls back to 32mm.
Whilst headlap is frequently mentioned when specifying slates, side lap
and nail hole position is rarely specified and so rarely measured or
supervised, but in most cases is a much more important dimension,
especially when slates are installed on rafter pitches below 30°.
Trimming the width of a slate, or re-holing a used slate should be done
in the knowledge that it may affect the weathering performance of the
slates, unless the rafter pitch is higher than 45° and the headlap is
- Always try and maintain the half
bond side lap when setting out and laying a slate roof.
- Avoid trimming the width of a slate
on a shallow rafter pitch .
- Unless you are slating a very steep
rafter pitch, avoid re-holing a slate more than 30mm away from the
edge of the slate.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774