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When I am asked to inspect or investigate
broken or slipped slates on a pitched roof, the most common problem that
I find is the top course of slates have not been installed correctly.
Most slaters think that what they have constructed is correct, but is
To understand what should be done, we need to go back to the first
principles of how a slate on a pitched roof stays on, and what forces
try to remove it. Firstly we have weight, which pushes the slate onto
the surface of the batten but, because of the angle of the roof, may
also cause the slate to slip down the roof slope by the force of
gravity. To prevent this happening, the slates are nailed or hooked to
There are two locations for nails: centre-nailing or
head-nailing. Head-nailing is normally undertaken with heavy and small
slates where the force of wind suction on the exposed surface of the
slate is insufficient to lift the slate. But with large, lightweight
slates, centre-nailing is used, as the nail position acts as a pivot –
which is more efficient – and the wind suction on the exposed surface of
the slate is resisted at the opposite end of the slate that rests on a
batten, turning uplift into downward force. Provided the slate is rigid,
this transfer of an upward force at the lower end to a downward force at
the top end should not bend or break the slate.
Types of slates and fixings
With fibre cement slates, which are thin and can flex in their length, a
copper disc rivet is used to transfer the majority of the wind upward
force on the exposed surface of the slate to the edges of the slates
below and into their centre-nail fixings. Therefore, provided the cut
course of slates that finish at a ridge or top edge abutment are
head-nailed and fixed with copper disc rivets at the tail, the wind
uplift forces will be resisted into the course of slates below.
With resin slates that are a bit thicker than
fibre cement slates, and will flex slightly less in their length, there
is no copper disc rivet fixing, so when the head of the slate is cut off
and the top slate is head-nailed, there is nothing but the weight of the
ridge tiles or the flashing to hold the slate down.
With natural slates, which are very rigid, once the
head of the slate is cut off, again there is nothing but the ridge tiles
or the flashing to hold the slates down against the wind uplift forces
on the exposed surface of the slate. Wind uplift forces on natural
slates that are fixed with slate hooks have their force resisted by the
slate hook through to the batten below and, therefore, when the heads of
the top slates are cut off, provided the slates are fixed with the slate
hooks, the slates will not lift.
Top edge detailing
We have seen that if a copper disc rivet is used at the tail of each top
slate this will hold the slate down, as will a slate hook, but to stop
the slates rotating about that fixing to the left or right, the top
slates also need to be head-nailed. But, where rivets or hooks are not
used, there are two other systems.
Firstly there is the shouldered method. This entails
cutting off the two top corners of the last full course of slates at 45°
to leave about half the width of the slate along the top edge. The
batten that is located under the top edge of the slate is positioned
about 25mm down from the top edge of the top of the slate so that it can
only be seen in the V-shaped spaces between the slates. The top course
of slates should now be cut such that the top edge finishes flush with
the top of the slate below.
The only way the cut slate can be fixed is
to punch two new nail holes in the slate to coincide with the centre
line of the batten, as far apart as possible. The effect of moving the
batten down, and the top edge up, is to make it difficult for the cut
top course of slates to rotate about the new nail positions.
Secondly there is the double-batten method. With the
double-batten method, a second, larger batten is placed above and
against the head of the normal top batten. The thickness of the second
batten should be equal to, or a little more than, the thickness of the
batten plus the thickness of the slate, (normally 32mm). The top cut
course of slates are cut with the top edge level with the top edge of
the thicker batten. These cut slates are then twice nailed as normal
into the thinner top batten against the top edge of the course of slates
below. This means that between the nail and the top of the slate will be
about 75mm, which is sufficient to stop the slate rotating upwards. If
the second top batten is the same thickness as the other battens (25mm),
then the top slate will kick up, or can be lifted by the wind, until the
head of the slate meets the second batten face.
Ridge tiles and flashings
Some slaters and tilers believe that a mortar-bedded ridge or a lead
flashing is sufficient to hold a head-nailed top slate down. This is not
the case, as it is a bit like pulling a nail out with a claw hammer. The
exposed surface of the cut slate is usually much longer that the lap of
the ridge over the head of the slate so, in most instances, the slate
will lever up the ridge tile and break or crack the mortar bedding. Even
with dry-fix ridge systems, the ridge may not come off but can be
levered over to one side. With flashings, clipping the lead flashing
helps prevent the lead lifting, but is not strong enough to resist the
combined leverage of the top slate and the lead flashing.
Head-nailed slates such as stone slate should cope with being
head-nailed at a ridge or top edge, especially if they are nailed onto
rigid sarking. Fibre cement slates that are fixed with copper disc
rivets should not require additional fixings at a top edge, but
double-lap, centre-nailed slates should be either fixed with slate hooks
and nails, shouldered and nailed, or a second, larger batten should be
installed to stop the head of the slate from moving once it has been
twice nailed to the lower batten.
- Gauge out the roof and see where the
last course of slates finishes below the ridge or top edge before
deciding which method of additional fixing you will use, as
sometimes there is insufficient room to install the double batten
- All ridge tiles should lap over the
top slate by a minimum of 75mm.
- In very windy locations, it may be
appropriate to use slate hooks and shoulder/double-batten the top
slates to fix them.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774