Before bituminous underlay became
popular, and to save money, it was quite common for builders to use
featheredge timber boarding as both the sarking and the battens under
clay plain roof tiles.
The idea of combining sarking with battens was very clever. Timber
boards were tapered across their width (like a fence paling) which was
correct for the batten gauge for plain tiles. The boards were laid
horizontally like battens with the thick edge (15-22mm) of the board
uppermost on the rafter to provide the edge onto which the tiles would
be laid. The thin edge of the board (5-10mm) would locate into a rebate
in the thick edge of the board below, making setting out of the gauge
very simple. Each board was nailed twice to each rafter to stop them
curling up. The cross section through a featheredge boarding gave a saw
Feather edge boards did give some protection from wind driven snow
and from wind, provided good lateral bracing of the roof structure was
in place, and they gave a reasonably smooth surface on the underside.
The timber boarding also provided a small amount of insulation and sound
deadening. What it did not do was keep out the water where there was a
broken tile. Water entering the roof ran down the board to the thin-edge
where it would seep through the horizontal joint and down the underside
of the boards to the eaves, rotting the fascia board or tilt fillet.
Alternatively, as the water seeped down the underside of the boards it
would drip off a protruding nail point.
The boards, being tapered from a nominal 19mm to 8mm, were
approximately 17mm thick at the level of the nail hole, making it just
possible to achieve the minimum 15mm of penetration needed for the nail.
Due to variation in board thickness, the depth at the nail hole position
could be as little as 14mm, allowing the nail to penetrate the underside
of the board.
Over the years many of the early featheredge boarded roofs have been
re-roofed with small format concrete interlocking tiles with a 306mm
maximum gauge, such as Redland 49 or Marley Ludlow Plus. Roofers could
strip off the original tiles and, providing the boarding was in good
condition, lay new interlocking tiles onto the original featheredge
boarding, using every third board to make a gauge of 300mm. Whilst some
contractors still use this method of re-roofing it is not considered to
be good practice.
British Standard 5534, Code of practice for slating and tiling: Part
1. Design, states: "The use of featheredge boarding as a support
for plain tiles is not recommended"
However, due to an oversight, the British
Standard does not make the same recommendation for interlocking tiles.
It recognises the fact that for a 600mm rafter spacing a timber depth of
25mm is required to ensure a safe grid on which men can work and thin
feather edge boards are not easy or safe to walk on. A tile fixed with
nails that penetrate the full depth of a 25mm tiling batten will provide
approximately 60 percent more frictional resistance than could be
achieved with featheredge boarding. Also the featheredge boarding does
not provide a barrier against water, or air pressure especially once the
timber has shrunk a little and restricts air movement directly below the
tiles, unlike battens with a flexible underlay.