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With more and more buildings being
designed with rooms in the roof, the more we are seeing rigid insulation
being placed above the rafters, just below the battens. By placing
insulation boards above the rafters , to reduce the cold bridging effect
of the rafters, the greater the need for counter battens.
Whenever a rigid board, or sarking, is placed
between the rafters and the battens of a slated or tiled roof, it is
essential to lift the battens clear of the rigid board to allow any
water on the underlay to escape. Regardless of the type of underlay,
water in the form of condensation, melted snow or rain entering the roof
before the last ridge tile has been installed will try and run down the
underlay. Unless it can run under the battens it will dam up against the
first batten it meets and run sideways until it can find a way through
the underlay (usually down a batten nail hole). Provided the battens are
clear of the underlay by more than 6mm, water will take the easiest
route and drain off into the gutter at the eaves. To achieve a
consistent gap between the battens and the underlay, timber counter
battens should be fixed through the rigid board and into the rafter
But that's not the only reason why counter
battens are needed. With rigid insulation boards it would be expensive,
especially with plain tiles, to fix each batten with helical nail
fixings, through the insulation into each rafter. It is much more
economical to use fewer helical fixings to secure the counter battens
through the insulation into the rafters below, and use cheaper nails to
fix the battens into the counter battens.
The thickness of the counter batten can vary
depending upon the rigid board material. All batten nails need to
penetrate a structural material by a minimum of 38mm. Where insulation
board is used that has no structural fixing quality, the counter batten
needs to be the full 38mm deep to provide the full nail penetration
depth. With rigid boards such as plywood and oriented strand board the
thickness of the board can be used as part of the nail penetration depth
provided the board is adequately secured to the rafters first. This can
either be achieved by using a nail long enough to pass through the
counter batten, rigid structural board and into the rafters by 38mm.
With this method the counter batten contributes to the batten nail
penetration depth. Alternatively the board is nailed directly to the
rafters before the battens are nailed through the counter batten into
the rigid board and rafter. With this arrangement the counter batten
only qualifies as a spacer and can not count as a part of the batten
nail penetration depth.
Weight of tiles on steep roof slopes with
counter batten used with insulation board, can cause the helical fixing
nails to bend, as the section of the nail passing through the insulation
is unsupported. In this situation the ends of each counter batten should
be fixed to horizontal restraining battens equal to the depth of the
insulation board. Provided each end of the counter batten is fixed in
this manner they will not rack and will allow the helical counter batten
fixing to transmit safely the tension and compression loads.
|The fixing of
counter battens through any form of board is not easy. With the roof
boarded out it is difficult to see the rafter that you are fixing into. Accurate marking of the
boards as they are installed, or setting chalk lines from top to bottom,
is essential to maintain the line of each rafter. If the underlay is
laid before the counter battens the rafter lines need to be transferred
to the surface of the underlay. The thicker the insulation board and the
thicker the counter batten, the greater the risk that the helical nail
fixing will miss the centre line of the rafter below. A fixing that
misses the rafter or splits the edge fibres of the rafter will not be
transferring any load back into the roof structure.
At the apex of a ridge the counter battens
should finish with a vertical mitre cut. This ensures that the top
tiling/slating batten on each side will not finish on a joint and is as
far away from the end of the counter batten as possible.
Provided underlay is installed over the counter
battens, the counter battens can be finished behind the fascia board.
But where the underlay is laid under the counter batten the bottom of
the counter batten should finish close to the bottom edge of the first
tile/slate batten to allow the underlay to pass out over the fascia
board without having to rise. A rise in the underlay at this point will
allow water on the underlay to be trapped and pond, where it could drain
through an end lap in the underlay.
Counter battens installed with rigid insulation
material should be a minimum of 38 by 38mm fixed through the insulation
into the structure below with helical fixings. The length of the fixings
should equal the thickness of the insulation plus 80mm and the ends of
the counter batten should be fixed to a restraining batten.
Counter battens installed with rigid structural
boards do not need helical fixings and do not require a thick counter
batten. However the length of the counter batten fixing nail should be
equal to the thickness of the counter batten plus 40mm provided the
rigid structural board is adequately fixed to the rafters.
Where the counter batten is used as a spacer
the length of the batten nail should be equal to the thickness of the
slate/tile batten plus counter batten + 40mm. The wider the rafters, the
thinner the combined thickness of the insulation and counter batten the
better the chances of the helical fixing nail entering the middle third
of the rafter and achieving an adequate pull out resistance.
As with any system of components, the more
layers the more options are available and the more things that can go
|Compiled by Chris Thomas, The Tiled
Roofing Consultancy, 2 Ridlands Grove, Limpsfield Chart, Oxted, Surrey,
RH8 0ST, tel 01883 724774