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The minimum rafter pitch for any roof should
be determined by the material and type of roof covering. But sometimes
other issues creep in and cause the roof covering used to be below its
minimum rafter pitch parameter. What are the consequences if this
It is fair to say that the new visitors’ centre built
at the top of Snowdon will experience more wind driven rain and snow
than a terraced house in Swindon, requiring the roof to be built to cope
with the vastly different weather conditions that prevail at the top of
But 80% of all roofs in the UK are located in
fairly sheltered lowland locations with similar climatic conditions, and
it is therefore not unreasonable that minimum rafter pitch
recommendations reflect the majority rather than the more exposed
minority. If the industry or manufacturer’s recommendation are ignored
then whoever makes that decision should take responsibility for the
consequences, if and when the roof leaks, or fails.
Why would anybody want to put a pitched roof covering onto a rafter
pitch that is too low for the roof covering? In many instances there is
pressure from the planning authority to keep below a designated ridge
height, and to use a particular roof covering, which may not be
compatible. Often the designer will have second guessed the planners’
demands and allowed for a hidden flat roof at ridge level. Predicting
the problem will always allow the designer to design out the problem.
It is often changes to the design after planning
approval that can cause problems, such as the inclusion of a splay
across the internal corner of an L-shaped building, forcing a new roof
slope from the splay back up to the ridge. This new roof pitch will be
shallower than the adjacent roof slopes, as the rafter length will be
longer for the same rise.
If the roof covering on the main roof slopes are at, or close
to, the recommended minimum, then the splay roof will fall under. One
solution is to pitch the roof at the same angle as the adjacent roof
slopes and form a triangular flat roof at ridge level.
Another common situation is building a ground floor
extension with a lean-to roof. The further the extension extends away
from the building, the lower the rafter pitch needs to be if it is to be
constructed below the window sills of the floor above. The only option
may be to form cut-outs in the roof and form an internal dormer with a
flat roof from the window sill out to the pitch line for each window
When met with the situation of the rafter pitch being below the minimum
for the roof coverings, many building control officers have allowed the
work to proceed if the underlay is doubled up. This option is perhaps
the easiest to undertake, but the worst of all solutions, as this
acknowledges that the roof covering will not keep out the rain, and the
underlay will be expected to drain away the water that gets through, to
the eaves. Firstly the battens are going to get wet, which will not do
them any good, secondly this makes the underlay the primary water
resistant layer, which it was never intended to achieve, and the product
testing would have reflected this.
With some types of underlay, if the material is wet or
damp for long periods, the material will begin to break down. If the
material shrinks, or is laid flat under counter battens, then the water
may soak under the counter battens and down the holes in the underlay
made by the batten/counter batten nails. At the eaves the reduction of
pitch and/or the introduction of a counter batten could result in a back
fall of the eaves tilt fillet, which allows water to pond behind the
fascia. At an inclined valley, water running down the underlay will need
to be channelled down the side of the valley construction past the ends
of each timber batten, forcing the valley construction to be redesigned.
This is possible, but far from ideal.
The use of corrugated sheeting over the
rafters and under the battens is popular for gable to gable mono
pitches, but makes the eaves detail difficult, as the guttering has to
be lowered to collect the water running off the corrugated sheeting.
This often results in the water running off the tiled roof covering,
over-shooting the gutters during deluges.
Tile manufacturers are reluctant to sanction an extension to their
guarantees for below minimum pitch situations. Building control officers
take no liability for passing such a construction. Some underlay
manufacturers may allow their product to be used within strictly defined
conditions. Architects and specifiers show it on their drawings as being
above the minimum pitch, so they are not liable. It is left to the poor
old roofer who follows on after the carpenter, and has to use the roof
covering that has been specified, which may be inappropriate, to take
responsibility for the construction problem not of their making.
What to do
Designers should design the roof with all rafter slopes having the same
rafter pitch, and with a 10 to 15° differential between the tile/slate
minimum pitch recommendation, and the actual rafter pitch. This helps
with room in the roof situations, saves on the length of expensive lead
flashings, and looks better than a shallow pitch; where you look up the
roof slope at the exposed leading edges.
Contractors should always check the
actual rafter pitches of each roof slope and report discrepancies to the
main contractor, or client, as soon as possible so that they can make a
decision, or make alterations, before the roof covering is installed.
All contractors should resist the temptation to change the specification
to save money. The right tile/slate for the pitch should be used, or the
right pitch for the tile or slate roof covering specified should be
If you analyse the situations where slates or tiles are installed below
their recommended minimum rafter pitch, in most instances there was a
lack of design, or a lack of specification, or a lack of supervision, or
a lack of understanding of the knock-on effect of a change during
construction, and the result is a compromise that is to nobody’s
advantage, when it goes wrong and the roof leaks. Provided you keep
above the minimum pitch parameters at all times, this should eliminate
- Check the drawings for any
discrepancies or apparent changes of specification.
- Check the pitch of each roof slope
against the proposed roof covering.
- Use patented under roof solutions to
overcome a minimum rafter pitch problem with suspicion, unless it
has a solid guarantee.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774