FLAT ROOFING
There’s something in the air
The British Standard for slating and tiling now recognises that air permeable underlays can take the place of
ventilation in a cold roof – as long as that underlay meets minimum performance standards. Graham Copson,
technical manager of Klober, explains
To ventilate or not to ventilate: that
is the question, and it’s one that
the industry has been discussing
for too many years. The good
news is that we now have a defi nite
answer to that question, as contained in
the latest amendments to BS 5534, the code
of practice for slating and tiling published
in February this year (BS 5534:2014 +
A2:2018). For the fi rst time, there are
minimum air permeability fi gures
included in the Standard for underlays,
that allow them to be used without any
other ventilation in a cold roof.
This is potentially good news for
roofi ng contractors. Pick the right
underlay, and there’s no need to spend
the time and money on installing
additional ventilation at the ridge.
Cold roof conundrums
Historically, there have been
problems with breathable
underlays. Around ten
years ago, we started
to fi nd new homes with
breathable underlays on
the roof, and yet there was
condensation in the roof space,
running down the underside of the
underlay and making rafters and stored
possessions damp.
We now know that the problem was
with the understanding and defi nition
of a ‘breathable’ underlay. We breathe
air, so it seems reasonable to think
that a breathable underlay lets air
through. But this isn’t necessarily so.
Just because an underlay is permeable
to water vapour, it doesn’t mean that
it is permeable to air too. Back in
2010, the National House- Building
Council (NHBC) reported problems
in unvented roof spaces with vapour
permeable membranes – condensation
was forming on the underside of the
underlay in cold, frosty weather.
In 2011, the NHBC added a clause
to its standards to say that high-level
roof void ventilation should be used
when installing vapour permeable roof
underlays and the ventilation had to be
equivalent to a 5mm continuous slot at
or near the ridge.
However, the following year, the
NHBC made a further clarifi cation.
In its Technical Extra 06, published
in February 2012, the NHBC said that
underlay that was certifi ed as ‘both
vapour and air open’ could be used
without ventilation,
providing that the
manufacturer could prove
that it enabled ventilation levels
equivalent to the 5mm gap.
This year’s amendment to BS 5534
goes a step further, setting down a
specifi c value for air permeability: a LR
(low resistance to water vapour) underlay
must have a water vapour resistance not
more than 0.25 MNs/g combined with an
air permeability of not less than 20m3/m2h
at 50 Pa. So, for example, Klober’s Permo
air would meet this new defi nition. It has
a water vapour resistance of 0.045MNs/g
and when it comes to air-resistance, tests
at the Building Research Establishment
delivered results of 63 m3/m2 hr at 50
pascals.
Fewer fi xings
When calculating wind uplift loads,
one must consider the effectiveness of
the roof substrate – boarding or sarking
and/or underlay – as ‘shielding’ the
tiles or slates from said uplift. This is
referred to as the ‘shielding factor’.
If an underlay allows air to pass
through, it implies that the ridge and
hip tiles will be taking more force from
the wind uplift, than if the underlay was
impermeable (although subject to less
force than if there was an open gap). That
is to say: it benefi ts from less shielding.
In calculation, the shielding factor
is expressed as ‘S’, and the February
amendment now sets down what S
should be for the various situations.
Where a non-ventilating underlay
is to be used, factor S would be 0.1. For
ventilated ridges (and roofs without
underlay) factor S is 1.75, and for roofs
with air permeable membranes factor
S is 1.5. This means that with an air
permeable membrane such as Permo air,
fewer fi xings are needed than if highlevel
ventilation had to be incorporated.
Ceiling airtightness
Another important consideration
when choosing underlay for cold roofs
is how airtight the ceiling below the
roof is. The latest amendment to BS
5534 tackles this issue by getting rid of
references to a ‘well-sealed ceiling’ and
instead defi ning a ‘continuous ceiling’
as one with 30mm2/m2 or less and a
‘discontinuous ceiling’ as one with more
than that fi gure.
Taking all things into consideration,
the choice of a vapour and air permeable
underlay is often the right choice for
both the roofi ng contractor and the
housebuilder. As well as time and
materials saved on the roof, this solution
means that the house is watertight
sooner, with all the positive impacts to
you that this can bring.
w www.klober.co.uk
78 www.rcimag.co.uk November 2018
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