Roofing

Underlays
Choosing an Underlay
The Quality Issues
Until the introduction of vapour permeable underlays (VPUs), roofing
membranes had been limited to heavy bituminous materials which,
for those installing them at least, had little to recommend them. In
terms of performance they were very much products of the ‘40s and
‘50s and were never improved in any way. However, though they had
limitations there was at least a choice between a British Standard
product (BS 747 Type 1F) and a lower grade material usually
described as a ‘non-BS felt’. Both were sold on a commodity basis but
once a roof had been felted and battened they were more or less
weathertight and allowed other trades to start work inside the
building.
For properties being refurbished, the protection a bituminous felt
provided led to roofs being left for short periods unprotected. By
contrast VPUs were designed to prevent infiltration of water and air in
the uncompleted building, but not for prolonged exposure. Though
Graham Copson, Technical
Manager at Klober Ltd
resistant to rainfall, they should not be expected to do a great deal more than
protect against light showers and UV. Leaving them exposed when a
downpour or heavy winds are expected is to invite problems, all the more so
with lightweight materials. VPUs are often quoted as being able to provide
around 2-3 months protection, but it’s doubtful whether this takes any account
of severe weather conditions.
Spectacular Failure
Since their introduction, in excess of 100 branded VPUs have gained BBA
accreditation. However, the variation in product quality and certificate content
is still often ignored or overlooked. As a result, specifiers, contractors and
merchants have been inclined to look at BBA certification alone as sufficient
evidence that products are broadly the same. The update to BS 5534 was
designed to clear this up by making publication of wind uplift performance on
wrappers mandatory. However, agreement to allow testing to be undertaken
in-house has been described by many as a spectacular failure. Indeed, some
manufacturers of lightweight underlays, with their reputation for ‘ballooning’,
continue to claim a level of functionality much the same as higher-performing
materials, despite what appears to be clear evidence to the contrary. The most
reliable data, including that for water vapour resistance and permeability is
that produced by an independent third party. Just a few companies, including
Klober, have commissioned agencies such as the BRE to do this.
The only tangible differentiation was achieved by the BSI and NHBC agreeing
to a need for supporting high-level ventilation for all but air-open underlays.
However, even this hasn’t prevented some from describing vapour permeable
materials as air-open. It seems staggering when even basic assessment of
performance data shows it not to be true. In contrast, very few membranes of
this type have so far come to the market, possibly because they are put to the
severest test of dispersing moisture which arises from wet processes
generated in the construction process. This critical ‘drying out’ period also has
to deal with moisture which may enter a structure from outside.
Developments
An underlay has, however, been developed which is designed to provide a
weatherproofing layer. In the same way as membranes for use in airtight
construction, it has self-adhesive edge strips to seal laps. This in itself is not
unusual but these developments also have a high mechanical load capacity
and are tear and puncture-resistant. Critically, this enables a tile or slate to be
installed at far lower than its normal minimum recommended pitch. To do this
it has to function as a waterproofing layer, its fleece also having a capillary
stop which controls the formation of condensation. For monopitch extensions
where existing pipework or windows limit the pitch, the scope to lay a plain
tile as low as 22.5 o gives a roofer flexibility to match what has been used on
the main building roof. Classed as a type HR underlay, (one with a water
vapour resistance in excess of 50 MNs/g), it does however require additional
supporting ventilation.
The other main area in which underlays have to offer high performance is in
airtight construction. This demands first and foremost use of an air barrier and
vapour control layer. Look for products that far exceed the requirements of
BS9250 (Code of Practice for design of the airtightness of ceilings in pitched
roofs). Fitted on the warm side of the insulation and rafters, this type of
product can be fixed to ceilings and walls of rooms-in-the-roof and on the
ceilings below traditional uninhabited loft spaces.
A different perspective on underlay development has come with a rigid
insulation board which has an integral vapour permeable underlay laminated
onto its outer surface. This has enabled two elements of roof construction to
be combined but more significantly, the boards have tongue and groove
edges to prevent formation of air paths and thermal bridges. Fitted over
rafters is not only quick and easy but provides added room in the roof space.
The board density also enables a higher level of thermal performance to be
provided than traditional boards of an equivalent thickness. A vapour
permeable, universal connection roll has also been developed with a vapour
permeable, weatherproof connection at the ridge, hip, valley or abutment
areas of the roof.
Such developments should leave behind those whose aim is to secure
market share by selling lightweight membranes on a commodity basis; as
standards in the industry are driven upwards, that should ultimately become
a reality. Meanwhile, specification of air-open underlays continues to draw
attention to the deficiencies of products which put contractors at risk of call
backs for problems such as ballooning and which need high-level ventilation
to prevent formation of condensation in the roof space.
For more information on the latest developments in VPLs contact Graham
Copson at Klober Ltd. on tel. 01332 813050, email: technical@klober.co.uk.
Enquiry 2
Page 32 Roofing Today
Roofing Today Page 33