Klober Ltd - Ventilation - Taking care with re-roofs
Some hints for builders and roofers on ventilation pitfalls with
re-roofs
The proportion of pitched roofing work accounted for by re-roofs
continues to grow due, in the main, to public sector housing
upgrades as we near the Decent Homes Standard initial 2010 target
date. Already, however, common problems related to provision of
adequate roof space ventilation are beginning to arise. Keeping
abreast of the swathe of building performance legislation, not least
that relating to ventilation isn’t easy, while product claims from
manufacturers in an endless quest for market share are often
confusing. The effects of condensation can, however, be devastating,
and risk of its presence in building structures continues to grow as
building standards improve.
Condensation, however, is also a symptom of modern living, use of
domestic appliances producing a greater amount of water vapour in
the home than ever before. Much of the older publicly owned
properties previously had little or no loft insulation for decades
after they were built, so condensation in the roof void was rarely a
risk. Now, with insulation being installed at thicknesses of
250-270mm, today’s pre-occupation with avoiding heat loss, and fewer
open fireplaces, the loft is colder and more prone to formation of
water vapour.
As a consequence of improved insulation standards, getting roof
ventilation right has never been as important. How this is done,
however, comes down to a straightforward choice of whether to
ventilate in the traditional way or use a vapour permeable underlay
to allow the structure to breathe. A few basic and inexpensive
precautions, however, will ensure the ventilation performs properly
and avoid the vast majority of problems that can arise. Fortunately,
most publicly owned stock is of broadly conventional roof design, so
any recommendations made apply to the greater proportion of it.
One of the commoner forms of roof refurbishment now employed uses a
non-breathable HR underlay with existing eaves ventilation. Problems
often arise if improved insulation, often installed some time after
the roof refurbishment, is pushed into the eaves. The ventilation
path becomes blocked, allowing stagnant air pockets to form and
creating an ideal environment for condensation to form. Ironically,
a re-roof provides the ideal opportunity to install accessories such
as a rafter tray or eaves vent system, which are fitted more easily
from outside than from within an enclosed and restricted loft space.
Products such as the Roll Out Rafter Tray and Loft Vent Tray,
supplied by Klober, and fitted with fascia or soffit vents, provide
the equivalent of a continuous 10mm or 25mm opening, and ensure
correct positioning of insulation at the eaves. The Loft Vent Tray
provides added convenience for the installer by adjusting itself to
the pitch of the roof.
Fascia vents are both cost-effective and desirable as they offer a
discreet and unobtrusive means of eaves ventilation. Their use,
however, does present a problem on mixed ownership terraced or
semi-detached properties. An over-fascia vent will increase the
board height and so has to be continuous, given the effect on lower
tile courses. Vent tiles, often criticized for being unsightly, are
the only option if a continuous roof line cannot be maintained or is
impractical to install. This problem has now been overcome, as
popular tile profiles such as the large format concrete Double
Roman, Double Pantile and Flat interlocking tiles such as the
Sandtoft Calderdale now have matching Profile Line® vent tiles.
These enable ventilation to be provided at high or low level and are
all but invisible from the ground. With the option of a similar vent
for the smaller 15 x 9 tile, so common in the South East, the
greater proportion of tiled and slated properties can now benefit
from unobtrusive roof ventilation In areas of planning sensitivity
in particular, this is a major advance.
Mixed ownership also presents the common problem of varied tiles and
slates within a roof line, creating the need for a bonding gutter.
These typically have a sanded strip for mortar bedding so that tiles
or slates can be butt joined. Frequently, however, the strip is
wrongly used as a valley with consequent potential for water to seep
in.
Use of vapour permeable membranes such as Klober’s Permo Forte has
also proved effective over the past 6-8 years, and was one of the
main reasons why the recently updated BS5250 was so eagerly
anticipated. Ordinarily, Permo Forte requires no additional
ventilation, but although most vapour permeable underlays now have
BBA Certification, problems can occur where comparison of
performance between similar products is not carried out. With 50 or
so such products currently covered by such accreditation, a decision
to switch source of supply can prove costly if what is supplied
proves not to be like-for-like. This is particularly true when
considering product strength and risk of fall through.
Provision of ventilation at the ridge may however be necessary and
is an added benefit that comes with dry fixing. This offers far
greater control over and consistency of workmanship, while reducing
need for remedial work due to mortar failure. Failed mortar joints
were highlighted by Granada Reports following the recent spate of
high winds. Not surprisingly, with the benefits dry fix brings to
those with long-term maintenance commitments for publicly owned
properties, complaints tended towards privately owned dwellings.
Where used, the venting area follows the full ridge length but
again, it is visually discrete. To make life easy for installers,
products such as Klober’s Uni-Dry Ventilating Ridge and Roll-Fix are
suitable for most half round and angled ridges in clay, concrete and
slate. Installation is pretty much unaffected by weather and without
the mortar there is far less weight on the roof. In practical terms
too, jobs are more likely to be completed on time. Improved visual
effect and neatness of finish are an added bonus.
Provision of adequate roof space ventilation need not be a
complicated or expensive process. Even so, evidence suggests that
there is still widespread inconsistency in the understanding of
condensation’s effects in a well insulated building. There is
already evidence of buildings having to be demolished where
uneconomic to repair as a consequence of vapour getting into the
structure. So, a little care with assessment of ventilation needs
while upgrading insulation could just prove to offer ‘best value’ in
every sense.
Email: info@klober.co.uk
Arundel Jones Associates Ltd
Hill Farm, Linton Hill, Maidstone, Kent ME17 4AL
Tel : 01622 745333
news@buildingdesign.co.uk
Registered in England and Wales No. 07334149
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