homebuilding.co.uk 85Roof ventilation
The major complexity facing roof
builders today is how to keep
the roof structure dry from the
inside. Now that we routinely
add thick slabs of insulation to a
roof, there is a very real risk of
water vapour condensing within
the roof structure, as there is a
pronounced temperature drop
through the roof in winter, from
inside to out.
To compound the issue, there
are alternative insulation options
which use various materials
placed in different zones within
the roof. Insulation can be
placed within the rafter zone,
underneath it, above it or, as
frequently occurs, a combination
of all three. Various terms
like cold roof, warm roof and
hybrid roof are used to describe
these arrangements and much
confusion results from the casual
use of these poorly defined terms.
The key point is that whichever
roof profile is used, it must
avoid any build up of moisture
within the structure that might
lead to condensation. The most
important requirement is to
seal the layer between the inside
of the house, where the warm
air carries more moisture, and
the roofspace above. There are
various membranes designed to
do this, known as either vapour
barriers or air barriers, and it
is important to have a strategy
in place so that any internal
membrane is designed to work
with the insulation above and any
additional external membranes
as well. Your first strategy is
to stop water getting into the
roofspace, but there should also
be a way of letting any trapped
water evaporate. Features like
downlighters should not penetrateformats of trussed rafter, each
designed to fit a particular house
style or design. The downside
of all this is that the cost and
speed advantage of the simple
roof truss gets lost as the trusses
grow in complexity and many
housebuilders found themselves
switching back to the traditional
cut roof. This was especially
true when features like dormer
windows were added to the
design, for the dormer can be as
complex as a whole roof and once
you start adding cut roof features
on top of a trussed roof any
hoped-for economies vanish.
Panelised roofs
The popularity of the trussed
roof was further dented in
the 1990s by the arrival of
the panelised roof which
incorporates insulation
embedded between two rigid
sheets of plywood or similar.
SIPs (structural insulated panels)
in particular are now popular.
A panel roof is craned into
place to rest on supporting
beams at the ridge and the walls
and sometimes at mid-span too.
They are extremely quick to
install, and reduce the amount
of follow-on work because the
insulation comes embedded
within the panel so there’s no
need to add it.
Panelised roofing comes into
its own on simple roof shapes
but gets progressively more
challenging as the roof shape
grows in complexity, and here
conventional roof carpentry finds
itself back in favour.
But where the roof space is
designed to be lived in, and
the roof shape is suitable, the
panelised roof is now the most
popular option.
casestudy:
cutting timber on-siteWhenresidentHomebuilding&
Renovatingwriterandchartered
surveyorianrockbuilthis100m^2
extension,hehadtoconsiderthe
roofstructurecarefully(below).“the
planners wanted the front of the
extension to be set 200mm lower than
the roof on the main house, whereas
at the back it needed to be the same
heightastheexistingrooftocreatethe
desireddualgabled‘twinpeaks’effect.
“theroofdesign comprises two
separatestructuresjoinedtogether,”
explainsian.
Soian opted for a traditional cut
roof, with the carpenter constructing
the skeleton and fitting the rafters, too.
Makingsurethenewroofabutsthe
existinghousepreciselycanbetricky.
being able to cut what you need on
site can mean any adjustments needed
can be done on site rather than trying
to alter prefabricated trusses, which
isn’t easy or particularly recommended
because it can weaken the timber.go to
@Myhomebuilding
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inspiration, technical tips
and expert advice