homebuilding.co.uk 87Design& Materials cutvSprefabvS paneliSeDrooftruSSeS
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getty images; istock; c/o shelagh krasno, sips roof supplied by westwind oak
the layer beneath an external roof,
because this will make a pathway
for warm moist air directly into
the roofspace.
Another point to consider
is whether the roof structure
should be ventilated beneath
the roof cover itself. This can
be complicated (and therefore
expensive) to do, but it adds
a further level of protection,
enabling a roof to dry outwards,
should water get in.
Whatever method you choose,
it is vital that a condensation risk
analysis is carried out on your
roof design before work starts.Flat roofs
The issues with flat roofs are very
similar. Although no such thing
as a flat roof truss exists, a lot of
flat roofing today is increasingly
supplied in panelised form. Flat
roofs are invariably covered in a
continuous waterproofing layer
such as felt, rubber or PVC,
always constructed to a slight fall.
Insulation can be placed within or
above the structure and attention
to water vapour movements from
the inside is critical.
The fashion for green roofs
continues to grow and there are
countless different designs for
green roof structures. In fact, the
issues are not so very different
to any other roof. Insulation and
moisture management remain
critical in these situations and
once again you should ensure
that a competent condensation
risk analysis is carried out before
construction gets underway.paneliSeD roofS
proS
lVery quick to install,
althoughusuallythis
requiresa craneonsite
lThe insulation required
for the roof is already
builtintothepanel,soa
greattime-saver
lGreat for designs calling
foropen roof space.conS
lThe roof load is borne
by supporting beams
so has to be designed
aroundthis,whichcan
limitdesignchoice
lTime and cost
advantagesarelostwith
complexroofshapes
lNot every site can take
a crane, though delivery
lorry cranes are now
available and capable
of lifting many tonnes —
up to 7m in height and
25m in length.traDitionalcut
roof(on-Site)proS
lDoesn’trequire
specialisedfabrication
lHighly adaptable
but best for complex
shaped roofs and might
be best employed for
renovation/extension
when matching up to an
existing roof
l Materials readily
available and work can
begin with lead-in times
associated with prefab
systems.conS
l Requires high skill
levels, therefore labour-
intensive and relatively
expensive
l Timber sections are
deeper, therefore use
much more timber.truSSeDrafterS
(prefab)proS
lOn simple pitched roofs,
rooftrussesarevery
quickandeasyto install
lBecause each individual
truss is a web of thin
timber sections, it uses
less timber than a cut
roof.conS
lOrdered via specialists
and fabricated off-site,
thereforerequires
orderingin advance
lDifficult (though not
impossible) to adapt
so converting the
roof space into a loft
conversionat a later
dateis challenging
lNot so clever on
complex roof shapes
where the speed of
installation slows down.