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How to design
a flat roof
Controlling condensation is increasingly
important for flat roof designers, say Robin
Lancashire and Lewis Taylor
T
oday we are constructing buildings that are better insulated than
ever before. More types of materials in greater quantities are
being used to deliver a warm, airtight structure, while maximising
space and meeting planning and building regulations. While this
is good for conservation of fuel and power, it makes construction
detailing more complex and increases the potential for building
defects connected with condensation.
The danger of condensation must not be overlooked. It reduces thermal
performance, can cause water staining and mould growth on internal
surfaces, but most importantly can cause damage to materials, which
may affect the structural integrity of the building. The TRADA telephone
advisory line receives many enquiries about roof detailing and there
appears to be some confusion in the industry about how roofs should
be constructed.
There are two main types of roof – pitched roofs and flat roofs. However
within each of these two main categories, there are many different ways
of constructing and detailing them. There is much talk of warm roofs
and cold roofs, but often little understanding or clarity as to what these
terms mean and the implications for design. The construction of pitched
roofs is a little better understood than flat roofs and there is generally
less of a risk of serious mistakes being made in the design. Here we
deal with the design of flat roofs.
Interstitial condensation risks
Condensation forms when warm, moist air meets a cold surface. As
the air cools, its ability to hold moisture reduces to a point where the
air is saturated (100% relative humidity). At this point the moisture
vapour condenses into liquid water which is deposited on the cold
surface. We are all familiar with condensation on the inside of glass
on a cold morning, annoying but visible and can be wiped away.
When condensation occurs within a structure it is called interstitial
condensation, a hidden problem which causes unseen damage. If this
cycle is repeated over time, very large quantities of moisture can be
deposited.
If interstitial condensation is deposited onto wood or wood based
materials, there is a risk that mould growth and decay can occur. Timber
will decay if it remains above 20% moisture content for a prolonged
period of time. When using timber in a structural application, this fact
must be at the forefront of a designer’s mind.
In order to maintain long-term durability of the structural timber
components, the design must mitigate the risk of interstitial
condensation, as well as providing ventilation to ensure that if any
moisture does come into contact with the timber, it is allowed to dry.
Through our site inspections, defects consultancy and TRADA advisory
line calls, we are starting to see more and more building designers
overlooking the basic principles of good design – even more now that
very large amounts of insulation need to be incorporated into structures
to achieve stringent U-Value targets.
There are three main types of flat roof – warm flat roofs, inverted
warm deck flat roofs and cold flat roofs. The terms warm and cold
have no bearing on the quantity of insulation used, but instead refer to
the position of the insulation in relation to the structure of the roof. In
a warm flat roof, ALL of the insulation is above the structure (such as
timber roof joists) meaning that the structure is on the warm side of the
insulation. In a cold flat roof, the insulation is typically installed between
and/or below the structure meaning that the structure itself is cold.
Warm flat roofs
A warm flat roof will typically consist of timber roof joists overlaid with
a wood-based deck material (OSB or plywood) and a plasterboard
ceiling below. A vapour control layer (also doubling as an air tightness
membrane) is installed above the deck, over which rigid insulation is
fixed. For weight-bearing purposes, this insulation is nearly always a
rigid foam based product. A waterproof roof covering is laid over the
insulation.
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Ti m b e r In d u s t r y
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69
condensation will form. If the vapour control layer is moved to the
underside of the joists, the structure then becomes something more
closely resembling a cold roof structure and should be treated as such
with a ventilation path introduced.
Inverted warm deck flat roof
The general construction and detailing of an inverted warm deck flat
roof is broadly similar to that of a standard warm flat roof construction.
However, the inverted warm deck flat roof places all of the insulation
(usually with a ballast system on top) over the top of the roof deck
and waterproof layer, meaning that both are protected from external
temperature variations and ultra violet degradation. This also avoids
the need for a separate vapour control layer, as the waterproof layer
provides this function. No ventilation is required.
Thought needs to be given to the thermal performance and durability of
the insulation materials when exposed to wetting.
Warm deck flat roof
With warm roof construction, all the structure is below the insulation
and vapour control layer, so is subjected to internal temperature
and humidity conditions. There is no risk of surface or interstitial
condensation in the structure, so there is no need to provide ventilation.
However, there is a risk of interstitial condensation within the insulation
layer, but this is typically mitigated by ensuring that the vapour control
layer has a moisture vapour resistance at least equal to that of the
waterproof roof covering. Because all of the insulation is above the
structure, the overall roof thickness is greater than other flat roof types.
In some situations, this may present planning and design issues.
BM TRADA recommends that warm flat roof designs do not include
timber materials (such as decking) anywhere between the vapour
control layer and waterproof roof covering. The risk of the timber
elements being exposed to high humidity or condensation and their
inability to breathe/dry, means that the potential for timber decay in this
area is high.
Hybrid flat roofs
To reduce total roof thickness, insulation can be specified above and
between the roof joists and deck. This type of flat roof is neither a true
warm roof nor a true cold roof and so is sometimes dubbed a hybrid
roof. Many still refer to it as a warm roof construction.
A very careful review of the risk of interstitial condensation must be
carried out. If the roof is constructed as a warm flat roof, but with a
very small amount of insulation added between the roof joists (below
the vapour control layer), then it should still function as a normal warm
roof. However, there is a tipping point to be found; as more insulation
is added between the joists, so the temperature of the underside of
the deck/vapour control layer above falls to a point where interstitial
Inverted flat roof
Cold flat roofs
A cold flat roof will typically consist of timber roof joists overlaid with
a wood-based deck. On top of this deck will be the waterproof roof
covering. Insulation is installed between and/or below the roof joists
with a vapour control layer and ceiling linings below this. Although a
vapour control layer is incorporated within the build up, condensation
could still form within it, typically in the upper layers of the roof on the
underside of the deck or waterproof roofing membrane. In order to
avoid this unacceptable condensation risk, a ventilation void is provided
between the top of the insulation and the underside of the deck. This
ventilation void is normally at least 50mm and is vented at either side of
the roof to promote a through flow of air.
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Go carefully
We understand that both warm and cold flat roofs have their place
but, where space allows, warm flat roofs are preferred. Where there is
no choice but construct a cold flat roof (for example due to thickness/
height constraints), the roof designs should always incorporate a vapour
control layer on the warm side of the insulation and a well-ventilated
void between the top of the insulation and the underside of the deck/
waterproof roof covering.
Cold deck flat roof
Some roofs, due to size, orientation, or edge detailing make providing
ventilation openings at opposing eaves very difficult or impossible. Even
when conditions are favourable, the practicality of achieving effective
ventilation over a length exceeding five metres is difficult and alternative
roof types should be used. Northern Irish and Scottish Building
Standards take a dim view of cold flat roofs, suggesting that they should
be avoided.
The lack of understanding of the dynamics of insulation and
condensation risk, coupled with poorly modelled condensation risk
calculations, means that there have been numerous occasions where
we have reviewed condensation risk calculations for clients specifying
unventilated cold flat roofs and quickly discovered that the data used
for the moisture vapour resistance of the waterproof roof covering is
incorrect. This inaccurate data then provides an artificially favourable
condensation risk calculation.
Other types of unventilated cold flat roof designs rely on the excess
moisture migrating though the structure being absorbed by natural
materials with favourable hygrothermal attributes (for example cellulose
or wood fibre-based insulation materials). This stored moisture is then
released back into the inside of the building when conditions allow;
either through the use of an ‘intelligent’ vapour control layer, or by
the omission of a high resistance membrane on the warm side of the
insulation. While calculations and modelling can demonstrate this to
be the case, we have seen instances where a build up of interstitial
condensation has resulted in the wood based materials in the roof
structure failing due to fungal decay.
We have heard others say ‘...ventilation gaps result in condensation, so
fill the roof void fully with insulation – no gap means no condensation’.
The thought being that if ventilation gaps are created, then a form
of reverse condensation can occur, where external air enters the
ventilation space and is then cooled resulting in condensation forming.
These instances are quite unusual and the conditions which allow
this to happen are often very short lived. BM TRADA advises against
unventilated cold flat roofs.
Robin Lancashire
Senior Timber Frame Consultant
BM TRADA
Lewis Taylor
Technical Consultant
BM TRADA
Supporting services
BM TRADA can assess timber frame construction
details against frameCHECK best practice and
building regulations. Telephone +44(0)1494
569966
Visit http://bookshop.bmtrada.com for this
related publication:
• Timber frame construction, 5th edition,
ISBN 978-1900510820, TRADA
Technology, 2011