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It is important to be aware of the
existing Building Regulations concerning the design and
constructions of flat roof structures. In new constructions and
some flat roof projects, It may be necessary to consult your local
council Building Control office with regard to compliance with
Building Regulations. For
information relating to current insulation requirements, refer to
Part L1 and L2.
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In a flat roof the waterproofing is
always supported by a structural roof deck. This is usually a
timber boarding of some type which in turn is supported on joists.
With the exception of garages, most
roofs above habitable parts of the house require insulation. The
most common method is where the insulation is located immediately
above the ceiling. This is known as a cold roof. (See Fig. 5). In
cold weather the roof could be prone to condensation if adequate
through ventilation is not provided. Refer to BS 6229: 1982.
Alternatively, the insulation may be placed above the roof deck.
This is known as the warm roof design (See Fig. 6). The insulation
used must be of a urethane/plywood composite board with the
plywood face on the top surface. |
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Timber Boarding
Where an existing deck comprises of
close boarded tongued and grooved timber, it is recommended that a
6mm plywood overlay deck should be installed to provide a smooth
homogenous surface for the self-adhesive membrane to bond to.
Concrete.
Common in roofs to blocks of
flats and some pre-war houses, concrete is a stable and reliable
deck material. If it needs repair, provision should be made for
drying out before any priming or waterproof covering is laid.
Chipboard.
Although frequently used for
residential flat roofs, it is not generally suitable unless the
roof has no ceiling, for example, as in a garage. If chipboard is
used it should conform to British Standard 5669; Part 2: 1989
Types C.3, C.4 or C.5.
Note: Pre-bitumenised decks
are also suitable.
Protection
from condensation
- Cold roofs should have
adequate through ventilation in the void space (see Fig. 5)
- All insulated roofs require a
a vapour control layer.
Protection
from the wind
- All roofs should be
constructed to resist wind forces.
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Essential for a
successful flat roof.
Like any part of a building's exterior, flat roofs should be
constructed to withstand natural and human forces with the minimum
of attention.
Protection
from rain and snow
- The finished roof should
have a slope of at least 1 in 80.
- It is best to drain the roof
to one or two edges.
- Conventional eaves gutters are
better than internal outlets
- Internal outlets should be
adequately sized to deal with storm conditions and be fitted
with clamping rings, leaf and gravel guards.
- The waterproofing should
extend up adjacent walls at least 150mm from the finished roof
surface in all situations.
- The top edge of felt should be
protected by a cover flashing.
Protection
from sun and frost
- It pays to insulate: heating
bills are lower and rooms are cooler in the Summer.
- Insulation of new flat roofs
must satisfy the Building Regulations, Part L1 and L2.
Protection
from people.
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If the use is
changed, the structure may have to be strengthened.
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Damage is often
caused by service trades such as window cleaner, for example
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