Download Guidance Note 7 - Roofspace Conversion

Building Control Service
Guidance Note 7
Roofspace Conversions
Converting existing roof spaces can be one of the
most cost effective ways of increasing living
accommodation in many houses. Rooms in the
roof can provide attractive internal spaces ideal for
extra bedrooms, studies, playrooms, etc, with the
added bonus that no garden is sacrificed for the
extra space.
.
Is the existing roof space suitable for
conversion?
Some shapes and forms of roof lend themselves
more readily to conversion than others. The key
factors to access are the roof shape, its internal
height and width, and the pitch.
Gabled roofs – roofs with gable end walls are
usually easier to modify to accommodate space in
the roof than hipped roofs. Party or gable end
walls together with any internal load – bearing
walls can usually accommodate any new beams
or purlins that may be required.
Hipped roofs – the conversion of hipped roofs
can be complicated unless internal load bearing
cross walls are available or extra beams are
inserted at floor level to support secondary beams
or purlins.
Roofs with intersecting pitches (Valleys) –
Intersecting pitched roofs with valleys can also be
difficult to convert unless existing internal cross
walls are present to provide support for new
timber beams or purlins. New columns or walls
may be required at these intersections to support
beams or purlins forming the new roof space.
Is the type of existing roof structure suitable
sor conversion?
Not all types of roof structure are suitable for
conversion, so it is essential to assess what type
you are dealing with.
Traditional rafter and purlin construction
Until the 1960s most roofs to dwelling were
usually made up by carpenters on site. Early
roofs used large section timbers to form frames to
carry purlins and rafters from these members.
Most of these earlier types of roofs relied upon
support from load bearing internal walls for all but
the smallest spans. These roofs are often suitable
for conversion.
Trussed rafter roof construction
Since 1945 a number of different types of
prefabricated trusses have been developed.
These usually do not rely on support from internal
walls. The first trussed rafter roofs are normally
placed at 600mm centres with the individual
timber members joined together with pressed steel
plates.
Although it is possible to convert trussed rafter
roofs it is generally more difficult than converting
purlin and rafter roofs due to the lack of internal
load bearing walls to support new floor
constructions and the fact the trussed rafter roofs
commonly have low roof pitches from 22.5o to 30o
making them difficult to convert into habitable
spaces.
For further information, please contact the Building Control Service
P O Box 102, Civic Centre, Sunderland SR2 7DN or email [email protected]
Telephone 0191 561 1550
Roofspace Conversions continued
Building Regulations – Building Regulation
approval will always be required when a loft is
converted into a habitable space.
Complying with the Building Regulations
The following is a general checklist for
conversion works to a roof space. It is not an
exhaustive list and additional items of
information may be required.
Structural stability (Part A) – New or altered
structural members to be designed by a
structural engineer.
Surface spread of flame (Part B2) – Walls
and ceilings to new accommodation and
circulation spaces associated with the
conversion must comply with the appropriate
spread of flame regulations - Class 1.
Fire resistance (Part B3) – This normally
affects the upper floor ceiling to a new floor
construction, which must achieve 30 minutes
fire resistance. If the total floor area of the
conversion does not exceed 50m2 and
contains no more than 2 habitable rooms a
modified 30 minutes standard of fire resistance
to the existing 1st floor construction may be
acceptable.
Means of escape (Part B1) –
i) In the case of a single storey house converted
to two storey, emergency egress windows with
an unobstructed openable area of 0.33m2 are
required. The bottom of the window not to be
more than 1100mm above the floor.
ii) In the case of a two storey house converted
to three storeys the existing stair should be
enclosed with walls and doors that are fire
resisting and the enclosure should either (i)
extend to a final exit or (ii) give access to at least
two escape routes at ground level, each
delivering to final exits and separated from each
other by fire resisting construction.
Any glazing to the stair enclosure should be fire
resisting.
iii) Mains powered automatic smoke alarms
should be provided to each storey level, the
alarms need to be interlinked and positioned in
circulation areas such as landings and halls.
Sound insulation (Part E) – Existing
separating walls particularly in older properties
may need to be lined to achieve the minimum
sound insulation required by Part E of the
regulations.
Ventilation of rooms (Part F1) – Windows in
habitable rooms must have an opening equal
to 1/20th of floor area and have controllable
background ventilation of 5,000 mm2
equivalent area. Bathrooms including shower
rooms require a mechanical extractor fan with
minimum extract rating of 15 litres/second.
Ventilation of roof space (Part F2) –
Ventilation is required at eaves level equal to a
continuous 25mm air – gap and at ridge level
equal to a continuous 5mm air – gap. A 50mm
(minimum) ventilation gap must be provided
over any insulation in the pitched roof slope to
ensure adequate cross ventilation.
A vapour control layer is also required behind
plasterboard linings to walls and sloping
ceilings.
Last Updated: 10th February 2011
For further information, please contact the Building Control Service
P O Box 102, Civic Centre, Sunderland SR2 7DN or email [email protected]
Telephone 0191 561 1550
Roofspace Conversions continued
Access (Part K) – There is no restriction on the
type of staircase, which may be a straight flight,
winding staircase, quarter or half-landing stair.
Loft ladders cannot be used to provide access to
rooms although space savers alternating tread
stairs may be provided if they provide access to
only one habitable room.
The minimum headroom for stairs to loft
conversions is 1.9m to centre of stair reducing to
1.8m at side of stair. The maximum pitch of stair
is 42o.
Thermal insulation (Part L) – Thermal
insulation is required to new walls, horizontal
and sloping ceilings.
Mineral wool and
expanded polystyrene are often used as
insulating materials. Current ‘U’ values are
0.18W/m2. K for sloping ceilings. Windows need
to be double glazed and achieve a U value of
1.6 W/m².K
Glazing (Part N) – Any glazing to windows
lower than 800mm should be specified as safety
glass (toughened or laminated).
Plumbing, heating and drainage – Cisterns
tanks etc may need to be raised and
accommodated. Soil stacks may need to be
extended to accommodate any new baths/wcs.
These works need to be considered at an early
stage of the project.
Roofspace conversions are generally
considered to be a simple procedure – this is not
the case. They should be designed and
constructed by people aware of the health and
safety aspects affecting building occupiers.
SUNDERLAND CITY COUNCIL BUILDING CONTROL SERVICE
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For further information, please contact the Building Control Service
P O Box 102, Civic Centre, Sunderland SR2 7DN or email [email protected]
Telephone 0191 561 1550