Depending on the roof structure and
planning constraints, a loft conversion is one of the most straightforward ways
of getting extra space. Almost all houses can benefit from this extension with
a bit of basic planning.
IS
MY LOFT SUITABLE FOR CONVERSION?
CONVERSION ASSESSMENT:
The features that will decide the
suitability of the roof space for conversion are the available head height, the
pitch and the type of structure, as well as any obstacles such as water tanks
or chimney stacks. An inspection of the roof space will reveal its structure
and physical dimensions.
HEAD HEIGHT:
Take a measurement from the bottom of
the ridge timber to the top of the ceiling joist; the useable part of the roof
should be greater than 2.2m.
PITCH ANGLE:
The higher the pitch angle, the
higher the central head height is likely to be, and if dormers are used or the
roof is redesigned, then the floor area can be increased.
TYPE OF STRUCTURE:
Two main structures are used for roof
construction — namely traditional framed type and truss section type. The
traditional framed type is typically found in pre-1960s houses
where the rafters and ceiling joists, together with supporting timbers, are cut
to size on site and assembled. This type of structure has more structural
input, so is often the most suitable type for conversion. The space can be
easily, and relatively inexpensively, opened up by strengthening the rafters
and adding supports as specified by a structural engineer. Post 1960s,
the most popular form of construction used factory-made truss roof sections.
These utilise thinner – and therefore cheaper timbers – but have structural
integrity by the addition of braced diagonal timbers. They allow a house roof
to be erected and felted in a day, which is a big advantage to a builder.
However, this type of truss suggests that there are no loadbearing structures beneath,
and so opening up the space requires a greater added structural input. This
will normally involve the insertion of steel beams between loadbearing walls
for the new floor joists to hang on and the rafter section to be supported on —
together with a steel beam at the ridge. It is advisable to seek advice from
specialist firms in this instance.
LOW
HEAD HEIGHT?
If the initial roof space inspection
reveals a head height of less than 2.2m, there are two available – but costly –
solutions that will require professional input.
SOLUTION 1: RAISE THE ROOF:
This would involve removing part or
whole of the existing roof, and rebuilding it to give the required height and
structure. This is structurally feasible, but the major problems are the high
cost and getting planning approval. If the whole roof area needs removing, a
covered scaffold structure, to protect the house from the weather during the
works, would also be required.
SOLUTION 2: LOWER THE CEILING IN THE ROOM BELOW:
The ceiling height in some rooms in
older properties may be 3m or more, so if the roof space height is limited
there is the option of lowering the ceilings below, providing it still allows
at least 2.4m. This will require all the existing ceilings in question to be
removed, causing much mess. With this method a plate will need to be bolted to
the wall using shield anchors or rawlbolts, for the new floor joists to hang
from. There is also a need for a suitable tie between the roof structure and
the dwarf wall formed, to prevent the roof spreading.
It’s a good idea to use the low space
under the eaves for storage — but you could create a more dramatic effect by
having a gallery overlooking the floor below.
NEW JOISTS
The existing ceiling joists are
unlikely to be adequate to take a conversion floor, so additional new joists
will be required to comply with the Building Regulations. The size and grade
would be indicated in Trada Span Tables or specified by a structural engineer,
who will have taken into account the span and the separation distance for a
given loading. The new joists span between load-bearing walls, and are normally
raised slightly above the existing ceiling plasterwork. This spacing must be
sufficient to prevent any new floor joist deflection from touching the ceiling
plaster below. The new joists run alongside the existing joists. Above window
and door openings, thicker timbers are used to bridge the opening, so that
pressure is not put on the existing opening lintel. RSJs may also specified to
distribute the load, and in some installations are used to carry the ends of
the new joists.
INSULATION
The roof structure can be insulated
in one of two main ways. The most straightforward is to use a ‘cold roof’
method. This involves partial filling the space between the rafters with phenolic
foam insulation such as Kingspan or Celotex, ensuring that there is 50mm
spacing between the roofing felt and the insulation (for ventilation via the
roof and soffit vents). In addition, a suitable thermal laminate plasterboard is
attached to the inside of the rafters. The rafter thickness is often less than
120mm, so a batten may be required along each rafter to allow the 50mm spacing
and the insulation. The roof section requires 350mm of mineral wool insulation e.g.
Rockwool.
The other main method is ‘warm roof’.
This method uses 100mm Kingspan or Celotex insulation or similar over the
rafters, and a covering capping, followed by the tile battens and tiles. This
is not really a practical option unless the roof coverings have been stripped
off. It could be used with a dormer, especially if it has a flat roof.
Continuity of insulation between walls and roof is required to avoid any cold
bridging. The dormer walls can be insulated between the studwork.
The internal partition walls use a
100mm quilt or 25mm Isover APR 1200 that will provide sound insulation. Plaster
- board is attached to one side of the wall then the insulation inserted,
followed by plaster - board on the other side.
Insulation can also placed between
floor joists, and this is typically 100mm-thick Rockwool fibre or similar —
mainly for its sound-reduction properties.
THE STAIRCASE
The ideal location for a staircase to
land is in line with the roof ridge: this will make best use of the available
height above the staircase. The minimum height requirement above the pitch line
is 2m, although this could be reduced to 1.9m in the centre, and 1.8m to the
side of a stair. In practice, the actual position will depend upon the layout
of the floor below, and where necessary the available height can be achieved
using a dormer or adding a rooflight above the staircase or, if appropriate,
converting a hip roof end to a gable.
Maximum
Number of Steps:
The Regulations specify that the
maximum number of steps in a straight line is 16. This is not normally a problem,
as a typical installation usually only requires 13 steps.
Step Size:
The maximum step rise is 220mm,
whereas the step depth or ‘going’ is a minimum of 220mm; these measurements are
taken from the pitch point. The step normally has a nose that projects 16-20mm
in front of the pitch line. However, the ratio of size must not exceed the
maximum angle of pitch requirement of 42°. Any winders must have a minimum of
50mm at the narrowest point. The width of steps is unregulated, but in practice
the winders are likely to limit the reduction in width.
Balustrading:
The height minimum is 900mm above the
pitch line, and any spindles must have a separation distance that a 100mm
sphere cannot pass through.
WINDOWS & DORMERS
The loft conversion will require a
means of getting natural light and ventilation, and the most straightforward
method is to use rooflights that follow the pitch line of the roof. This type
is fitted by removing the tiles and battens in the position that the rooflight
will be fitted. The rafters are cut to make way for the rooflight after
suitably reinforcing the remaining rafters. The rooflight frame is then fitted
within the new opening, and flashings added before making good the surrounding
tiling. This type of window is the most economic, and more likely to be allowed
without planning permission, under your Permitted Development rights.
Conservation rooflights, which are slightly more flush to the roofline and are
made of metal, can also be specified.
Dormers not only give natural light
but can add space to a loft conversion; they can be at the ends or sides. They
are particularly effective where the pitch angle is high, as the useful floor
area can be increased. The mansard type will give maximum conversion roof space
because it projects the maximum available head height, thus giving a greater
usable floor area. A hip to gable conversion has a similar effect.
Dormers and other similar conversions
are normally installed by opening up the roof, and cutting the required
specified timbers to size on site. They normally involve compound angle cuts.
Care also needs to be taken with the roof and side coverings, to get a good
weatherproof structure.
Some companies will make the dormers
off site in their workshop and lift into place. This process allows quick
installation, and quick weatherproofing.
Dormers can have gabled or hip roofs,
and with careful design can enhance a roof line. In practice, a mixture of the
available types can result in the maximum light and space, and provide a fire
exit.
FIRE SAFETY
The plasterboard ceiling in the upper
rooms will delay the spread of fire to the roof space in an unconverted house.
However, when an opening is introduced for the staircase the risk is shared
with the conversion — therefore, safeguards must be in place to reduce the
risk.
All habitable rooms in the upper
storeys served by a single staircase should have an escape window with an
obstructed openable area of at least 0.33m², a minimum 450mm high x 450mm wide,
and not more than 1.1m above the floor level. For loft conversions to existing
two storey houses, more stringent provisions apply, due to the greater risk
associated with escape via high-level windows. These require a new 30- minute
fire-resistant floor to the loft conversion, and a protected 30- minute
fire-resistant stair enclosure discharging to its own final exit, with fire
doors to all rooms (except bathrooms and WC). The fire doors do not need to be
self-closing.
At least one mains-operated smoke
alarm with battery backup must be installed in the circulation space of each
storey. All alarms are to be interconnected.
(Homebuilding & Renovating)
