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Anatomy of a Loft Conversion

A typical loft conversion with a box dormer

This page is intended to give the homeowner an idea of the typical structural features involved in converting a loft space into a habitable room.

Firstly, there's no such thing as a typical loft conversion! We have designed hundreds of loft conversions over the years, and they're all different due the variations in the original construction and the requirements of the occupants. In one case we designed two halves of a semi-detached house simultaneously, but the structures had to be totally different!

This example is based on a 19th Century end-terrace house, and has a "box dormer" on the rear of the roof. This requires a ridge beam to support the apex of the roof, as the rafters are no longer coupled as in the original roof. Parts of the structure are hidden to enable viewing.
Ridge Beam Support Post at ChimneyTo avoid supporting the steel ridge beam in the chimney stack (which is often weak and sometimes poorly supported if a chimney breast has been removed), we install a steel post against the face of the chimney.
Chimney supported on steel beam

Sometimes a steel beam is needed at the new floor level to support the post, if the load-bearing spine wall is not in line with the ridge of the roof.
In this case, the beam fulfils a second function to support the chimney stack, as the chimney breast has been removed below the new floor level. Often these are found to have been previously removed without installing any support to the 2 tonnes of old brickwork above!

Steel Beam supporting front of roof and floorStairwell opening trimmed out with multiple joistsTo enable the collars to be removed from the loft space, the purlin supporting the front rafters needs to be supported. This is normally effected by a studwork "ashlering" wall taking the load down to the floor structure. Where headroom permits deeper joists to be employed, a steel beam is not always necessary as shown here.

The new floor joists are installed between the ceiling joists with a small clearance above the ceiling. The ceiling joists are usually suspended from the new floor in order to resist sagging. The stairwell opening is trimmed out with a double or triple joist.

The floor joists are mounted in steel joist hangers from a timber plate bolted to the top of the beam. They are restrained in place by timber packers between the joists (not shown).Floor Joists suspended in steel hangers

Dormer Window framed out in studworkRidge Beam detail to maximise headroomThe dormer structure is framed out in studwork, with timber lintels for the windows supported on "cripple" studs. The sheathing is not shown, but is vital for stabilising the framework. OSB or plywood panels are nailed to the studs and noggins (short pieces of timber) are placed to support the edges of the sheets. The sheet ends should be staggered to improve racking resistance.

The detailing of the dormer roof is often critical, as headroom is usually limited. There are a number of different ridge beam details that can be adopted to make the most of the available space.
© Charles Tallack Engineering Consultancy 2012

Engineers Notes

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