Trussed Roof Loft Conversions

• Traditional cut roof: Built on site from separate rafters, purlins and ceiling joists. It is found in most houses built before the 1960s, including Victorian, Edwardian and interwar homes. The loft is largely open, and the roof is carried by purlins and supporting walls, so it is usually simpler to convert.
• Trussed roof: Made from factory-built timber trusses, usually W-shaped (known as fink trusses). The trusses are placed at close centres and joined with metal plates. It is common in houses built from the 1960s onwards. Every part of each truss is working, so the webs that fill the loft cannot simply be cut out.

A quick way to tell: a cut roof has a few large timbers and plenty of open space. A trussed roof has many thin timbers, closely spaced, with diagonal members crossing the loft.

Yes, in most cases. The trusses are removed, and the roof is carried in a different way. The usual approach is:

1. 1

   Structural design: a structural engineer designs the new roof structure and floor, based on the span, the roof pitch and the walls available to carry the loads.

2. 2

   Steel beams: new steel beams are installed, usually at purlin level and sometimes at the ridge. They rest on padstones built into the load-bearing walls.

3. 3

   New floor joists: deeper joists are installed, because truss ceiling ties are too slender to carry a room. They span between walls or steel beams, independent of the ceiling below where possible.

4. 4

   Temporary support and truss removal: the roof is propped while the new structure takes the load, then the truss webs are cut out in the sequence the engineer specifies.

5. 5

   Conversion: with the space open, the loft is converted like any other.

Where the roof pitch is very shallow, or the structure is not suitable for re-engineering, the whole roof may be rebuilt. This can be done as a new cut roof, or as part of a larger roof extension.

Never cut or remove any part of a roof truss without an engineer's design and building regulations approval. It can weaken the whole roof.

Yes, always. A trussed roof loft conversion changes how the whole roof is supported, so a structural engineer must design the steel beams, floor joists, padstones and the removal sequence. The engineer also checks that the walls and foundations can carry the new point loads. Their calculations are submitted for building regulations approval, and building control inspects the work. We work with structural engineers on every trussed roof project and build exactly to their design.

Once the trusses have been re-engineered, most loft types are possible:

• Velux loft conversion: Where the roof pitch gives enough head height.
• Dormer loft conversion: The most common choice, because many trussed roofs have a shallower pitch and need the extra height a dormer gives.
• Hip-to-gable loft conversion: For hipped trussed roofs on semis and detached houses, often with a rear dormer.
• Shell loft conversion: The structural work, steel beams, floor and windows, with the interior left for you to finish.

Trussed roofs are often built at a shallower pitch than older cut roofs, so head height is the key check. The new, deeper floor joists also raise the floor level slightly. At the survey we measure the height from the new floor level to the underside of the ridge. We then tell you honestly whether roof windows alone will work, or whether a dormer is needed to give comfortable standing height.

The roof structure itself does not change the planning position. That depends on the type of conversion you build on top. Roof windows and rear dormers are often permitted development on houses, within the usual limits. Every trussed roof conversion needs building regulations approval, including:

• the structural calculations
• a protected escape route and interlinked smoke alarms
• the staircase
• insulation
• ventilation

On semi-detached and terraced houses, steel beams bearing into a party wall need a notice under the Party Wall etc. Act 1996.