What changes to the roof in a loft conversion?

What the roof was built to do, and what it must now do

In an unconverted loft the roof has one job: keep the weather out. Insulation is laid between the joists at floor level, the space above is cold and ventilated, and the rafters, the angled timbers under the battens and tiles, only have to hold the covering and resist wind. Nobody lives under them.

Once the loft becomes a bedroom or study, the roof slope becomes the wall and ceiling of a heated room. The insulation has to move up into the line of the rafters, the structure has to carry people and furniture, and the covering has to stay watertight with far less margin for a leak. A drip that used to fall on bare joists now falls on a plasterboard ceiling.

That shift is why a conversion is more than an interior fit-out. It touches the roof in four places: the structure, the insulation and ventilation, the openings, and the weatherproofing around them. Each is dealt with in turn below, along with where a roofer works and where a structural engineer or builder takes the lead.

Rafters, purlins and what has to be strengthened

Many older Bath houses have a traditional cut roof, built on site from rafters, ridge board and purlins. Purlins are horizontal timbers laid along the length of the slope, propping the rafters halfway along their span. These roofs are often well suited to conversion because the loft is open and the timbers are substantial, though the floor joists below usually need to be doubled up or replaced, since they were only ever sized to carry a ceiling.

Newer houses are often framed with factory-made trusses, and those are much harder to convert. The web timbers that zigzag across the loft are part of the structure. They cannot be cut out for headroom without something taking their place, which normally means steel beams and new framing designed by an engineer. Our page on roof trusses, rafters and purlins explains how to tell which kind you have and why that decides how much work follows.

Whichever type you have, the structural design belongs to an engineer and the builder. What we look at as roofers is the condition of the timbers that stay: rot at the wall plate, old beetle damage, or rafter feet softened by years of a slow gutter leak. It is far cheaper to find these before the plasterboard goes up than after.

Insulation, ventilation and the membrane

This is the part of a conversion that most often goes wrong, and it is a roofing detail as much as an insulation one. In a warm sloping roof the insulation is fitted between and often over or under the rafters. Part L of the Building Regulations sets a target U-value, the measure of heat loss, of 0.15 W/m2K when a roof element is renewed in an existing dwelling, so a thick layer is needed and the rafters are rarely deep enough on their own.

The danger is trapped moisture. Warm, damp air from the room below can pass into the roof and condense on cold timber. How much ventilation is needed depends on the underlay laid under the battens. Marley's guidance, based on BS 5250, gives different eaves and ridge ventilation figures for non-breathable and breathable underlay, and the details change again with pitch and span. Our guide to membranes and roof ventilation goes through the differences.

If the old roof has no underlay at all, as many Bath roofs do not, a conversion is the moment to add one. That means lifting the covering on the affected slope, which links to the next point.

Roof windows, dormers and the covering

A converted loft needs daylight and, on a two-storey house, a way out. Under Approved Document B, each habitable room on an upper storey needs a way out: either a window big enough to escape through or access to a protected stair. A roof window serves as that way out only within 4.5 m of the ground, so the new loft room in a house of two storeys needs a protected stair. Your designer sets this out, and your building control body checks it.

For the roofer, each opening is a new set of details. A roof window needs rafters trimmed and framed around it, an insulated and watertight surround, and flashings dressed to the covering. A flashing is a lead strip turned over a joint so water runs off it. We do not restate the permitted development rules here, so for planning on windows see our roof windows service. A dormer is bigger work: a new box projecting from the slope, with cheeks of its own, roof and lead or tile hanging, and every junction with the old roof is a place water can enter.

On the slopes that stay as they are, the existing covering usually remains. Where the tiles or slates are tired, or where a membrane needs adding, the slope is stripped and re-covered. Matching the existing tile or slate matters in a street where neighbouring roofs are visible.

Regulations, consent and who does what

A loft conversion is building work that needs Building Regulations approval, which covers structure, fire safety, stairs, escape and insulation. Check with B&NES Building Control or a registered building control approver, the private alternative that replaced the approved inspector regime in April 2024, before work starts. Planning is a separate question. If the house is listed or in the Bath conservation area, which covers about half of the city, speak to B&NES planning first, because the two consents are separate.

In practice a conversion has several trades. An engineer designs the structure, a builder or specialist firm does the carpentry and fit-out, and a roofer handles the covering, the new openings and the weatherproofing. Where we are brought in for the roof side, we survey the slopes first, tell you what is sound, and send a written fixed price once that is done.

If the underlay or battens need attention at the same time, the guide on roof underlay and the main pitched roofing page are the places to start.