Dormer loft conversion structural engineering
A dormer loft conversion cuts a large hole in the roof slope and builds a box out of it, so the roof around the opening has to be re-supported and the new walls carried on the floor below. We prepare the calculations and drawings that prove it and satisfy Building Control, for clients across the UK.
What is different about engineering a dormer conversion?
A dormer removes a large section of roof, so the rafters around it are trimmed and beams carry the roof over and under the opening. The dormer's own walls and flat roof then have to land on the new floor, which makes the frame more involved than a rooflight conversion.
Cutting the opening and trimming the roof
A dormer means removing a run of rafters to form the opening, which breaks the load path that carried the roof at that point. The rafters either side of the hole now have to pick up the ones you have cut, so they are doubled or replaced with steel trimmers, and a beam usually spans across the head of the opening to carry the roof above the dormer. Below the opening, a trimmer carries the cut rafter feet and the front wall of the dormer. Getting this frame right is what stops the roof sagging around the new box.
We work out the roof loads either side of the opening, size the trimming rafters and the head and cill beams, and design how the dormer's own roof, whether flat or pitched, is carried back to the main roof structure. Because a large box dormer can remove a big slice of the roof, we check that what remains still spans and that the ridge and purlins are supported where the dormer interrupts them, adding a ridge beam or steel where the original members no longer have a clear run.
Carrying the dormer walls and flat roof
The cheeks and front wall of a dormer are new structure that has to land on something solid. On a flat-roof box dormer the flat roof joists span between the cheeks or onto a beam over the front opening, and those cheeks carry down to the new floor structure below. We size the flat roof joists for the imposed and, importantly, the snow and access loads to BS EN 1991-1-3, and design the beam over the dormer window so the wide glazed opening at the front does not sag.
Because the dormer walls are often full height and carry the flat roof, the floor beneath them has to be strong enough to take that line of load. We make sure the new loft floor joists or a beam pick up the cheek walls, so the dormer does not rely on the old ceiling joists. Where the dormer sits close to the eaves or the party wall we check the load lands over supporting masonry and carries cleanly to the foundations.
The new floor and the load path down
Like any loft conversion, a dormer needs a new floor designed for habitable loads, usually joists spanning between steel beams that bypass the old ceiling timbers. The difference is that the dormer adds its own weight and its walls onto that floor, so we coordinate the floor steel with the dormer frame above so the two work together. Each beam reaction is traced through the walls below and checked down to the foundations.
The calculations are prepared to the Eurocodes, the relevant UK National Annex and Approved Document A, and cover the dormer frame, the roof trimming, the new floor and the load path to the foundations as one package. We work from your architect's or loft specialist's drawings, coordinate the steel with the head height and the stair, and issue a set your builder and Building Control can work from.
What we check.
The points our calculations resolve for a project like this.
- Trimming rafters and head and cill beams around the dormer opening
- Ridge or purlin steel where the dormer interrupts the original roof members
- Flat roof joists sized for snow and access loads over the dormer
- Beam over the wide dormer window to control deflection of the glazing
- Cheek walls carried onto the new floor rather than the old ceiling joists
- Load path from the dormer frame and floor beams down to the foundations
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What is different about engineering a dormer conversion?
A dormer removes a large section of roof, so the rafters around it are trimmed and beams carry the roof over and under the opening. The dormer's own walls and flat roof then have to land on the new floor, which makes the frame more involved than a rooflight conversion.
How much does a dormer loft conversion cost to design?
A single fixed fee covers the calculations and drawings, fixed before we begin. The fee reflects the size of the dormer, the trimming and beams needed around the opening, and the new floor design.
Does the dormer roof need special design?
Yes. A flat dormer roof is designed for imposed, snow and access loads, and the beam over the front window is sized to keep the wide glazing from sagging. We detail how the dormer roof ties back into the main roof.
Will a big box dormer weaken my roof?
Not if it is trimmed correctly. We replace the load path through the removed rafters with trimming members and, where the dormer cuts across the ridge or purlins, add a ridge beam or steel so the remaining roof still spans safely.
Where do the dormer walls carry their load?
The cheek walls and front wall carry down onto the new loft floor structure, not the old ceiling joists. We make sure the floor joists or a beam pick up that line of load and take it through the walls to the foundations.
Related in home & residential structural engineering.
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