Trimmers, Noggins & Stair Openings

Cutting an opening in a timber floor — for a stairwell, a loft hatch, or around a chimney breast — interrupts one or more joists that were carrying load continuously across the floor. A trimmer joist sits across the end of the opening and picks up that interrupted load, carrying it to the joists either side. Because a trimmer is carrying the load of every joist it interrupts, it is typically a doubled or tripled timber rather than a single joist, hung on proper joist hangers rather than nailed through.

We fix trimmers to the specification confirmed for the job — read from span tables for the grade and loading involved, or from a structural engineer’s calculation where the opening is large or the load above it unusual. We do not estimate a trimmer size by comparing it to a previous job; the opening width, joist span either side, and what the floor above carries all affect the figure.

No — forming the opening and fitting the trimmers is our structural carpentry scope. The staircase itself — stringers, treads and balustrade — is a separate job, covered on our staircase upgrades and staircase repairs pages. We coordinate trimmer position and opening dimensions directly with your staircase supplier or installer so the two pieces of work meet exactly.

Building Regulations Approved Document K sets out the requirements for stair headroom, pitch, and tread rise and going, and these vary depending on whether the stair serves a loft conversion or a standard storey. We cut the opening to the dimensions your stair supplier or architect specifies against those requirements, rather than estimate clearance ourselves — getting it wrong means an opening that is the wrong size once the stair is delivered.

The structural principle — trimming interrupted joists and carrying their load around the opening — is the same, but loft conversion stair openings often bring extra constraints: reduced headroom under the roof slope, a specific stair type (alternating tread, for example) to clear a tight space, and interaction with the loft floor strengthening the conversion needs anyway. We cover the loft-specific version of this in more detail on our loft conversion carpentry pages, including the stair opening scope there.

Yes. Framing around a chimney breast, a duct, or any other obstruction that joists cannot run through uses the same trimming principle as a stair or hatch opening — trimmer joists carrying the interrupted load around the obstruction. Fire regulations around a chimney breast also require specific clearances between timber and the flue, which we build to.

Yes, in almost all cases. Forming a structural opening in a floor falls under Building Regulations Part A, and where it is for a stair, Part K governs the stair itself. We leave the trimmers and joist hangers accessible for Building Control inspection before the floor is decked or the ceiling below is closed up.

We carry out trimmer and stair opening carpentry across Berkshire, Surrey and South Buckinghamshire, including Ascot, Bracknell, Windsor, Maidenhead, Reading, Wokingham, Sunningdale, Virginia Water, Camberley, Stoke Poges and Gerrards Cross.