Berkshire Bespoke Carpentry
Rafter, Purlin and Ceiling Joist Carpentry
The individual structural timber members that make up a cut roof — rafters, purlins and ceiling joists, set out and cut on site to the engineer's design. Send your plans for a fixed-price quote, or arrange a site visit.
Local to Berkshire
Experienced Carpenters
32 years in the trade
Fully Insured
High-Quality Finish
Our Approach
The Members That Make Up a Cut Roof
A cut roof is built from individual structural timber members, not a single prefabricated frame — and rafters, purlins and ceiling joists are the three that do most of the structural work. This page goes deep on those members specifically: what each one does, how it is set out and cut, and where it sits within the finished roof structure.
For the complete picture of a cut roof as a whole — rafters, hips, valleys, ridge and joists built together as one structure — see our cut roofs page, the complete service this carpentry is part of. This page is about the individual members within that service, in more detail than the whole-roof page covers.
Rafters carry the roof covering’s load down to the wall plate, cut to length and birdsmouthed to seat correctly at the eaves. Purlins run across the slope to support rafters at a mid-span point, reducing the unsupported span each rafter has to carry. Ceiling joists tie the base of the roof together, resist the outward spread the rafters would otherwise create, and often carry the ceiling or floor below. Each member has a specific structural job, and each is sized and graded individually by the engineer according to the span and load it carries.
Every rafter, purlin and ceiling joist we cut follows a structural engineer’s or architect’s design. We do not calculate spans, sizes or loadings ourselves — our role is to set out and cut each member precisely to the specification we are given, in the timber grade it calls for, and to leave the structure accessible for the Building Control inspection that structural roof carpentry requires.
For a closer look at this kind of work, see our rafters and ceiling joists project, or browse all roof carpentry projects.
What We Do
Rafter, Purlin and Joist Carpentry We Carry Out
Rafter Setting Out and Cutting
We set out every rafter from the engineer’s drawing, cutting each one to length with a birdsmouth joint to seat correctly over the wall plate, and a plumb cut at the ridge. Rafter spacing and size follow the specification; the cutting and fixing is where our work adds the precision.
Purlin Installation
Purlins are lifted and fixed across the roof slope to support rafters at the mid-span point the engineer has specified, reducing deflection over longer spans. Where a purlin bears onto a wall or a structural support below, we build that bearing to the design detail given.
Ceiling Joist Fixing
Ceiling joists tie the base of the rafters together and resist the outward spread a pitched roof would otherwise create, as well as carrying the ceiling below or the floor above in a loft conversion. We fix joists to the spacing and connection detail the engineer specifies.
Birdsmouth and Wall Plate Connections
Where a rafter meets the wall plate is one of the most loaded points in the structure. We cut birdsmouth joints to the correct depth and fix rafters to the wall plate with the restraint straps or connectors the design calls for.
Rafter Reinforcement and Sistering
Where an existing rafter is undersized, sagging, or needs strengthening for a new load — a loft conversion floor, a roof light, added insulation — we sister a new section of timber alongside it or replace it entirely, to the engineer’s specified size and fixing.
Collar Ties, Noggins and Strutting
Collar ties, noggins between joists, and strutting between rafters all play a structural role in stopping movement and distributing load evenly. We fix these to the centres and specification the design sets out, not as an afterthought once the main members are up.
Recent Work
Rafters, Purlins and Joists in Progress
Scope
What We Leave to Other Specialists
Rafter, purlin and ceiling joist carpentry, as we carry it out, is structural timber work only. We do not supply or fit roofing membranes, felt, battens for tiles, roof tiles or slates, leadwork, flashing, or guttering. Those trades follow on from the carpentry once the structure is signed off.
We also do not design or calculate the sizes, spans or grades of rafters, purlins or joists. Every member we cut is sized and specified by a structural engineer or architect, and the completed work is inspected by Building Control. We build precisely to that design; we do not act as the designer.
This page covers the individual structural members. For dormer framing, hip and valley junctions, or the whole-roof picture, see our roof carpentry hub and our cut roofs page.
How It Works
From Drawings to Fixed Timber Members
Send Plans or Visit Site
Email your architect’s or engineer’s drawings, or arrange a site visit. We review the member sizes, spans and spacing the design calls for before quoting.
Fixed-Price Quote
A written, itemised quote within two working days, covering rafter, purlin and joist specification, quantities, and sequencing against other trades.
Timber Ordering
We order graded structural timber to the specification for each member — rafters, purlins and joists are rarely all the same grade — plus any steel-flitched sections required.
Setting Out and Fixing
We set out and cut each member on site, fixing birdsmouth joints, purlin bearings and joist connections to the design detail, in sequence with the rest of the build.
Sign-Off and Handover
The completed structure is left accessible for Building Control inspection. Once signed off, we hand over to the roofing trades or the next carpentry stage.
Standards
Standards and Regulations Worth Knowing
Rafters, purlins and ceiling joists fall within Building Regulations Part A (structure), and structural work on these members is inspected by Building Control before the structure is covered. We work to the design provided by a structural engineer or architect and leave the structure accessible for the inspector to check.
Timber for each member is specified by grade — typically C16 or C24 under BS EN 338 — chosen by the engineer according to the span and load that particular rafter, purlin or joist is carrying. A purlin spanning a long, unsupported distance may need a higher grade, a larger section, or a steel flitch plate; we supply timber to the grade specified for each member rather than a single grade across the whole roof.
We do not hold a Building Control self-certification scheme membership, so this work is inspected by the local authority building control team or an approved inspector in the ordinary way. Some engineers specify additional requirements — restraint straps, noggins at specific centres, strutting between rafters — and we build to whatever the design calls for.
Pricing
What Does This Cost in Berkshire?
Rafters, purlins and ceiling joists are not priced as separate line items — they are priced as part of the whole roof structure they belong to, since the quantity and specification of each member depends entirely on the roof as a whole. The relevant ranges from our roof carpentry pricing page, where this work is included, are below. They are carpentry-only and exclude roofing, weatherproofing, engineer’s fees, and Building Control fees.
| Roof Carpentry Work | Typical Range (carpentry only) |
|---|---|
| Cut roof structure, single-storey extension (rafters, ridge, joists) | £3,500 – £7,000 |
| Cut roof structure, two-storey extension / larger span | £6,000 – £12,000 |
These ranges cover the complete structure — rafters, purlins where specified, ceiling joists, and the ridge — rather than any one member on its own. Send your drawings and we will give you an exact, itemised price for the specific members your roof needs.
Get Started
Send Your Plans or Drawings
Email your architect's or engineer's drawings and a brief description of the project, and we will come back with an itemised quote or arrange a site visit.
Prefer email? Write to info@berkshirebespokecarpentry.co.uk or call 07399 651 836.
Common Questions
Rafters, Purlins & Joists — Frequently Asked Questions
What is the difference between this page and your cut roofs page?
Cut roof carpentry is the whole-roof service — rafters, hips, valleys, ridge and joists built together as one structure. This page goes deep on the individual members within that structure: rafters, purlins and ceiling joists specifically, and how each is set out, cut and fixed. If you are looking at the bigger picture of a complete new roof, see our cut roofs page; if you want detail on the structural members themselves, you are in the right place.
What does a purlin actually do?
A purlin is a horizontal structural timber (or steel-flitched timber) that runs across the slope of a roof, supporting the rafters at a mid-span point and reducing how far each rafter has to span unsupported. Without a purlin, longer rafters would need to be oversized to avoid excessive deflection. Purlins are specified by the structural engineer, including their size, position and how they bear onto a wall or structural support below.
Do you cut rafters, purlins and joists to our existing roof, or to a fixed standard size?
Both apply, in different ways. The size and grade of each rafter, purlin and ceiling joist comes from the structural engineer’s design — we do not choose or calculate those ourselves. But the length, angle and birdsmouth cut of every rafter is set out individually on site to match the actual roof geometry, which is rarely perfectly uniform even on a new extension.
What is a birdsmouth joint, and why does it matter?
A birdsmouth is the notch cut into the underside of a rafter where it sits over the wall plate, allowing the rafter to seat flush and transfer its load down into the wall rather than resting on a single edge. Cut incorrectly — too deep, or in the wrong place — it weakens the rafter at exactly the point carrying the most load. We cut birdsmouths to the depth and position the design allows for.
Can you reinforce or add ceiling joists in an existing roof without a full rebuild?
Yes, this is common in loft conversions and renovation work — adding new ceiling joists or collar ties to strengthen an existing roof, or sistering a new joist alongside an undersized one, without removing and rebuilding the whole structure. The specification for any added or reinforced member still needs to come from a structural engineer, since it changes how load moves through the existing roof.
Do you design the sizes of rafters, purlins and joists, or just cut them?
We build to a structural engineer’s or architect’s design; we do not calculate rafter, purlin or joist sizes ourselves. Span tables, loading calculations and the resulting timber sizes and grades are structural engineering, not carpentry, and getting them wrong has real structural consequences. If you do not yet have a design, we can recommend engineers we have worked alongside.
What timber grade is used for rafters, purlins and ceiling joists?
Whatever the structural engineer specifies for that particular member — typically C16 or C24 graded timber under BS EN 338, with the grade chosen according to the span and load each rafter, purlin or joist is carrying. A purlin spanning a long gap may need a higher grade or a steel-flitched section than an ordinary rafter; we supply timber to the grade specified for each member rather than a single grade across the whole roof.
Related Services
Often Paired With This Work
Roof Carpentry
Rafters, purlins and joists are part of our wider roof carpentry service — see the full picture of cut roofs, dormers, ridge beams and loft conversion work.
Cut Roofs
The complete service this carpentry is part of — a cut roof built rafter by rafter on site, tying into an existing roofline or carrying a dormer.
Hips and Valleys
Hip and valley junctions are a more complex application of the same rafter carpentry, where two roof slopes meet at an angle.
Timber Roof Repairs
Where existing rafters or joists are sagging, rotten, or storm-damaged, we repair or reinforce them rather than cutting a new roof from scratch.
Send Your Plans for a Rafter, Purlin or Joist Quote
Email your drawings or arrange a free site visit. Fixed-price written quote within two working days.
We carry out rafter, purlin and joist carpentry across Berkshire, Surrey and South Buckinghamshire: