Roof Truss Replacement: What's Involved

Summary

Since the 1960s, the great majority of new-build UK roofs have used prefabricated trussed rafters rather than the traditional cut roof (see pitched roof structure for cut roof construction). A trussed rafter is a factory-made triangulated timber frame — top chords (rafters), a bottom chord (ceiling tie), and internal web members — connected at every joint with punched metal fastener plates (gang-nail plates) pressed in under factory conditions. The whole assembly is engineered as a single structural unit: remove or weaken one member, and the load paths the rest of the truss was designed around no longer work as intended.

That engineered-as-a-unit nature is exactly what makes truss replacement different from repairing a traditional cut roof. A cut roof's rafters, ceiling joists, and purlins are individually sized members that a competent tradesperson can, within limits, assess and replace piece by piece using published span tables (see joist span calculator). A trussed rafter is not sized from a general span table at all — every truss design is calculated for its specific span, pitch, spacing, and loading by the manufacturer (or a truss engineer) using proprietary design software, and the manufacturer's original calculations are what Building Control relies on for compliance. Replacing a truss "by eye" against the damaged one, without going back to a designed drawing, is a common and serious defect.

This guide covers why trusses need replacing, the main truss types encountered on domestic work, the structural sign-off process, and the temporary works (propping, bracing, weatherproofing) needed to do the job safely. It assumes replacement of one or a small number of trusses within an existing roof — a full re-truss (for example, converting to attic trusses as part of a loft conversion) is a larger structural project that should be engineered from the outset. See re roofing for full roof strip-and-recover work and building control for the wider Building Control notification process.

Key Facts

• Trussed rafters are factory-engineered products, not site-sized structural timber — manufactured to BS EN 14250 (timber structures — product requirements for prefabricated structural members assembled with punched metal plate fasteners) and designed to BS EN 1995-1-1 (Eurocode 5)
• Common domestic truss types: Fink (W-truss) — the most common, economical, no usable loft space; Fan truss — similar to Fink with additional webs for wider spans; Attic truss — designed with a clear room space and structural floor within the truss depth, allowing a habitable loft room without the separate structural work of a conventional loft conversion (though Building Regulations for the habitable room, e.g. Part B fire, Part K stairs, Part L, still apply); Hip truss / hip end truss — used at the hipped (sloped) end of a roof, working with jack rafters/creeper trusses to form the hip
• Gang-nail (punched metal) plates connect every joint — these plates are pressed in under factory pressure and cannot be reliably replicated on site; a truss with a damaged or lifted plate connection is a structural defect, not a cosmetic one
• Bracing is part of the structural design, not an optional extra — every truss roof design includes a bracing schedule: diagonal rafter bracing (top chord), longitudinal bracing (ceiling tie/bottom chord level), and diagonal bracing at internal web members on longer-span trusses. Missing or under-specified bracing is a frequent underlying cause of truss movement and premature failure, and must be checked (and reinstated correctly) on any replacement job
• Truss replacement is always a designed intervention — either like-for-like replacement using the original manufacturer's drawings (or a compatible replacement from any truss manufacturer, provided the design is checked against the original loading), or a bespoke replacement designed by a structural or truss engineer where original drawings are unavailable
• Common causes of truss failure/damage: wet rot at bearing points (often caused by blocked or inadequate roof ventilation trapping moisture — see roof ventilation), storm/wind damage, fire damage, historic unauthorised alterations (webs or struts cut for loft access/storage without engineering approval — very common and often only discovered years later), and woodworm/insect damage in older installations
• Never remove more than one truss at a time without a specific temporary works plan — adjacent trusses rely on the shared bracing and roof covering for lateral restraint; removing a truss without adequate temporary propping to the trusses either side risks a progressive collapse of the roof plane
• Temporary propping typically uses raking or vertical acrow props under the adjacent trusses at ceiling tie and/or rafter level, with temporary binders tying several trusses together, before the damaged truss is cut free and removed
• Restraint straps tie the truss down to the wall at the wall plate — typically at maximum 2m centres, matching general pitched roof practice (see pitched roof structure)
• Building Regulations Part A (structure) applies to any structural alteration of a roof, including truss replacement — this is notifiable work requiring a Building Notice or Full Plans application and Building Control inspection
• CDM Regulations 2015 apply where the project involves more than one contractor, which is typical for truss replacement (scaffolder, roofer, structural engineer, sometimes a specialist truss-fixing crew)
• Since April 2024, "approved inspectors" in England are registered as Registered Building Control Approvers (RBCAs) under the Building Safety Act 2022 framework — the Building Control Body for the job is either the local authority or an RBCA
• BS 5975 (code of practice for temporary works procedures) is the relevant reference for the temporary propping/bracing design during the works — a genuinely engineered temporary works plan, not an improvised prop arrangement, is expected for anything beyond a single, straightforward truss

Quick Reference Table

Detailed Guidance

Diagnosing why a truss has failed

Before pricing or designing a replacement, establish the actual cause — replacing a truss without addressing the underlying cause of failure just sets up the replacement to fail the same way:

• Wet rot at bearing points — usually caused by long-term moisture ingress at the eaves, often from inadequate roof space ventilation (see roof ventilation) or a failed felt/underlay allowing water to track along the rafter to the wall plate. Probe every bearing point on affected and adjacent trusses, not just the obviously damaged one.
• Historic unauthorised alterations — cut struts or webs, often done to create loft storage access without engineering sign-off, is one of the most common causes of truss distress discovered on survey. Look for cut or spliced members, especially where they interrupt a diagonal web — this is a structural alteration that should have gone through the same design process as a genuine replacement.
• Storm or wind damage — check racking (out-of-plane movement), lifted or sheared gang-nail plates, and displaced or snapped bracing, not just the visibly broken member.
• Fire damage — even where char is limited to a small area, heat exposure can weaken adjacent connections and timber beyond the visibly charred zone; a fire engineer or structural assessment is usually warranted for anything beyond very minor, localised scorching.
• Insect damage (woodworm) — common in older stock; most historic infestation is inactive, but any structurally significant section loss found during probing needs the same designed-replacement approach as any other damaged member.

Getting the replacement designed

Trussed rafters cannot be safely "matched by eye" on site. There are two routes to a compliant replacement:

Like-for-like from the original manufacturer (or a compatible manufacturer). If the original truss design drawings are available (from the manufacturer, the original developer's records, or a previous Building Control file), a truss manufacturer can supply an identical or directly equivalent replacement built to the original specification. This is the fastest and usually cheapest route.

Bespoke design from a structural or truss engineer. Where original drawings aren't available, where the roof is being modified as part of the replacement (a different loading, an added dormer, a change to attic trusses), or where the damage suggests the original design or installation may itself have been inadequate, a structural engineer or specialist truss engineer needs to survey the roof, establish the actual loading and span, and produce a new design — including a full bracing schedule and connection details for the replacement truss into the existing structure.

Either route produces the documentation Building Control will expect to see: the truss design (span, pitch, timber sizes, loading assumptions), the bracing schedule, and the connection/restraint details at the wall plate and to adjacent trusses.

Temporary works: propping, bracing and weatherproofing

This is the stage most likely to be under-priced and under-planned, and it's also the safety-critical part of the job.

1. Assess the scope before touching anything. A single, clearly isolated truss with sound neighbours either side is a smaller intervention than a run of several damaged trusses, or a truss adjacent to a valley, hip, or structural opening — scope the temporary works to match, and get an engineer involved for anything beyond a single straightforward truss.
2. Prop the adjacent trusses first. Install raking or vertical acrow props under the ceiling tie and/or rafter of the trusses either side of the one being replaced, before any cutting starts. Temporary binders (horizontal timber runs) tying several trusses together add extra lateral restraint while the bracing at the affected truss is disturbed.
3. Establish weatherproofing before stripping the covering. Plan the sequence so the roof covering over the affected area is only opened up for as short a time as practical, with tarpaulin or temporary sheeting ready to go over the opening at the end of each working period.
4. Remove the damaged truss. Cut and remove the truss in a controlled sequence once adjacent support is confirmed — never remove the roof covering and strip out a truss's bracing connections before the temporary props are in place and checked.
5. Fit the new truss. Lift into position (by hand for smaller domestic trusses, by crane or mechanical lift for larger spans), check plumb and level, fix at the wall plate with the correct restraint straps (typically at maximum 2m centres, matching general pitched roof restraint practice), and connect to adjacent trusses per the design.
6. Reinstate bracing exactly to the design/bracing schedule. This step is very commonly under-done on repair jobs — bracing that "looks similar" to what was there before is not the same as bracing installed to the manufacturer's or engineer's specified pattern and fixing. Missing or incorrect bracing reinstatement is one of the most frequent defects found on truss repair work at Building Control inspection.
7. Reinstate the roof covering — underlay/membrane, battens, and tiling/covering to match the existing roof, checked for correct lap and fixing.
8. Building Control inspection and sign-off, generating the completion certificate for the structural alteration.

For anything beyond a single, straightforward truss, the temporary propping and bracing sequence should itself be designed as a temporary works plan (see BS 5975) rather than improvised on site — this is standard practice on larger commercial trussed roof projects and is good practice for any domestic job involving more than an isolated single-truss replacement.

Attic truss considerations

Where a damaged conventional (Fink) truss roof is being replaced with attic trusses to create habitable loft space at the same time, this is a materially bigger project than a like-for-like replacement — it changes the loading (floor load within the attic truss, not just ceiling load), the fire safety requirements (Part B — protected escape route, fire doors, potentially a fire-rated ceiling below), the means of escape and stair provision (Part K), and the insulation strategy for a habitable space (Part L). Treat this as a loft conversion project structurally designed from scratch, not a truss "upgrade" — see pitched roof structure for related structural principles and consult Building Control early.

Frequently Asked Questions

Can I replace a single damaged truss without an engineer if I match it exactly to the one next to it?

Not safely, and not compliantly. Even a "like-for-like" replacement needs to be sourced against the original design (drawings from the manufacturer or Building Control records) rather than measured off the adjacent truss by eye, because gang-nail plate positions, timber grades, and web configurations are all part of an engineered design that isn't always obvious from a visual match. Building Control will expect evidence of a designed replacement, not a description of visual matching.

Why can't I just cut the strut out of a truss to get more loft storage space?

Every member in a trussed rafter — including the internal webs that seem to be "in the way" of loft storage — is part of the calculated structural design. Cutting a web or strut without an engineer's redesign removes load-carrying capacity the truss was depending on, and is one of the most common causes of later truss failure, sometimes not becoming apparent for years. If loft storage is wanted, use a proprietary raised-storage system that spans independently between trusses without cutting any structural member (the same principle covered for ceiling joists in joist span calculator), or get an engineer to design a genuine modification.

Do I need Building Control involvement for a single truss replacement?

Yes. Any structural alteration to a roof, including replacing a truss, is notifiable under Building Regulations Part A and needs a Building Notice or Full Plans application to the relevant Building Control Body (a local authority or a Registered Building Control Approver), with inspection at the appropriate stages and a completion certificate at the end.

How long does a typical single or small-group truss replacement take?

For a straightforward, accessible single truss with sound neighbours, allow roughly 1-2 days for the structural work itself once the design/drawings are in hand and scaffolding or access is set up — plus the lead time to get manufacturer drawings and a replacement truss fabricated (commonly 1-3 weeks) or an engineer's bespoke design produced. Weatherproofing, roof covering reinstatement, and Building Control inspection add further time either side of the structural work itself.

Regulations & Standards

• Building Regulations Approved Document A — structure; governs the notification and design standard for any structural roof alteration, including truss replacement

• Building Regulations Approved Document B — fire safety; relevant where attic truss conversion or adjacent alteration affects means of escape or compartmentation

• BS EN 14250 — timber structures — product requirements for prefabricated structural members assembled with punched metal plate fasteners (the manufacturing standard for trussed rafters)

• BS EN 1995-1-1 (Eurocode 5) — structural design of timber structures, the design basis for truss engineering

• BS 5975 — code of practice for temporary works procedures, relevant to the propping and bracing plan during replacement

• CDM Regulations 2015 — health and safety duties where more than one contractor is involved in the project

• Building Safety Act 2022 and The Building (Registered Building Control Approvers etc.) (England) Regulations 2024 — from 6 April 2024, "approved inspectors" in England became Registered Building Control Approvers (RBCAs); the Building Control Body is a local authority or an RBCA

• Structural Timber Association (STA) — trussed rafter technical guidance (incorporating the former Trussed Rafter Association)

• GOV.UK — Approved Document A: Structure

• TRADA — Timber Research and Development Association — timber engineering guidance

• HSE — Temporary works guidance

• NHBC Standards — Roofs — trussed rafter roof workmanship standards

• pitched roof structure — traditional cut roof structure, rafter sizing, and restraint strap practice

• roof ventilation — roof space ventilation, a common underlying cause of bearing-point wet rot in trusses

• building control — Building Control notification process for structural roof work

• re roofing — full roof strip-and-recover projects, where truss replacement may sit alongside covering renewal

• joist span calculator — structural timber span methodology for solid-timber floor and ceiling members