Roof Trusses: Types, Loading Notation, Bracing Requirements and Modification Rules

Summary

Engineered roof trusses have been the dominant roofing system in UK housebuilding since the 1960s, replacing traditional cut-roof construction in the vast majority of new-build dwellings. A truss is a pre-engineered, factory-manufactured structural element where the geometry of the members and the placement of the connector plates are calculated to carry specific loads over a defined span. The efficiency of a truss comes from the triangulation of forces — each member is primarily in tension or compression, not bending, which allows lighter timber sections to carry larger spans than a simple beam.

Because the design is so precise, any alteration — even apparently minor ones like removing a diagonal web member or cutting a notch for a pipe — disrupts the load path and can cause catastrophic failure. This is the single most important rule in roof truss work, and it is reinforced by NHBC Chapter 7.2, which requires that any alteration is approved in writing by the original truss designer or a suitably qualified structural engineer before work begins.

Understanding the range of truss types, how loading is expressed, and what bracing is required allows carpenters and site managers to specify correctly, erect safely, and communicate with designers and building control inspectors.

Key Facts

• Fink truss — the most common domestic roof truss; W-shaped internal web pattern; spans up to approximately 12m without intermediate support
• Attic/room-in-roof truss — designed with a habitable room space in the centre; heavier sections, more expensive, requires habitable use to be declared at design stage
• Mono-pitch truss — single slope; used on extensions, garages, and lean-to roofs; typically limited to 6–7m span
• Hip-end system — girder truss at the hip end with jack trusses spanning from it to the wall plate; the girder truss must be designed for the additional concentrated load from the jack trusses
• Gang-nail plates — the proprietary name for toothed metal connector plates (MiTek, Gang-Nail, Posi-Joist brands); pressed into timber with rollers under high pressure in the factory; field repairs are not possible
• WS2 loading — standard domestic loading notation: 0.75 kN/m² imposed (occupancy load) + 0.75 kN/m² snow (ground snow load in most of England); steeper pitches and Scotland require higher snow loads
• BS EN 1995-1-1 (Eurocode 5) — current structural timber design standard; superseded BS 5268 but both remain in use for design reference
• 400mm or 600mm centres — standard truss spacing; 600mm is most common for domestic roofs; 400mm used where additional load paths or heavier finishes are specified
• Temporary bracing — must be installed immediately during erection; permanent bracing is often insufficient during construction
• Rafter bracing — diagonal bracing across rafter pairs; resists racking of the roof plane
• Longitudinal binder — horizontal member running along the ridge or near ridge tying trusses together in the longitudinal direction; prevents domino collapse
• NHBC Chapter 7.2 — specific chapter requiring all truss modifications to be approved in writing by the truss designer; required for Buildmark warranty compliance
• Service zone — the depth between the bottom chord of the truss and the ceiling plane; should not be penetrated without design approval
• Delivery and crane — trusses are delivered pre-made; spans over 8m typically require a crane or mechanical lifting; trusses stored flat can deform

Quick Reference Table

Detailed Guidance

Truss Design and Loading Notation

Trusses are designed by specialist truss manufacturers (MiTek, Gang-Nail, Posi-Strut) using proprietary software that implements BS EN 1995-1-1. The designer inputs the span, pitch, spacing, timber species and grade (typically C16 or C24), and the design loads.

Design loads for standard UK domestic roofs (WS2):

• Permanent (dead) load: 0.75–1.0 kN/m² (tiles + battens + sarking board)
• Imposed (occupancy) load: 0.75 kN/m² (maintenance access)
• Snow load: 0.75 kN/m² at sea level in England; increases with altitude and latitude (Scotland may require 1.0+ kN/m²)
• Wind load: derived from BS EN 1991-1-4 based on site location and exposure

The design output is a drawing showing member sizes (typically 47×97mm or 47×120mm C16), plate sizes and positions, and the maximum permitted reaction loads at each support. This drawing should be retained on site and filed with the Building Control documentation.

Fink Truss

The fink (or W-type) truss is the standard domestic roof truss, representing the majority of trusses installed in the UK. It has a central vertical compression member (the king post) and two diagonal web members on each side, forming a W shape when viewed from the end.

For typical domestic spans (7–10m) at 600mm centres with a 35° pitch and WS2 loading, the bottom chord is usually 47×97mm C16 and the rafters (top chord) are 47×120mm C16. The connector plates are sized by the design software — do not compare plate sizes between manufacturers as different designs are not interchangeable.

The fink truss leaves no usable attic space — the web members fill the space. This is its primary limitation.

Attic and Room-in-Roof Truss

Where habitable loft space is required, an attic truss (room-in-roof truss) is used. The internal geometry creates a clear rectangular or trapezoidal habitable space with raised tie members at ceiling height and vertical members at the sides. The timber sections are significantly heavier (often 47×145mm or 47×170mm) and the truss is substantially more expensive to manufacture.

The critical point with attic trusses is that the habitable use must be declared to the truss designer at the point of order — the design loads are different (habitable floor load is 1.5 kN/m², not 0.75 kN/m²) and the truss cannot be retrospectively upgraded. Building Control will require evidence of the truss design certificate when inspecting the floor structure.

Attic trusses also require a floor deck (typically 22mm tongue-and-groove P5 chipboard) to be designed into the system, as the bottom chord members double as joist supports. The span of the floor is determined by the truss design, not independently.

Hip-End Truss Systems

A hip-end roof has sloping surfaces on all four sides. Using trusses for a hip end requires:

1. A girder truss — a heavier, doubled or tripled truss at the hip position designed to accept the concentrated loads from the jack trusses
2. Jack trusses — shortened trusses running perpendicular to the main span, connecting to the girder
3. Hip boards — timber members running from the ridge to the corners of the building

The girder truss must be specifically designed for the combined loading. It is not acceptable to double up standard fink trusses as girder trusses without a calculation.

Bracing to BS 8103-3

BS 8103-3:2009 specifies the minimum bracing requirements for trussed rafter roofs in domestic buildings. Three types of bracing are required:

1. Longitudinal binders (ridge line and mid-rafter)

• Run along the length of the roof parallel to the ridge
• Typically 97×22mm or 100×25mm CLS softwood
• Fixed to the top chord of every truss with a single 3.35mm nail minimum
• Minimum two binders per roof slope for spans under 8m; more for larger spans

2. Rafter bracing (diagonal)

• 97×22mm or 100×25mm boards fixed diagonally across the rafter braces
• Minimum 45° angle; max 60° from horizontal
• Must start and finish at a gable wall or cross-wall
• Placed in groups of braces at quarter points of the roof length

3. Binder bracing (bottom chord)

• Bottom chord longitudinal binders resist buckling of the bottom chord in compression
• Required where bottom chord length between internal web joints exceeds 3m

All bracing must be fixed during erection — it is temporary bracing until the roof covering provides diaphragm action. A roof left unbraced overnight or over a weekend in wind can collapse, and this has caused fatalities on site.

The Absolute Prohibition on Unauthorised Modification

This cannot be overstated. Modifying a roof truss without the written approval of the original truss designer or a qualified structural engineer is:

• A breach of Building Regulations (Approved Document A)
• A breach of NHBC Buildmark Chapter 7.2 (which voids the warranty)
• A breach of CDM 2015 (the designer is responsible for the structure in use)
• Potentially criminal under the Building Act 1984 if it causes collapse

Common illegal modifications seen on site:

• Cutting out a web member to create access to the attic (water tank, aerial cable)
• Notching the bottom chord for a ceiling joist or purlin
• Drilling through the top chord for cables or pipes
• Adding point loads (water tanks, mechanical plant) without design checks
• Cutting back truss tails to accommodate a fascia detail not on the original drawing

If a modification is needed — for example, inserting a loft hatch or routing services — the truss designer must be contacted. Most manufacturers provide a free modification check service for straightforward requests.

Spacing, Delivery and Crane Considerations

Standard truss spacing is 600mm centres for most domestic roofs, or 400mm centres where loads are higher or span tables require it. Trusses are pre-cut to the ordered span and pitch; they cannot be trimmed on site (the plates are already fixed).

For spans up to approximately 7m, trusses can be manhandled into position by two or three operatives using a temporary ridge board as a guide. For spans over 7–8m, the size and weight make mechanical assistance practical. Many deliveries of trusses over 8m are accompanied by a crane booked by the manufacturer.

Trusses must be stored on site correctly: ideally vertical in a stillage or rack, or horizontal with support at intervals of no more than 1.5m to prevent deflection. Never stand a truss on its apex or store it for extended periods without proper support.

Frequently Asked Questions

Can I cut a truss to fit around a chimney stack?

No — not without a structural engineer's written approval. The truss must be designed from scratch to incorporate the opening, or the chimney must be designed to transfer loads independently. Inform the truss designer of any chimney openings before ordering.

My client wants a loft hatch — can I just cut through the bottom chord?

No. A loft hatch in a trussed rafter roof requires a trimmer beam arrangement approved by the structural engineer. The bottom chord must be kept intact between its support points. The hatch opening must be framed using additional trimmer members designed to transfer the cut chord loads.

What happens if I drill through a connector plate?

This weakens the connection significantly — connector plate teeth rely on an uninterrupted grid of embedments. Even a single hole can reduce the joint capacity by more than the calculated margin of safety. The truss designer must be consulted; in most cases the plate would need to be replaced or supplemented with an alternative fixing.

Are old BS 5268 trusses less safe than new Eurocode 5 designs?

No — BS 5268 was a well-established permissible stress design code and trusses designed to it are structurally sound. The change to Eurocode 5 (EC5) introduced a different calculation method (limit state) but the resulting physical dimensions of typical domestic trusses are similar.

How long can I leave a roof without bracing?

Temporary bracing must be installed as each truss is erected — do not leave a partially braced roof unattended in windy conditions. The Health and Safety Executive (HSE) and truss manufacturers' guidance requires that no section of the roof should ever be left with fewer than five trusses braced together as a stable group.

Regulations & Standards

• BS EN 1995-1-1:2004+A2:2014 (Eurocode 5) — design of timber structures; replaces BS 5268

• BS 8103-3:2009 — structural recommendations for lightweight framing; includes truss bracing requirements

• BS EN 14250:2010 — product requirements for prefabricated roof trusses using connector plates

• Approved Document A: Structure (2004, amended 2013) — Building Regulations structural requirements

• NHBC Standards Chapter 7.2 — roofing; truss erection and modification requirements

• CDM Regulations 2015 — designer duty to ensure structures in use are safe

• MiTek UK: Truss Design Guidance — leading truss manufacturer; span tables and design resources

• TRADA: Roof Truss Guidance — Timber Research and Development Association technical guidance

• NHBC Standards — Chapter 7.2 roofing requirements

• HSE: Roof Truss Safety — temporary works and erection safety

• Planning Portal: Building Regs — Approved Documents and Building Control guidance

• timber spans — span tables for floor joists and rafters

• engineered timber — LVL, glulam and I-joist specification

• part a structure — when structural engineers are required

• nhbc warranty — NHBC Chapter requirements and warranty implications