Pitched Roof Structure: Couple, Close Couple and Purlin Roof Types, Rafter Sizing Tables, Birdsmouth Joint and Ridge Board

Summary

The traditional cut and pitched roof has been the dominant UK domestic roof form for centuries. Despite the growth of prefabricated trussed rafter roofs since the 1960s, cut roofs remain common for extensions, bespoke properties, loft conversions, and any situation where the roof space must be habitable or where prefabrication is impractical. For tradespeople, understanding cut roof geometry and structural principles is essential for pricing, defect diagnosis, and complying with Building Regulations Part A.

The three main cut roof types represent progressively sophisticated solutions to the problem of rafter thrust — the outward force that rafters exert on the supporting walls as they try to push outwards under load. The couple roof does not address this; the close couple roof uses a ceiling tie to triangulate the structure; the purlin roof introduces intermediate support to allow longer spans or smaller rafter sections.

Building Regulations Part A requires structural stability, and Part A2 specifically addresses loading on roofs from dead load (self-weight, tiles, insulation) and imposed load (snow, maintenance access). Rafter sizing must be justified by reference to published span tables or structural engineering calculations. For straightforward domestic roofs within the scope of standard span tables, building control will accept table-based selection without an engineer's report.

Key Facts

• Couple roof — simplest form; rafters meet at ridge only, no ceiling tie; limited to very short spans (under 3.5m) as rafter thrust is not resisted; rarely used today
• Close couple roof — ceiling tie (joist) at wall plate level triangulates the roof; eliminates outward thrust; typical for domestic roofs up to approximately 5.5m span
• Collar roof — ceiling tie fitted one-third to halfway up the rafter span; raises the ceiling height but reduces structural efficiency; span limit approximately 4.5m
• Purlin roof — purlins (horizontal beams) provide mid-span support for rafters, reducing effective rafter span; allows larger spans and/or smaller rafter sections
• Ridge board — non-structural member at apex; receives rafter ends; minimum depth half the rafter depth + 12mm; typically 32mm × 200mm or similar
• Wall plate — 75mm × 100mm C16 timber bedded in mortar on top of masonry wall; anchored to wall with restraint straps at maximum 2m centres
• Birdsmouth joint — notch cut in rafter to seat on wall plate; maximum depth one-third of rafter depth; typically 40–50mm for standard rafters
• Rafter minimum size — C16 graded timber minimum; C24 for higher loads or longer spans
• Common rafter spacing — 400mm or 600mm centres
• Dead load assumption — concrete tiles: 50–80 kg/m² (0.5–0.8 kN/m²); slates: 25–50 kg/m² depending on thickness
• Snow load (UK) — 0.6 kN/m² for most of England; higher for Scotland and exposed upland locations (check BS EN 1991-1-3)
• Purlin size — determined by purlin span and load; typically 75mm × 150mm to 75mm × 225mm C16/C24; must bear at each end on a structural wall or on a binder beam
• Hanger — vertical member from purlin to ceiling joist; typically 50mm × 100mm; prevents ceiling joist sag; spaced maximum 1.8m
• Hip rafter — diagonal rafter from corner of wall plate to ridge end; size typically one size up from common rafter; 50mm × 200mm is common

Quick Reference Table

These are approximate values; always verify against TRADA span tables or BS 5268-7.5 for the specific load case.

Detailed Guidance

Close Couple Roof Construction

The close couple is the standard cut roof for domestic extensions and small new-build properties. Construction sequence:

Step 1: Wall plates Bed 75mm × 100mm timber wall plates in mortar along the top of both bearing walls. The plates must be continuous (scarfed joints at wall ends), level, and at the correct separation for the design roof span. Fix with galvanised mild steel restraint straps at maximum 2m centres, hooked over the plate and fixed to blockwork.

Step 2: Ceiling joists (collar ties) Fix ceiling joists across the wall plates at rafter centres (or 400/600mm centres) before the rafters go up — the ceiling joists triangulate the structure by acting in tension. Ceiling joists are typically 47mm × 100mm to 47mm × 150mm depending on span. They must be notched or bolted at the wall plate and at the ridge end (if a trimmer is used). Ceiling joist size is determined by the ceiling load (typically 0.5 kN/m²) and span.

Step 3: Ridge board Erect a temporary ridge prop at the correct height and position. The ridge height is calculated from the wall plate separation and the roof pitch angle:

Ridge height above wall plate = (half span) × tan(pitch angle)

For a 4m span at 35°: ridge height = 2m × tan(35°) = 2 × 0.7 = 1.4m above wall plate.

The ridge board is positioned at this height and temporarily supported while rafters are fixed.

Step 4: Common rafters Cut rafters to the calculated rafter length. Rafter length (along slope) = half span / cos(pitch angle).

For 35°: rafter length = 2m / cos(35°) = 2m / 0.819 = 2.44m (plus any overhang)

Birdsmouth joint: cut a notch at the lower end of each rafter to seat on the wall plate. The horizontal (seat) cut is typically 40–50mm (must not exceed one-third of rafter depth). The vertical (plumb) cut is perpendicular to the rafter slope. The seat must bear fully on the wall plate with no gap.

Fix rafter at ridge: skew-nail or use framing anchors (joist hangers, rafter clips). At wall plate: use galvanised skew nails 2 × 75mm minimum.

Step 5: Collar ties Where the ceiling ties are below the rafters (forming a collar roof), they are fitted at one-third to one-half of the rafter height. They are still in tension and must be adequately nailed/bolted to both rafters.

Purlin Roof Construction

For spans above approximately 5.5m (close couple limit) or where smaller rafter sections are preferred for economy, introduce a purlin to provide intermediate support.

Purlin design: The purlin receives rafter loads from above via the rafters bearing on it. The purlin itself spans between supporting points — either gable walls, load-bearing partitions, or binder beams. Purlin design (size and species) is determined by:

• Purlin span (distance between bearings)
• Spacing of rafters bearing onto it (load per metre of purlin length)
• Species and grade (C16, C24)

A rough working rule: for common timber purlin in a domestic roof, span (m) should not exceed 40 × depth (mm) for C16 timber. A 75mm × 175mm C16 purlin should not span more than 40 × 175/1000 = 7m — but this is a rough guide only; use TRADA tables for actual design.

Strut: a strut transfers load from the purlin down to a load-bearing element below. Struts must:

• Be at 45° or less from vertical (steeply angled struts are ineffective)
• Bear onto a wall, padstone on a purlin, or properly designed binder — never onto ceiling joists or rafters alone
• Be minimum 75mm × 100mm section
• Be notched at top to sit on the purlin; fixed with skew nails

Hanger: prevents the ceiling joist from sagging under ceiling load. Fitted vertically from the underside of the purlin to the top of the ceiling joist at maximum 1.8m intervals.

Hip Roofs

Hip roofs have no gable ends — all four sides slope to the ridge (or to a central point on a pyramid hip). Additional components:

Hip rafter: runs from the corner of the wall plate at 45° to the end of the ridge. It supports jack rafters (short rafters that run from the wall plate to the hip rafter) on two sides.

Hip rafter sizing: the hip rafter is loaded by jack rafters from both sides, so it carries more load than a common rafter. Size typically one step up from the common rafter (e.g., 50mm × 200mm where commons are 50mm × 150mm).

Jack rafters: same depth as commons; length varies from zero at the corner to full rafter length near the ridge end. Each jack rafter has a birdsmouth at the wall plate and a compound plumb-and-skew cut at the hip rafter.

Valley rafter: at the junction between two roof slopes meeting at an internal angle (e.g., a T-plan house). Valley rafters run from the wall plate corner up to the ridge at 45°. Like hip rafters, they are loaded from both sides.

Checking Existing Roof Structure

When assessing an existing pitched roof (for loft conversion, repair, or extension abutment):

1. Spread: measure the span at wall plate level and compare to the designed span. Significant spread (wall plates more than 20mm further apart than the original design) indicates the ceiling ties have failed or the structure is inadequate
2. Deflection: sight along ridges and purlins. Significant sag (more than span/200 = 15mm for a 3m purlin) indicates overload or member deterioration
3. Rot: probe all wall plates and any timber in contact with masonry. Wet rot is common at the birdsmouth if the fascia and soffit have allowed water ingress
4. Woodworm: check for flight holes in older roofs; most historic woodworm is inactive but structural weakening in heavily affected members is possible

Frequently Asked Questions

Can I remove a purlin or strut to create more loft space?

Only with a structural engineer's design for alternative support. Purlins and struts are structural — removing them transfers load to the rafters, which are almost certainly undersized for the increased span. This is one of the most common causes of roof collapse during DIY loft conversions. Building Regulations Part A requires formal notification and structural design for any loft conversion.

How do I calculate the ridge height for any pitch?

Ridge height above wall plate = (half rafter span) × tan(roof pitch in degrees)

Quick reference:

• 22.5° (shallow): multiplier 0.41
• 30°: multiplier 0.58
• 35°: multiplier 0.70
• 40°: multiplier 0.84
• 45°: multiplier 1.00
• 50°: multiplier 1.19

Does a timber roof need fireproofing treatment?

Structural timber in a roof does not normally require fire treatment unless it is within 600mm of a party wall or separating wall, where fire-retardant treatment may be required by Building Regulations Part B. Roof timbers in contact with a chimneystack must maintain a 40mm minimum gap under Part J. Always check Part B requirements for loft conversions — fire-protecting the loft floor is usually required.

Regulations & Standards

• Building Regulations Part A — structural stability; rafter sizing and connection requirements

• Building Regulations Part B — fire safety; separating walls and roof timbers

• BS 5268-7.5:1990 — structural use of timber; span tables for traditional pitched roofs (note: superseded in full by Eurocode 5, BS EN 1995, but tables still widely referenced)

• BS EN 1995-1-1 (Eurocode 5) — design of timber structures; timber connection and member design

• TRADA Technology — span tables for solid timber members in floors, ceilings and roofs; widely used for building control compliance

• BS EN 1991-1-3 — Eurocode 1; snow loads on buildings

• TRADA Span Tables — Span Tables for Solid Timber Members in Floors, Ceilings and Roofs (for dwellings)

• Approved Document A — Building Regulations structural requirements

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

• NHBC Standards Chapter 7.2 — pitched roofs; materials and workmanship

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