Cut Roofs Explained

Cut Roofs Explained: A Complete Guide to Traditional Site-Built Timber Pitched Roofs

Cut roofs are assembled on site from individual timber members to suit the building geometry and structural design. This illustrated guide explains structural forms, load paths, member roles, setting out, trimming, insulation, ventilation, erection and inspection for low-rise housing in England.

Key Topics Covered

• Core Principles & Structural Forms: Defines traditional site-assembled timber roof framing. Compares structural configurations including simple couples, close-couple roofs with low-level ceiling ties, raised collars, purlin-and-strut supported systems, and complex hipped or valley intersections.

• Gravity Loads & Horizontal Thrust Mechanics: Explores gravity load delivery from tiles and battens down through rafters, purlins, struts, and internal loadbearing walls. Details horizontal thrust resistance mechanics using continuous ceiling joists, tie beams, binders, and diagonal stability bracing.

• Anatomy of Primary & Secondary Members: Structural breakdowns covering rafters, ceiling ties, purlins, struts, non-beam ridge boards, binders, collars, wall plates, and galvanized restraint straps. Details trimming rules for openings around dormers, chimneys, and rooflights.

• Setting Out, Insulation & Moisture Strategy: Step-by-step erection sequences from wall-plate level checks to roof covering installation. Evaluates cold-roof (ceiling-level) and warm-roof (rafter-level) insulation layouts, airtightness, vapour-control layers, and eaves-to-ridge ventilation paths.

• Compliance & Defect Prevention: Aligned with Building Regulations Approved Documents A (Structure), B (Fire Safety), C (Site Preparation & Moisture), L (Conservation of Fuel & Power), Regulation 7, and CDM/Work at Height safety rules. Features an illustrated troubleshooting matrix for critical site defects (e.g., undersized rafters, struts bearing on non-loadbearing ceiling joists, interrupted ceiling ties, deep uncalculated rafter notches, blocked eaves ventilation), a 5-stage inspection checklist with pre-concealment hold points, a carpentry/structural glossary, and FAQs.

To compare factory-manufactured construction, see our Trussed Roofs Explained guide. The government’s Approved Document A provides technical guidance on structural loading and roof construction.