Roof Pitch & Rafter Calculator UK

Roofing in Practice: What the Calculator Doesn't Tell You

Trussed Rafters vs a Cut Roof

The vast majority of new-build housing in the UK uses factory-manufactured trussed rafters (gang-nailed trusses). And for good reason: they're typically 30–50% cheaper than a traditional cut roof, faster to install (a gang of two can roof a standard semi in a day once the trusses are on site), and remove the need for a skilled carpenter to cut every member individually. The engineering is done off-site to BS 5268 or Eurocode 5, the truss manufacturer provides the calculations, and the design is fixed. Order them early — typical lead time from a specialist truss manufacturer is 2–4 weeks, and this is one of those items that causes serious programme problems if left until the last minute.

A cut roof — where a carpenter works from a rod (a full-size setting-out drawing) and cuts every rafter, jack rafter, hip rafter, and purlin on site — is the right choice when you need flexibility: a loft conversion, a complex extension roof that has to work around existing structure, a listed building where you're replicating original carpentry, or a design with a particularly complex geometry. Cut roofs give you usable loft space without the web members that fill a trussed rafter void. They cost more in labour but they can solve problems that trusses cannot.

Pitch, Planning, and Local Character

Roof pitch is one of the most scrutinised elements of a planning application, particularly in areas with a strong local character. In many conservation areas, areas of outstanding natural beauty, and national parks, planners will specify minimum pitch, tile type, and sometimes ridge height to ensure new construction is subordinate to and consistent with the existing streetscape. A flat or very low-pitched extension on a terrace of traditional 45° pitched roofs will generate objections; a pitch and tile that matches the surroundings is significantly more likely to sail through.

For permitted development extensions, the ridge height of the new roof must not exceed the ridge height of the original roof (or the highest part of the original roof for rear extensions). Get this measured accurately on site before the roof structure goes up — it's a detail that's awkward and expensive to correct after the fact. If you're on the boundary of compliance, a pre-application enquiry with the local planning authority is a much better investment than a retrospective application.

Eaves, Details, and NHBC Requirements

The eaves overhang does two jobs: it throws rainwater clear of the wall face, and it provides a housing for the fascia, soffit, and guttering. A 300mm overhang is a sensible minimum for most domestic construction. Less than this and you're relying on the guttering to catch water that the overhang should be throwing clear; more can look out of proportion unless the design specifically calls for it. The eaves detail also has to accommodate ventilation: NHBC and Building Regulations require a minimum 25mm clear ventilation gap at the eaves for cold-roof construction (50mm for a warm roof), and this has to be maintained by the installation of ventilation tiles or fascia ventilators.

For extensions that abut a neighbour's property, party wall and flashing details require careful thought. The junction between the new roof and the neighbour's wall needs to be watertight and allow for differential movement — a lead soaker and cover flashing, or a step and cover flashing arrangement, properly dressed and secured. Get a competent roofer to detail this rather than relying on a generic specification: a failed flashing is one of the most common sources of water ingress and neighbour disputes in domestic construction.

How Much Does a New Roof Cost in the UK? (2025)

For a complete re-roof on an existing house (strip off old tiles, replace felt, battens, and tiles on the existing roof structure), costs in 2025 typically run £70–130 per m² of actual roof area. A standard 3-bed semi with a roof area of approximately 80 m² would typically cost £6,000–11,000 for a complete re-roof with concrete interlocking tiles. Clay plain tiles add 25–40% to the total (higher material cost, slower to lay). Natural slate runs £100–160 per m² installed. London and South East prices are typically 25–35% above the Midlands baseline.

For a new-build extension roof — trussed rafters, felt, battens, tiles, UPVC fascia, soffit, and guttering fully installed — expect £90–160 per m² of floor area as a working guide, depending on pitch and tile specification. A full re-roof on a 3-bed detached (roof area ~120 m²) runs approximately £9,000–17,000. Fascia and soffit replacement alone (UPVC, no structural work) typically costs £800–2,500 for a semi-detached. Always confirm whether the price includes scaffold — on any two-storey work, scaffold is essential and represents £600–1,500 of the total. Searches for "how much does a new roof cost UK 2025" and "re-roof cost per m² UK" are among the highest-volume queries in the roofing category, reflecting the scale of demand for reliable pricing guidance.