Hip-to-Gable Calculator — Steelwork, Brickwork & Tiles

A hip-to-gable conversion extends the sloping hip end of a roof outward to create a vertical gable wall, providing additional headroom and floor space in the loft. The hip rafters are removed, a new gable wall is built, and a structural ridge beam supports the extended roof. It is typically combined with a rear dormer. See our Dormer Sizing Calculator for the dormer element and Loft Conversion Cost Calculator for overall budget.

Hip-to-gable can be Permitted Development under GPDO Class B, but the added roof-space volume must stay within the cumulative limit: 50m³ for a detached or semi-detached house and 40m³ for a terraced house. Any previous loft/dormer additions count towards this allowance. On a semi-detached house it is normally PD on the non-party-wall side only. The work must not extend beyond the plane of the existing roof slope on the principal (highway-facing) elevation and must not exceed the highest part of the existing roof; materials must match. Hip-to-gable is exempt from the 20cm eaves set-back that applies to other roof enlargements. PD rights are removed in conservation areas, AONBs, national parks and similar designated land — check with your local planning authority.

A structural ridge beam is the primary steel, spanning from the existing party wall (or internal support) to the new gable wall. This replaces the removed hip rafters and supports all the new and existing roof loads. A steel beam in the gable wall may also be needed. All steelwork must be designed by a structural engineer.

A half-brick single-skin gable wall needs about 60 bricks per m² with standard 10mm joints (metric brick 215×102.5×65mm). The gable is typically triangular, so the area is (Width × Height) / 2. This calculator adds a 10% wastage allowance — the upper end of the usual 5–10% UK convention — because a raking triangular gable produces many cut bricks at the verge. Matching existing bricks can be challenging, so a mix of reclaimed and new bricks, or rendering / tile-hanging, may be more practical.

When the hip is removed, the existing hip tiles are no longer needed; new roof tiles are required for any extended roof slope. If you tile-hang (vertically clad) the gable wall with plain tiles, allow approximately 53 tiles per m² — BS 5534 vertical tiling uses a maximum 115mm batten gauge with a 35mm minimum head-lap, which works out near that figure. The much lower ~10 tiles per m² only applies to large-format interlocking tiles, which are a roof covering rather than a typical tile-hung wall finish. Matching existing roof tiles may mean sourcing from reclamation yards if the pattern is discontinued.

The calculator works out the added roof volume as a triangular gable cross-section tapering to a point at the old hip apex over the ridge-extension distance: Volume = (Gable Width × Gable Height / 2) × (Ridge Extension / 3) × Number of Sides. For example, a 6m wide, 3m high gable with a 4m ridge extension on one side of a semi-detached house gives a gable wall area of (6 × 3) / 2 = 9m², so the volume added is 9 × (4 / 3) = 12m³. If Ridge Extension is left at 0 the calculator defaults it to half the Gable Width, a rough approximation for a typical 45-degree hip; enter the actual ridge-beam extension from your drawings for an accurate figure. Check the total against the Permitted Development limit on the Loft PD Volume Calculator before assuming no planning application is needed.

As an indicative 2026 UK all-in build rate, a hip-to-gable conversion (typically built with a rear dormer) costs approximately £1,400 to £2,100 per m² of new floor area, staircase included (MyJobQuote/houseUP, June 2026). This calculator estimates the gable wall brickwork, tiles and rafters only, not the whole-project cost. Use the Loft Conversion Cost Calculator for the full build estimate, and add roughly £5,000 to £12,000 for the bathroom fit-out if one is included.

No. The calculator flags that a structural ridge beam is required but does not size it. Beam size depends on the span, roof pitch, tile weight, snow load and how the load transfers into the party wall or existing structure, so it must be designed by a structural engineer. A hip-to-gable conversion is structural work: alongside any Permitted Development check for planning, it needs Building Regulations approval (Part A for the structure, alongside Part B fire safety and the other parts a loft conversion engages) and a structural engineer’s calculations before work starts, whether or not planning permission is required. See the RSJ Installation Calculator for the installation sequence once your engineer has specified the beam.