What is the minimum head height for a loft conversion?

UK Building Regulations (Approved Document K) require a minimum head height of 2.1 metres over at least 50% of the usable floor area for habitable rooms. Areas with less than 1.5m head height are generally considered unusable and are often used for storage (fitted wardrobes, eaves cupboards). The 2.1m minimum is measured from the finished floor surface to the underside of the roof structure or ceiling.

How do I measure ridge height for this calculator?

Measure from the top surface of the ceiling joists (which will become your loft floor) vertically up to the underside of the ridge board. Do not measure from the existing ceiling below — you need the dimension from what will be the finished floor level in the loft. Allow about 50-75mm less if you are adding a floor finish on top of the joists.

What roof pitch is best for a loft conversion?

Roof pitches of 35 degrees or more generally provide enough head height for a loft conversion without needing to raise the ridge or add dormers. Pitches of 30-35 degrees are borderline and may need dormers to gain usable space. Below 30 degrees, a standard conversion is difficult and a dormer or mansard conversion is usually the only option.

What is a purlin and does it affect my conversion?

A purlin is a horizontal timber beam that runs along the length of the roof, supporting the rafters at mid-span. In a traditional cut roof, purlins sit about halfway up the rafter and can restrict head height. During a loft conversion, purlins often need to be replaced with structural steel beams (typically a universal beam or RSC channel) to free up the headroom.

Can I raise the ridge to gain more head height?

Raising the ridge is technically possible but is a major structural alteration that almost always requires planning permission (it is not covered by Permitted Development rights). It also changes the external appearance of the building significantly. Dormers, mansard conversions, or hip-to-gable conversions are usually more practical ways to gain head height and floor space.

Loft Head Height Calculator — Is My Loft Tall Enough to Convert?

Check whether your loft has sufficient head height for a conversion. Calculates usable floor area above 2.1m per UK Building Regulations Part K and identifies feasibility.

Ridge Height

From top of floor joists to underside of ridge board

Eaves Height

From top of floor joists to wallplate level

Roof Pitch Angle

Angle of roof slope from horizontal

Loft Width (optional)

Internal width eave to eave (auto-calculated if blank)

Purlin Position from Centre (optional)

Horizontal distance from ridge centre to purlin — leave 0 if unknown

How We Calculate This

This calculator uses trigonometry to determine the head height at any point across the loft width, based on the roof pitch angle, ridge height, and eaves height.

The formula

Height at distance x from centre = Ridge height - (x × tan(pitch angle))

Where x is the horizontal distance from the centre ridge to any point across the loft floor.

Usable width calculations

Distance for 2.1m height: (Ridge height - 2.1) ÷ tan(pitch)
Distance for 1.5m height: (Ridge height - 1.5) ÷ tan(pitch)
Usable width: 2 × distance from centre (both sides of ridge)

Part K feasibility check

For a habitable room, UK Building Regulations Approved Document K requires a minimum head height of 2.1m over at least 50% of the floor area. The calculator checks this by comparing the width above 2.1m to the total available floor width. If more than 50% of the width has 2.1m+ clearance, the conversion is likely feasible (subject to structural checks).

Space zones

Above 2.1m: Habitable space — can be counted as living area
1.5m to 2.1m: Useful for furniture, desks, beds, and circulation
Below 1.5m: Eaves storage only (wardrobes, cupboards)

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Loft Head Height Calculator — Is My Loft Tall Enough to Convert?

How We Calculate This

The formula

Usable width calculations

Part K feasibility check

Space zones

Frequently Asked Questions

Related Calculators

Loft Boarding Calculator

Loft Ladder Sizing Calculator

Loft Load Capacity Calculator

Loft Ventilation Calculator

Dormer Sizing Calculator