In roofing, a soffit (or soffit board) is the horizontal board fixed to the underside of the rafter feet at the eaves, spanning from the fascia board at the outer edge to the wall face. It closes the gap below the roof overhang, protects the rafter ends and the top of the cavity wall from weather, and provides the surface through which eaves ventilation (continuous slot or circular vents) passes. In a wider sense, 'soffit' refers to any exposed horizontal underside of a structural element - the underside of a staircase, a lintel, or a concrete beam.

What is the difference between a soffit and a fascia?

The fascia is the vertical board fixed to the cut ends of the rafter feet at the eaves - gutters are fixed to it. The soffit is the horizontal board below the fascia, closing the underside of the roof overhang. Together, fascia and soffit form the eaves trim: the fascia faces outward (vertical), the soffit faces down (horizontal). On a house with no overhang (a flush verge), there is no soffit - the fascia fixes directly to the wall. On most traditional houses with a standard overhang, both boards are present.

What materials are soffit boards made from?

Traditional soffit boards were solid timber (softwood, painted). Modern replacements are almost universally UPVC (unplasticised polyvinyl chloride) - white or coloured, in solid or vented profiles. UPVC soffits are maintenance-free, rot-proof, and significantly longer-lasting than painted timber. Fibre cement soffit boards are an alternative offering greater rigidity and a more traditional appearance. Aluminium soffit panels are used on commercial and higher-specification residential projects. Where a soffit is to be rendered or plastered (in a concrete eaves soffit, for example), the soffit is the underside of the concrete structure itself.

Does a soffit need ventilation?

Yes - in cold roof constructions (where insulation is at ceiling level and the roof space above is uninsulated), the roof void must be ventilated to prevent condensation forming on the cold roof structure. Ventilation is typically provided at the eaves via a continuous 25mm ventilation slot in the soffit (or equivalent area of circular vents) on both sides of the roof. High-level ventilation at the ridge is also required. UPVC soffits are available with an integral ventilation slot, or with pre-drilled vent holes. Blocking eaves ventilation is a common cause of condensation problems and roof timber decay in loft spaces.

Roofing noun

Soffit

/ ˈsɒfɪt /

Also known as: soffit board, eaves soffit, roof underside

Definition

A soffit is the horizontal board fixed to the underside of the rafter feet at the eaves, spanning between the fascia board at the outer edge and the wall face, closing the underside of the roof overhang. It protects the rafter ends and cavity wall top from weather and provides the surface through which eaves ventilation is admitted to the roof void. More broadly, 'soffit' refers to any exposed horizontal underside of a structural element - a lintel, beam, staircase, or bridge deck.

In practice

Modern soffit boards are almost universally UPVC - available in standard widths (150mm, 225mm, 300mm, 400mm) to suit different overhang depths, and in solid or vented profiles. A continuous 25mm ventilation slot (or equivalent area of individual vents) must be maintained across the full length of the soffit to ventilate the roof void in cold roof constructions. UPVC soffit boards clip into a starter channel on the wall face and are fixed to the underside of the rafter feet with stainless steel or UPVC-compatible fixings.

When replacing existing timber soffits with UPVC, the rafter feet must be in sound condition - rotten ends should be cut back and spliced with treated timber before the new soffit is fitted. Insulation pushed into the eaves to meet Part L thermal requirements must not block the ventilation gap above it - proprietary eaves ventilation trays (laid between the rafters above the insulation) maintain the ventilation path. On deep overhangs, the soffit may be divided into two panels with a continuous vent strip between them.

Building Regulations

Ventilation (Part F)Continuous 25mm slot or equivalent at eaves - cold roof

Warm roofNo eaves ventilation required - insulation at rafter level

Fire (Part B)Soffit materials must meet minimum Class C-s3,d2 on relevant buildings

Approved Document F requires cold roof voids to be ventilated to prevent condensation. The minimum ventilation at eaves is equivalent to a continuous 25mm gap for roofs with pitches above 15 degrees. Approved Document L requires insulation at eaves level to achieve continuity with wall insulation without blocking eaves ventilation - eaves vent trays are the standard solution. Soffit replacement is permitted development in most cases, but changing the appearance of a listed building or property in a conservation area may require consent.

Full Building Regulations guidance

Soffit

Just so you know...