How to fit soffit boards and vents

What does a soffit board do?

A soffit closes the underside of the roof overhang. It runs from the back of the fascia to the wall face, keeps birds, wasps and wind-driven rain out of the eaves, and carries the ventilation that keeps the loft dry.

Hollow soffit is what most houses get. It is a ribbed board, 300mm wide and 10mm thick, in 5m lengths. Flat general purpose board is the alternative, stocked from 100mm to 405mm wide, plain or factory-vented. Use hollow soffit on a wide overhang and general purpose board where the run is narrow or awkward.

How much ventilation does a soffit need?

A pitched roof with insulation at ceiling level needs a 10mm continuous gap at each eaves. Roofs under 15 degrees, and roofs where the insulation follows the slope, need 25mm.

Those figures are free air area, not hole size. A 10mm continuous gap equals 10,000 square millimetres for every metre of eaves. That arithmetic matters when you start counting circular vents, because a handful of holes rarely gets close.

Why does a blocked loft rot?

Warm damp air leaks out of the house into the loft and condenses on the cold underside of the roofing felt. With no airflow it has nowhere to go.

It drips onto the insulation and soaks the ceiling joists. Timber held above 20 per cent moisture content starts to rot, and mould follows. Nobody sees it until a ceiling stains or a rafter foot goes soft. The usual cause is loft insulation stuffed hard into the eaves. Rafter trays hold a 50mm air path open above it. Fit them while the soffit is down, because that is the only easy access you will get.

Do I need vented board or circular vents?

Vented soffit board is the dependable answer. The slots run the length of the board, so airflow is continuous and the spacing cannot be got wrong.

Circular vents suit topping up an existing run or a short section. Drill with a 70mm hole saw and push them in. Free air area is typically quoted between 3,000 and 4,500 square millimetres each, so you need two or three per metre to match a 10mm continuous gap. Check the figure the maker prints on the pack rather than guessing. If it is not stated, ring us before you order.

Soffit ventilation, sizing and fixing figures

Item	Figure	Note
Pitched roof, insulation at ceiling level	10mm continuous each eaves	Equals 10,000 sq mm per metre run
Roof under 15 degrees, or insulation following the slope	25mm continuous each eaves	Equals 25,000 sq mm per metre run
Clear air path over the insulation at the eaves	50mm minimum	Rafter trays hold it open
Circular soffit vent free air area	Typically 3,000 to 4,500 sq mm	Check the maker’s stated figure
Circular vents to match a 10mm continuous gap	2 to 3 per metre run	70mm hole saw, push fit
Hollow soffit board	300mm x 10mm x 5m	Joiner and starting sections also 5m
General purpose board widths	100 to 405mm, 9mm thick	Plain or pre-vented
Nail centres into timber	400mm maximum	50mm in from each board end
Expansion allowance	10mm per 5m length	uPVC moves in summer heat
Wastage to order	5 to 10 per cent	Take the higher figure on a cut-up roof

How do I cut hollow soffit to width?

Rip it lengthwise with a fine-tooth blade in a circular saw, running against a straight edge. A jigsaw works but wanders over a 5m board.

Measure from the wall to the back of the fascia at both ends of every run. Old houses are never square, and a run can lose 15mm over 5m. Cut 6mm short at each end for expansion. The starting section and the fascia both cover that gap, so a slightly shy cut is safer than a tight one.

How do I join and fix soffit boards?

A joiner section takes two board ends and hides the joint. It is 45mm wide in 5m lengths, so one length does several joints. A starting section fixes to the wall and holds the inner edge without nailing through the board face.

Fix with stainless steel colour-head pins. Use 40mm into joiner and starting sections, 50mm where you are going into rafter feet. Galvanised nails bleed rust down white board inside a winter, so do not use them. Nail at 400mm centres and 50mm in from each end. Leave the head just proud so the board can move.

Can I fit soffit off a ladder?

Not safely above single storey. You are at the roof edge, both hands on a 5m board, with nothing to hold on to. That is where roofline injuries happen.

Hire a tower or a scaffold for anything higher, and work with a second person on full lengths. On a bungalow with firm level ground a pair of ladders and a stage is workable if you are used to it. If the eaves sit over a conservatory, a bay or sloping ground, get a scaffold. Every house differs, so ring us and describe the job.