Roofing Battens & Felt: Spec & Cost

Battens and felt (or modern breathable membrane) sit between your tiles and the structural roof deck, performing two quite different jobs: the felt sheds any water that gets past the tiles, while the battens hold the tiles at exactly the right gauge to produce a weathertight overlap. Get either element wrong and you can end up with a roof that looks fine but leaks, or tiles that lift in high winds.

This guide sets out the current UK specifications, material costs, and practical choices for both battens and underlay, whether you’re quoting a full re-roof, adding a dormer, or just replacing a section of slipped tiles.

Roofing Batten Sizes and Specification

UK roofing battens should comply with BS 5534:2014+A2:2018, the code of practice for slating and tiling pitched roofs.¹ It is not a legal requirement, but following it aids compliance with the Building Regulations.² It sets out requirements for battens, including their dimensions, quality, preservative treatment and marking.³

Standard Batten Sizes

The 25 × 50 mm batten is by far the most common for modern interlocking tiles, because it is the minimum for single-lap tiles once rafter centres exceed 450 mm.⁴ The 25 mm thickness is a minimum: the standard’s tolerances allow a batten to be up to 3 mm over its nominal size, not under it.⁴ Battens should be at least 1.2 m long and supported by at least three rafters, and battens for larger spans need to be designed by calculation.⁴

Treatment and Durability

Tile battens should be pre-treated with preservative to Use Class 2 (UC2), using a pressure-impregnation process, for a desired service life of 60 years.⁵ Colour is not proof of grading or treatment: check that each batten carries the BS 5534 marking, and ask the supplier for a treatment certificate.⁴ Do not use untreated or simply painted timber as a roofing batten: non-compliant battens should be rejected from site.⁵

For exposed coastal areas, or where ventilation is restricted, some specifiers choose UC3 treatment. The cost difference between UC2 and UC3 treated battens is usually small, so ask your merchant to price both.

Roofing Underlay: Felt vs Breathable Membrane

This is where the biggest specification decision lies. The two broad categories are:

1. Type 1F: Reinforced Bitumen Felt (“Traditional Felt”)

The traditional underlay for decades. Black, heavy, and essentially impermeable to vapour. Still widely used in repairs where the existing underlay is this type, but no longer considered best practice on new installations.

• Weight: about 1.5 kg/m² (a typical 15 m × 1 m roll weighs around 23.5 kg)⁶
• Vapour resistance: high, so it counts as an HR underlay, and a cold roof under it needs roof-space ventilation: under BS 5250, eaves openings equivalent to a continuous 10 mm gap on pitches above 15° (25 mm at 15° or less), plus 5 mm at high level where the pitch exceeds 35°, the span exceeds 10 m, or the roof is a lean-to or mono-pitch⁷
• Cost: about £3.00–£3.60/m², since a 15 m × 1 m roll sells for roughly £45–£54 at roofing merchants⁸⁹
• Lifespan: typically a few decades; it becomes brittle with age

2. LR (Low Resistance) Breathable Membrane

The modern standard for new roofs and full re-roofs. A polypropylene laminate that allows water vapour out while resisting liquid water ingress. In some cold-roof designs, with a well-sealed ceiling and a membrane certified for it, the loft ventilation can be reduced or left out; follow the membrane’s certification and BS 5250.

• Weight: 100–160 g/m² depending on grade
• Vapour resistance: below 0.25 MN s/g by definition (an Sd value of roughly 0.05 m or less), so highly vapour-open⁴
• Cost: £1.05–£2.40/m² depending on grade and brand
• Lifespan: long, but guarantees vary by product, so check the manufacturer’s terms
• Ventilation: with a well-sealed ceiling and an air-permeable roof covering, the normal 3 mm gap at the eaves can be enough where the membrane’s third-party certification allows it; otherwise add 5 mm ventilation at high level⁷

3. HR (High Resistance) Underlay

HR underlays resist the passage of vapour, as traditional felt does, so a cold roof void under them needs ventilating in the same way.⁴ Modern HR membranes are lighter and tougher than bitumen felt, but most roofers now use LR.

Material Costs Summary

At a national merchant, a 4.2 m length of BS 5534 graded 25 × 50 mm treated batten costs £6.52 including VAT, about £1.55 a metre.¹⁰ A 1.5 m × 50 m roll (75 m²) of 115 g breather membrane costs £81.97 including VAT, about £1.09/m².¹¹ Galvanised 2.65 × 40 mm clout nails cost £9.98 for a 1 kg pack of about 570 nails.¹²

For a typical 3-bed semi-detached with a 60 m² roof area, battens and underlay together cost roughly £210–£450 in materials: about 180 m of 25 × 50 mm batten at £0.80–£1.60 a metre (£144–£288) plus around 66 m² of breathable membrane, allowing for laps, at £1.05–£2.40/m² (£69–£158).

Batten Gauge: Why It Matters

Batten gauge is the centre-to-centre spacing between battens and determines how much each tile overlaps the one below. The tile manufacturer specifies the gauge for their product at a given pitch, deviating from this, even by 10 mm, can compromise the weatherproofing of the entire roof.

Most modern concrete interlocking tiles are variable-gauge, but the gauge must not exceed the manufacturer’s maximum, which depends on the headlap needed at the roof’s pitch, normally 75 mm to 100 mm.¹³ Always check the manufacturer’s laying guide before setting out, it will give gauge vs pitch in a table.

Nailing and Fixing Requirements

BS 5534 significantly tightened fixing requirements in 2014.¹⁴ Every single-lap (interlocking) tile now has to be mechanically fixed, wherever the house is, and tiles at the perimeter of the roof need a minimum of two fixings.¹⁴ Ridge tiles and hip tiles must also be mechanically fixed, rather than relying on mortar alone.¹⁴ The exact specification comes from a wind uplift calculation for the site, which tile manufacturers will produce for a given roof.¹⁴

Clip fixings and disc fixings are acceptable alternatives to nailing for manufacturers who supply them. A roofer who quotes to nail only every few courses is working to the old practice, not the current code.

Batten nails should give at least 40 mm penetration into the rafter; BS 5534 guidance usually calls for galvanised wire nails 65 mm long and at least 3.35 mm in diameter.³⁴

Counter Battens and Ventilated Roofs

On an insulated (warm) roof or where a ventilated cold roof is specified, counter battens run vertically along the rafters before the horizontal tile battens are fixed. This creates a ventilated air gap between the underlay and tiles, typically 25–50 mm, that helps manage condensation and prolongs tile life.

Counter battens are the same treated timber (usually 25 × 50 mm or 38 × 50 mm) and add approximately £1.40–£5.90/m² to material cost, plus extra labour. At 400–600 mm rafter centres a roof needs about 1.7–2.5 m of counter batten per m², and a 38 × 50 mm treated batten costs around £1.85–£2.35 a metre at timber merchants.¹⁵¹⁶ On a roof with a breathable LR membrane, counter battens are technically optional, but they improve drainage of any incidental water through the underlay and are widely considered good practice.

When to Upgrade Battens and Felt

If you’re having tiles replaced on a roof more than 25 years old, it is almost always worth replacing the battens and felt at the same time. Labour is the dominant cost in a re-roof; the materials themselves are modest. Leaving rotted or brittle felt in place under new tiles is a false economy, the felt will fail before the tiles do.

Re-covering an existing roof normally needs a Building Regulations application if it affects 50 per cent or more of the roof, or 25 per cent or more of the whole building envelope.¹⁷ An application is also needed if the new covering is 15 per cent or more heavier than the old one, because the roof structure may need checking.¹⁷ Because a roof is a thermal element, re-covering it should normally also include improving its insulation.¹⁸ Smaller repairs below those limits do not normally need an application, but BS 5534 also covers normal re-roofing work and repairs, so follow it even on patch repairs.¹