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 must comply with BS 5534:2014+A2:2018 — the standard for slating and tiling. The standard specifies both minimum dimensions and timber grade requirements.
Standard Batten Sizes
| Tile/slate type | Typical batten size | Minimum grade |
|---|---|---|
| Concrete/clay interlocking tiles (most common) | 25 × 50 mm | GS/MGS (BS 4978) |
| Plain clay/concrete tiles | 38 × 25 mm (light) or 50 × 25 mm | GS/MGS |
| Natural slate (small/medium) | 25 × 50 mm | GS/MGS |
| Natural slate (large, >500 mm) | 38 × 50 mm | GS/MGS |
| Heavy stonework or fibre cement | 38 × 50 mm | SS grade |
| Counter battens (ventilated cold roof) | 25 × 50 mm or 38 × 50 mm | GS/MGS |
The 25 × 50 mm batten is by far the most common for modern interlocking tiles. The 38 mm depth dimension is the one that matters structurally — a deeper batten provides more resistance to nail withdrawal and better handles heavy tiles.
Treatment and Durability
BS 5534 requires battens to be pre-treated with preservative to Use Class 2 (UC2) minimum — this is a pressure-impregnated treatment applied before delivery. Green-tinted timber you see at builders’ merchants is typically treated to UC2 or UC3. Do not use untreated or simply painted timber as a roofing batten — it will rot within a few years in a cold roof void.
For roofs in Scotland, exposed coastal areas, or where ventilation is restricted, UC3 treatment is worth specifying. The cost difference between UC2 and UC3 treated battens is marginal — typically £0.02–£0.05 per linear metre.
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 standard from the 1970s through to the mid-2000s. 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: 170–180 g/m²
- Vapour resistance: Very high — requires ridge ventilation at 1/300 of the roof area (or 1/150 at eaves) under BS 5250
- Cost: £0.50–£0.80/m²
- Lifespan: 20–30 years before brittleness in cold weather
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. Used correctly, it removes the need for separate ridge/eaves ventilation in cold roof constructions.
- Weight: 100–160 g/m² depending on grade
- Sd value: Typically 0.1–0.3 m (highly vapour-open)
- Cost: £0.80–£1.60/m² depending on grade and brand
- Lifespan: 50+ years (most carry a 25-year material guarantee)
- Building Regs: Satisfies the ventilation requirements of BS 5250:2021 without ridge vents when installed correctly
3. HR (High Resistance) Underlay
A hybrid — more vapour-open than traditional felt, less so than LR membranes. Used where some condensation risk exists but a full LR membrane isn’t specified. Less common now; most roofers simply use LR.
Material Costs Summary
| Material | Cost per m² (supply only, 2026) | Cost per lin. metre (battens) |
|---|---|---|
| Type 1F bitumen felt | £0.50 – £0.80 | — |
| HR breathable underlay | £0.65 – £1.10 | — |
| LR breathable membrane (standard) | £0.80 – £1.20 | — |
| LR breathable membrane (premium) | £1.20 – £1.60 | — |
| 25×50 mm treated batten | — | £0.25 – £0.38 |
| 38×50 mm treated batten | — | £0.32 – £0.50 |
| Stainless steel clout nails (per kg) | £4 – £7 | — |
For a typical 3-bed semi-detached with a 60 m² roof area, battens and underlay together cost £250–£550 in materials, depending on spec and pitch.
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.
For most modern concrete interlocking tiles pitched at 22.5°–35°, gauge runs between 280 mm and 345 mm. Always check the manufacturer’s laying guide before setting out — it will give gauge vs pitch in a table. BS 5534 Clause 6.4 requires this verification to be documented on commercial projects.
Nailing and Fixing Requirements
BS 5534 significantly tightened fixing requirements in 2014. Every tile in exposed zones (per BS EN 1991-1-4 wind zones) must be mechanically fixed — not just the verge and ridge tiles. In practice, this means:
- Zone 1 (sheltered inland): Every 5th course nailed, plus all verge and ridge tiles
- Zone 2–3 (exposed): Every tile nailed in the top two courses, every other tile elsewhere
- Zone 4 (very exposed coasts): Every tile nailed
Clip fixings and disc fixings are acceptable alternatives to nailing for manufacturers who supply them. A roofer who doesn’t ask about your location’s wind zone before quoting should be questioned — fixing to the old standard (every nth tile) is now non-compliant on full re-roofs.
For battens, nails must be stainless steel or hot-dip galvanised to resist corrosion. BS 5534 prohibits the use of bright (uncoated) steel nails on exposed roofwork.
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 £0.30–£0.50/m² to material cost, plus extra labour. 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.
Any new roof or full re-roof submitted for Building Regulations approval (required when more than 25% of the roof is being renewed) must use compliant materials and fixing. Partial repairs on the like-for-like principle remain exempt, but BS 5534 is still considered best practice even on patch repairs.
