Re-Roofing a House: Strip and Recover vs Overlay, Sarking Felt, Battens & Building Control

Summary

Re-roofing is one of the highest-value roofing jobs in the UK domestic sector — and one where the quality of hidden work (felt, battens, fixings) determines whether the customer gets a 25-year roof or one that fails in 10 years. The visible tiles are usually only 40–50% of the job's importance; the understructure and fixing system is critical.

The UK saw a significant backlog of re-roofing work develop in the 2020s as properties built in the 1960s–1980s reach the end of their original roof's service life. Many of these roofs used sarking felt that has now completely failed (glass-fibre reinforced bitumen felt degrades over 25–40 years). When the felt fails, the roof loses its secondary weather barrier — the first defence if a tile lifts or breaks.

Building Regulations notification is a commonly overlooked requirement. Any re-roofing of more than 25% of a dwelling's roof covering requires notification to Building Control, which will inspect the energy performance compliance and structural adequacy.

Key Facts

• Building Regulations notification — Required for re-roofing >25% of a dwelling's roof area. Non-notifiable if <25% or if a registered roofing contractor self-certifies via the Competent Roofer scheme
• Competent Roofer scheme — Operated by NFRC (National Federation of Roofing Contractors). Members can self-certify re-roofing work without Building Control notification
• Sarking felt — Must comply with BS EN 13859-1 (flexible sheets for waterproofing). Current standard: Type 1F (HP200 breathable underlay) preferred over old-style non-breathable 1F felt
• Underlay types — Traditional non-breathable (black bitumen felt), breathable (polypropylene or polyethylene membrane), and high-performance reflective underlay. Breathable underlay preferred for cold roof construction to prevent condensation
• Batten sizes — 25×50mm for tiles up to 10kg/m² on rafter centres up to 600mm. 38×50mm for tiles over 10kg/m² or where loads require. Always follow BS 5534 Table 3
• Batten fixing — One 3.35mm × 65mm annular ring shank nail or screw through each batten into each rafter. Not staples alone (inadequate in exposed locations)
• Tile fixings — BS 5534:2014 requires mechanical fixing of all tiles in certain locations (all verges, eaves, ridges, and in high-exposure zones). Interlocking concrete tiles: clip-fixed at verges. Clay/plain tiles: wire-tied or clipped
• Stripping — All debris must be removed from roof void after stripping (old felt, broken tiles, bird/insect material). Vacuum where accessible
• Valley types — Open (lead or mortar) or closed (cut tiles with undercloak). Lead valleys must be Code 4 minimum per BS EN 12588
• Ridge type — Mortar-bedded or dry-fixed mechanical ridge. Dry-fix ridges now standard practice per BS 5534 (mortar alone is inadequate at high exposure)

Quick Reference Table

Detailed Guidance

Survey Before Quoting

Before quoting, inspect:

1. Roof structure: Enter the loft and inspect rafter condition, size, and spacing. Look for signs of rot, woodworm, insect damage, or previous structural repairs
2. Sarking felt: What type is it? Is it intact or falling away? Does it have split or collapsed sections (common with glass-fibre bitumen felt after 30+ years)?
3. Battens: Probe for rot with a screwdriver. Check size — pre-1970 battens are often 25×38mm (inadequate for current standards). Check fixing — many older battens are nail-fixed at 2–3 fixing points per rafter only
4. Flashings: Lead or mortar. What condition? Does the valley need replacing?
5. Tiles: What proportion can be retained? Salvaged concrete interlocking tiles typically look worse than clay but often have 30+ years of life remaining
6. Gutters and fascias: Does the customer want these replaced at the same time (logical to do so while scaffolding is up)?

Stripping the Roof

Work from ridge to eaves. Start at the ridge — remove ridge tiles first (mortar bedded: chip away mortar; dry-fixed: remove mechanical fixings). Remove tiles course by course working down the roof. Pass all materials down by hand or using a roof chute — do not drop tiles or battens.

Felt and battens: Cut felt at ridge and at the bottom of each batten. Roll up old felt and pass down. De-nail old battens and remove. Stack separately from tiles.

Debris: Clear the roof void (loft space) of all fell felt, dust, and debris before installing new felt. This is often skipped — don't. Old compacted felt debris in the loft is a fire risk and harbours pests.

Installing Breathable Underlay

Modern breathable membranes (e.g., Cromar BS, Marley Supafelt Breathable, Klober Permo Dry) are installed by rolling across the rafters, overlapping at least 150mm at horizontal joints (lower course first, upper course lapping over) and 100mm at vertical joints.

Drape the membrane into the rafter bays rather than pulling tight — the sag (typically 10–15mm) is intentional, allowing any condensation to run to the eaves rather than sitting in the batten cavity.

Eaves: Fold the underlay over the fascia into the gutter, or use a proprietary eaves guard. The underlay at the eaves must direct any water into the gutter — not behind the fascia.

Verges: Underlay must extend beyond the verge rafter (or bargeboard) by at least 50mm. Fold under the verge tile or clip in position.

Abutments and penetrations: At abutment walls (e.g., where the roof meets a party wall), lap the underlay up the wall by minimum 150mm and fix with adhesive flashing or lead flashing over the top.

Battens

Size selection: Follow BS 5534 Table 3. Standard domestic roof with concrete interlocking tiles at 600mm rafter spacing: 25×50mm batten minimum.

Fixing: At every rafter crossing. Use 3.35mm × 65mm annular ring shank nails or 3.5×65mm screws. Two fixings per batten at each rafter crossing on verge battens.

Setting out: Calculate the gauge (horizontal spacing between battens) to suit the tile's minimum and maximum recommended lap. For most concrete interlocking tiles: gauge is approximately 325–345mm. Set out from eaves to ridge and cut the top course to accommodate the overhang of the ridge tile.

Eaves battens: Double up at eaves to raise the first course of tiles to the correct angle.

Tile Relaying and Fixings

BS 5534 compliance: BS 5534:2014 requires mechanical fixing of all tiles in high-exposure zones and at all perimeter locations (eaves, verge, ridge). In sheltered locations, every 5th course minimum must be clipped or wired. Most roofing contractors now clip-fix all tiles on re-roofing work for warranty and insurance purposes.

Hip tiles: Mechanically bedded or dry-fixed. Hip rails for dry-fix hips must be specified for the tile manufacturer's system.

Ridge tiles: Dry-fixed ridge using purpose-made ridge rail and batten end caps is now standard practice. Mortar-bedded ridge using a semi-dry mortar mix is still used — but BS 5534 recommends mechanical fixing at a minimum alongside mortar bedding.

Verge details: Verge undercloak (concrete or GRP undercloak tile) under the verge tile. Verge tiles must be clipped or nailed at every course.

Energy Compliance (Building Regulations Part L)

When a re-roof is notified to Building Control, Part L1B applies. For re-roofing a cold roof (insulation at ceiling level):

• The existing insulation must be checked and brought up to a minimum standard where practically and reasonably achievable
• If the loft is insulated to less than 100mm of mineral wool, a significant improvement (to 270mm mineral wool) should be achieved if access allows

Building Control officers typically require sight of the existing insulation level and confirmation of any improvements. If the loft is already adequately insulated or access is genuinely impractical, the existing specification may be accepted.

Frequently Asked Questions

Do I need scaffolding for a re-roof?

Yes — re-roofing without scaffolding is a serious breach of the Work at Height Regulations 2005. Ladders alone are not adequate for the duration and load of a full re-roof. Scaffold must be erected by a competent scaffolder and inspected on handover.

Can I overlay existing tiles with a new layer?

Overlaying one layer of tiles on top of existing is technically possible and was common in the 1980s as a cost-cutting measure. However, it adds significant weight (modern concrete tiles: 50–70kg/m²) that most UK domestic roof structures were not designed for, prevents inspection of the existing structure, and usually means the sarking felt is not renewed. It is no longer recommended practice and most reputable roofers will decline to overlay.

The customer wants to switch from concrete tiles to slate. Is this straightforward?

The main consideration is weight. Natural Welsh slate at 9–12mm: approximately 28–32kg/m². Concrete interlocking tiles: 40–55kg/m². The roof structure, designed for the heavier concrete tiles, will be more than adequate for the lighter slate — this is good. However, the gauge (batten spacing) will change significantly, and the lap specification is different. Use the slate manufacturer's fixing specification and confirm the rafter sizing supports the new slate weight with any structural engineer sign-off if required by Building Control.

Regulations & Standards

• BS 5534:2014+A2:2018 — Code of practice for slating and tiling, including fixing requirements

• BS EN 13859-1 — Flexible sheets for waterproofing: underlay for slating and tiling

• BS EN 12588 — Lead and lead alloys: rolled lead sheet for building purposes

• Approved Document A (Structure) — Roof structure and re-roofing >25% notification

• Approved Document L1B — Energy conservation: existing dwellings, roof thermal performance

• Work at Height Regulations 2005 — Scaffolding requirements for roofing work

• NFRC Good Practice Guide — Re-Roofing — Industry standards for re-roofing including BS 5534 compliance

• Marley Eternit Technical Guidance — Tile fixing and batten specification

• Klober Breathable Underlay Technical Data — Underlay selection and installation guidance

• roof tile types — Tile selection and weight reference

• roof ventilation — Cold roof ventilation requirements

• working at height — Scaffold and working at height regulations

• building control — When Building Control notification is required