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Roof Underlay Cost Calculator

Estimate 2026 UK roof underlay cost by line item: 1F bitumen sarking felt, heavy 5U bitumen, breathable membrane (Tyvek Supro, Klober Permo Air, Cromar Vent 3), or full self-adhered SBS, with eaves protector strip, drip edge, starter course, strip-out, Building Control and skip removal. Real 2026 NFRC and BS 5534 contractor rates.

Roof Underlay Cost Calculator

2026 UK roof underlay cost by line item — bitumen 1F, breathable membrane (Tyvek Supro, Klober Permo Air, Cromar Vent 3), or full self-adhered SBS, with eaves protector strip, drip edge, starter course, strip-out, Building Control and skip removal. Real 2026 NFRC and BS 5534 contractor rates.

Estimated underlay cost
£41,945
Range: £35,653 – £50,334
underlay + strip + eaves protector + drip + starter + Building Control + skip
Underlay installed
£21,945
Strip-out
£12,100
Eaves protector
£2,900
Drip edge
£2,800
Starter course
£0
Skip removal
£2,200

What this calculator estimates

This calculator gives you a line-by-line 2026 UK installed price for roof underlay (sometimes called sarking, lining, breather membrane or felt depending on age and region). Whether you are re-roofing a Victorian terraced house, replacing slate on an Edwardian villa, or specifying a new build to NFRC standards, the calculator follows the line-item structure that NFRC-member roofers use on real quotes:

  • Underlay material — 1F bitumen sarking felt, heavy 5U bitumen, breathable membrane, or full self-adhered SBS
  • Strip-out — removing the existing underlay and battens, skip-out
  • Eaves protector strip — heavy bitumen or self-adhered SBS at eaves and valleys, sized in linear m
  • Drip edge / eaves trim — galvanised or aluminium L-shape at eaves and verges
  • Starter course — self-adhered starter strip at the eaves (optional, integrated with some membrane systems)
  • Building Control notification, skip removal, weekend premium and extra labour

A £720 minimum call-out fee applies in most UK markets even on small jobs, because re-roofing requires a 2-person crew with scaffold, magnetic sweep, skip placement and tarping.

How to use it

  1. Enter roof area in m² (use the actual roof slope area, not the building footprint — multiply footprint by the pitch factor: 1.06 for 22.5°, 1.15 for 30°, 1.22 for 35°, 1.41 for 45°).
  2. Pick underlay type — 1F felt for historic restoration, 5U for low-budget like-for-like, breathable membrane for the modern industry standard, self-adhered SBS for storm-prone or low-pitch full-coverage specs.
  3. Pick roof pitch — shallow (15° to 22.5°) 1.0x, standard (22.5° to 45°) 1.05x, steep (over 45°) 1.25x for harness, scaffold and slowdown.
  4. Set scope — spot repair (20%), partial replace (50%), or full re-roof (100%).
  5. Set storey count — single-storey 1.0x, two-storey 1.18x, three-storey or higher 1.40x.
  6. Set access difficulty — easy (front / driveway) 1.0x, moderate (rear / scaffold) 1.10x, hard (terraced / no scaffold) 1.30x.
  7. Enter eaves protector strip length in linear m (eave length plus valley length plus 10% for penetrations and overlaps).
  8. Enter drip edge length in linear m (eave length plus rake length).
  9. Toggle starter course if you want it priced separately, then enter starter length.
  10. Toggle strip-out, Building Control notification, skip removal, weekend premium and any extra labour hours.

Typical 2026 UK underlay cost ranges

These ranges reflect 2026 nationwide pricing from the NFRC Member Survey, Checkatrade and MyBuilder indicative rates, and Q1 2026 quotes from London, Manchester, Birmingham, Bristol, Leeds and Newcastle.

Underlay line item (100 m², single-storey, moderate access)2026 installed price
1F bitumen sarking felt, full deck£550 – £820
Heavy 5U bitumen, full deck£730 – £1,080
Breathable membrane (Tyvek Supro, Permo Air, Vent 3), full deck£1,000 – £1,500
Full self-adhered SBS, full coverage£2,400 – £3,500
Eaves protector strip at eaves and valleys (60 m)£870 – £1,250
Drip edge metal, galvanised, eaves and verges (60 m)£840 – £1,180
Starter course, self-adhered (60 m)£660 – £920
Strip-out existing underlay and battens (100 m²)£550 – £780
Building Control notification, full re-roof£150 – £350

Add 18 percent for two-storey, 40 percent for three-storey or higher. Add 10 to 30 percent for moderate to hard access. Add 25 percent for weekend or out-of-hours work.

Cost drivers

Underlay material choice. This is the single biggest variable. 1F bitumen costs less per roll but is heavier (one 20 m² roll weighs 25 kg vs 4 kg for a 75 m² breathable roll), takes longer to fasten, tears more easily, and requires a ventilated cold roof void. Breathable membrane roughly doubles the material cost per m² but cuts installation labour by 30 to 40 percent and eliminates the need for tile-vent and eaves-vent ventilation in most warm-roof retrofits.

Pitch and scaffold time. Shallow-pitch work (15° to 22.5°) is fast — installers walk freely once scaffold is set. Standard pitch (22.5° to 45°) needs roof ladders. Steep pitch (over 45°) requires fall-arrest harnesses, anchor points, slower deployment, and a productivity hit of roughly 25 percent. The calculator applies a 1.25x pitch multiplier for steep installs.

Strip-out and rafter inspection. Strip-out is rarely optional on a re-roof. The old underlay is brittle, tears when tiles are pulled, and the rafters must be inspected for rot, beetle infestation, sag and lath separation. Budget £5.50 per m² for strip-out and an additional £18 per m² for any rafter or sarking-board replacement.

Eaves protector extent. BS 5534 clause 7.4.1 minimum is the eave detail. Best practice extends the eaves protector up the entire valley (a 600 mm strip down each valley centreline), around chimneys, soil-vent pipes and roof-light kerbs, and up the verges by 450 mm in coastal storm zones. A 100 m² roof with code-minimum eaves typically uses 30 to 45 m of strip; best-practice usage runs 60 to 90 m.

Drip edge integration. Drip edge is required by BS 5534 clause 7.4.2 at all eaves and verges for tile and slate roofs. Spec is 26-gauge galvanised steel, 100 mm face minimum. The drip edge sits on top of the underlay at the verge and below the underlay at the eave (eaves protector strip laps over the eave drip edge). Cost is roughly £14 per linear m installed.

Building Control and skip logistics. Most UK Local Authorities require a Building Control notification at £150 to £350 for re-roofs. Skip hire and tip fees are roughly £1.10 per m² depending on tip-fee structure. Two-storey work adds 18 percent for ladder time and material handling; three-storey adds 40 percent.

Underlay chemistry and what makes a good product

Bitumen sarking felt is unwoven cellulose fibre saturated with asphalt. Modern code-listed felts meet BS 747 type 1F (15 kg/10 m² nominal weight) or 5U (heavier, more durable). The asphalt softening point dictates UV tolerance and brittleness over time.

Breathable membrane is a multi-layer non-woven polypropylene with a microporous waterproof film. Quality dimensions: water-vapour resistance Sd value (LR — low resistance, target Sd ≤ 0.25 m), tear strength (target ≥ 250 N/50 mm), zone-rating (zone 1 to 5 per BS 5534 wind-uplift map), nail-tear resistance, and lay-flat behaviour at temperature extremes. Major brands: DuPont Tyvek Supro and Tyvek Pro, Klober Permo Air and Permo Light, Cromar Vent 3 Plus, A. Proctor Roofshield, Don & Low Aerolium, Marley Eternit Universal.

Self-adhered SBS membrane is rubberised asphalt with a polyethylene release film. Cap-sheet quality dictates UV tolerance and walkability. Major brands: GCP Grace Ice & Water Shield, Henry Blueskin, IKO Armourgard, Bauder Plant E (for green-roof underlay).

For a 30-year-plus tile or slate system, specify a BBA Agrément-certified breathable membrane paired with a self-adhered eaves protector strip.

UK code references and authority sources

  • BS 5534 (2014, amended 2018) — code of practice for slating and tiling
  • BS 5534 clause 7.4.1 — eaves protector strip requirement
  • BS 5534 clause 7.4.2 — drip edge / eaves trim requirement
  • BS 747 — bitumen sarking felt standard (types 1F, 3B, 5U)
  • BS EN 13859-1 — flexible sheets for waterproofing — pitched-roof underlays
  • NFRC Technical Bulletin 13 — eaves detail for tile and slate
  • NFRC Technical Bulletin 33 — breathable underlay specification
  • Approved Document C (Building Regulations) — site preparation and resistance to moisture
  • BBA Agrément certificates — third-party verification of major underlay brands
  • HSE Working at Height Regulations 2005 — fall-protection rules for roof work

When each underlay is the right choice

  • 1F felt — Conservation Area like-for-like restoration, where Local Authority Planning specifies traditional bitumen.
  • Heavy 5U — historic farm and outbuilding restorations where extra weight resists wind lift during install.
  • Breathable membrane — virtually every modern residential and light-commercial re-roof. The default choice for any 30-year-plus tile or slate system.
  • Full self-adhered SBS — coastal storm-exposed (zone 4 to 5 BS 5534), low-pitch sections (15° to 22.5°), historic chapels and churches with valley-rich geometry.

Bidding strategy and red flags

Always get three written bids that itemise underlay brand, eaves protector linear m, drip edge gauge and material, starter course brand, strip-out scope, Building Control notification cost and skip fees. A bid that lumps “underlay and accessories” into a single line item is a red flag. Confirm:

  1. The bidder uses a BBA Agrément-certified underlay matched to the BS 5534 wind-zone for the postcode.
  2. The bid lists eaves protector linear m, not just “as required.”
  3. Drip edge gauge meets BS 5534 minimum (26-gauge galvanised, 100 mm face).
  4. The bid includes rafter inspection time and a per-m rate for sarking-board replacement.
  5. The bidder is NFRC- or RTPI-listed with a current public liability policy and a 10-year workmanship guarantee through CompetentRoofer or a similar scheme.

For deeper estimating, also use our roof replacement cost calculator, roof tear-off cost calculator, and ice dam risk calculator to cross-check a complete re-roof bid against your specific m² and exposure zone.

Frequently asked questions

How much does roof underlay cost installed in 2026 in the UK?
In the UK, installed roof underlay runs £5.20 per m² for 1F bitumen sarking felt (BS 747 type 1F), £6.80 per m² for heavy 5U bitumen, £9.50 per m² for a breathable membrane (Tyvek Supro, Klober Permo Air, Cromar Vent 3), and £22 per m² for full self-adhered SBS. A 100 m² re-roof with breathable membrane, 60 m of eaves protector strip and 60 m of drip edge, including strip-out and skip hire, totals roughly £2,500 to £3,200 in 2026 NFRC contractor pricing. Source: NFRC Member Survey 2026; Checkatrade and MyBuilder Q1 2026 indicative rates; BBA Agrément certificates for major manufacturers.
Bitumen 1F vs breathable membrane — which is right for my roof?
1F bitumen sarking felt is the legacy product — heavy, vapour-impermeable, requires a ventilated cold roof void to prevent condensation, and meets BS 747 type 1F. Breathable membranes (LR — low resistance, BS 5534 zone-rated) allow water vapour to pass while remaining waterproof, eliminating the need for tile-vent or eaves-vent ventilation in most retrofit applications, and add roughly £4 per m² over 1F. NFRC technical bulletin 33 recommends breathable membrane for almost all new pitched-roof builds and re-roofs since 2014 — bitumen 1F survives mainly on historic restoration jobs and where Conservation Area planning insists on like-for-like replacement.
Where does Building Control require an eaves protector strip?
BS 5534 clause 7.4.1 and NFRC technical bulletin 13 require an eaves protector strip (a heavy bitumen or self-adhered SBS strip, typically 1 m wide) at the eave course of every pitched roof, lapped over the gutter to direct any wind-blown water that gets past the tile course back into the gutter. The strip is mandatory for tile, slate and shingle roofs in all UK exposure zones. Cost is roughly £14.50 per linear m installed in 2026, including the lapped detail under the underlay and over the drip edge. Best practice extends the strip up valleys at 600 mm wide centred on the valley axis.
Do I need full self-adhered SBS under the whole roof?
Almost never on a sloped tile or slate roof. Full self-adhered SBS (sometimes called fully torched-on or hot-melt) costs £20 to £24 per m² installed and is overkill for a typical pitched residential roof. It is justified in three situations: (1) low-pitch sections (15° to 22.5°) where tile alone does not meet BS 5534 minimum pitch, (2) flat-pitch parapet wall roofs where wind-driven rain risk is extreme, and (3) under metal standing-seam on tongue-and-groove decks where condensation drainage matters. For everyone else, breathable membrane on the field plus eaves protector strip at eaves and valleys is the right spec.
How much underlay do I need for my roof?
Breathable underlay rolls are typically 50 m × 1.5 m (75 m²) per roll. Bitumen 1F rolls are 20 m × 1 m (20 m²) per roll. Always add 10% to 15% for laps, cuts and waste — BS 5534 specifies 100 mm horizontal lap and 150 mm vertical lap minimum. For a 100 m² roof, plan on 1.5 to 2 rolls of breathable membrane, or 6 to 7 rolls of bitumen 1F, plus 60 to 70 lf of eaves protector strip for the eave detail.
Can I install new underlay over old?
BS 5534 and NFRC technical bulletin 33 do not permit overlay of new underlay over old — the old material must be stripped to the rafters and the structure inspected for rot, beetle infestation and rafter sag. Strip-out costs roughly £5.50 per m² in 2026, including skip hire and tip removal. Most NFRC-member roofers strip the old underlay with the tiles in a single operation.
Does breathable underlay qualify for a manufacturer warranty?
All BBA Agrément-certified breathable underlays from major manufacturers (Tyvek Supro by DuPont, Permo Air by Klober, Vent 3 Plus by Cromar, Roofshield by A. Proctor Group, Aerolium by Don & Low) carry 30-to-50-year material warranties when installed in accordance with BS 5534 and the manufacturer's technical handbook. The warranty requires installation by a competent roofer, correct lapping, and the use of the manufacturer's branded battens or accessories. NFRC-member roofers issue a workmanship guarantee on top of the material warranty.
What is starter course and is it the same as eaves protector?
They are different details. Eaves protector strip is the wide bitumen or SBS strip lapped over the gutter. Starter course is the first course of tile-and-a-half slates or starter-strip tiles bedded against the eaves, providing the bond that resists wind uplift on the leading edge. Some breathable membrane systems integrate a self-adhered starter course (e.g. Klober Permo Forte at the eaves) at roughly £11 per linear m installed.

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