Roof Estimate Calculator (UK)

How this roof estimate calculator works

Most cost calculators give you one number. A real contractor estimate has six layers stacked on top — direct cost, overhead, profit, contingency, VAT, and the schedule of payments. This calculator shows all six. Enter the building footprint in m², the pitch in degrees, the material, complexity and region, then adjust the markup chain to match the kind of contractor you’re comparing — a small one-van operator runs 12% overhead and 10% profit; a national contractor runs 25% and 15%.

The output is a written-quote-style proposal: itemised line breakdown in GBP, three-tier good/better/best, gross margin %, a labour-hour estimate based on crew size, and a 30/40/30 stage-payment schedule.

The contractor markup chain — how the price gets built

Every UK roofing estimate starts as direct costs and gets transformed into a sale price through a fixed sequence of percentage markups:

direct cost   = materials + labour + strip-out + skip + membrane + batten/timber + dry-fix + permit/notice
+ overhead    = direct × overhead%        (van, public liability, CDM, office, marketing)
+ profit      = (direct + OH) × profit%   (the contractor's net margin)
+ contingency = subtotal × contingency%   (unseen rot / asbestos allowance)
+ VAT         = subtotal × 20%            (zero-rated only on new-build or specific energy bundles)
= grand total

The markup is multiplicative, not additive. A 20% overhead on a £15,000 direct cost is £3,000, not the £1,500 some homeowners assume — which is why a £18,000 materials-and-labour figure on the back of a survey form turns into a £24,000 final invoice. The markup chain pays the van, the £8m public-liability cover, the CITB levy, the office, the slow-month payroll and the 1-in-5 jobs that uncover hidden rot under the eaves.

The math, derived from first principles

Roof surface area is the building footprint times the slope factor:

slope factor = sqrt(1 + tan(pitch°)²)
roof area    = footprint × slope factor

A 200 m² footprint at a 35° pitch has a slope factor of 1.221, giving a surface area of 244 m². Concrete interlocking tile material runs about £20/m² installed, labour about £35/m² installed; strip-out adds £15/m² and skip + disposal £9/m². With breather membrane, batten replacement, code upgrades and a building notice the direct cost lands around £18,500.

Apply the markup chain (18% overhead, 12% profit, 5% contingency, 20% VAT) and the proposal total comes out to roughly £30,800 — about 67% above raw direct cost. The VAT alone is a third of that gap. That’s why the gross-of-VAT figure on a UK estimate looks dramatically higher than the same job in mainland Europe with their 5–10% reduced-rate VAT.

Region tier — why London quotes are 25–30% higher

The calculator uses three region tiers:

The high-tier multiplier is more than a labour-rate adjustment — it captures parking suspensions (£45–£90/day in inner London), scaffold permits (£200–£600 from the borough), congestion / ULEZ charges for skip lorries (£12.50/day each), tighter Party Wall Act notice timing on terraced and semi-detached properties, and the licensing and insurance overheads that thin the contractor pool to higher-priced operators. NFRC’s 2026 cost survey shows the spread between regional and London pricing is now 28%.

Crew size and labour-hour estimate

The calculator divides labour cost (labour line + strip-out line) by the burdened hourly rate (default £32/hr — covers wages, NI, holiday pay, pension auto-enrolment, public liability and tooling), then divides by crew size × 8 hours to estimate working days.

A 4-person crew on the 200 m² concrete-tile reroof above pulls roughly 175 labour-hours, so 5–6 working days. That matches the NFRC technical bulletin benchmark of 30–40 m² stripped and re-laid per crew-day on a moderately complex residential job. If a competing tender promises completion in two days with a three-person crew, the maths doesn’t add up — they’re either skipping the strip-out or not replacing the rotten battens.

Building Regulations notice and CDM 2015

Approved Document L1B 2024 requires a Building Regulations notice for any reroof that affects more than 50% of a single elevation, plus mandatory upgrade of insulation to current standards (U-value ≤ 0.16 W/m²K for cold roofs in retrofit). The calculator estimates the building-notice fee at 3% of materials+labour or a £450 floor — appropriate for most residential notices including the inspection.

CDM 2015 applies to every domestic reroof that takes more than 30 working days OR involves more than 20 person-days of work. Most reroofs trip the second threshold easily. The principal contractor must issue a construction phase plan and notify the HSE — that’s part of the overhead, not a separate line item, but it’s why a £80 ‘too-cheap’ bid never includes proper site documentation.

Stage payments — what’s normal in the UK

Most NFRC contractors use a 30/40/30 split:

• 30% deposit on signing — locks the start date, covers material deposits with the merchant.
• 40% midpoint — strip-out complete, sarking and battens fixed, watertight membrane installed.
• 30% final — completion, all sign-off documentation handed over, NFRC workmanship guarantee issued.

Consumer Rights Act 2015 caps any deposit at a ‘reasonable’ amount before goods/services are provided. Reputable trade bodies interpret this as no more than a third. Always pay by debit or credit card or bank transfer with a written invoice (Section 75 protection on credit cards over £100). Never pay cash; never pay 100% up front.

The 3-tier good/better/best

A flat single-price quote forces the homeowner into a yes/no decision against a competitor they can’t directly compare. A 3-tier offer sells the recommended option by anchoring it between a cheaper baseline and a premium upsell. The three tiers in this calculator approximate:

• Good (86%): concrete interlocking tile (Marley Modern, Redland Mini Stonewold), 30-year material guarantee, dry-ridge BS 8612 compliant.
• Better (100%, recommended): concrete interlocking + BBA-certified breather membrane, dry-fix ridge and verge, 30-year materials + 10-year workmanship.
• Best (118%): natural slate or clay plain tile (Welsh Slate, Marley Acme), 75–125-year material guarantee, BBA breather, 25-year workmanship guarantee.

Sources used in this calculator

• NFRC (National Federation of Roofing Contractors) — 2026 contractor cost survey, technical bulletins.
• Construction Industry Training Board (CITB) — labour-rate index for roofing trades.
• BS 5534:2014+A2:2018 — slating and tiling for pitched roofs and vertical cladding.
• BS 8612:2018 — dry-fixed ridge, hip and verge components.
• Approved Document L1B 2024 — energy efficiency for existing dwellings.
• Approved Document C 2024 — site preparation and resistance to contaminants and moisture.
• CDM 2015 (Construction (Design and Management) Regulations) — health & safety duties.
• Work at Height Regulations 2005 — scaffold and edge-protection requirements.
• Checkatrade and MyBuilder 2026 cost reports — homeowner-survey medians.
• BBA (British Board of Agrément) — material certification.

Use this calculator alongside the others

For a single material-cost estimate without the markup chain, use the roof cost calculator. For a low/mid/high consumer cost band with insurance scenarios, use the roof replacement cost calculator. To compare 10 materials side by side at the contractor-bid level, use the roofing cost calculator. When you’ve gathered three real tenders, compare them line by line against this output and ask each contractor to justify any line that’s more than 15% off.