Gambrel Roof Calculator (UK)

What this calculator returns

This Dutch barn / gambrel calculator takes building length, width, break height and the two pitches and returns:

1. The horizontal projection of the lower (steep) section
2. The horizontal projection of the upper (shallow) section
3. On-slope surface area for each pitch, doubled for both sides
4. Total roof area in m² and roofing squares
5. A material allowance with waste applied

All inputs and outputs are metric. UK roofers work in degrees and millimetres, so the calculator accepts pitches in degrees (and shows X/12 alongside for anyone working from imported drawings).

Step 1 — Measure the building footprint

Walk the perimeter and measure to the verge and eaves edge, not to the masonry. Dutch barns commonly have generous overhangs — 300–500 mm at the eave and 100–200 mm at the verge. For an L-shape or any non-rectangular plan, treat each rectangle as its own gambrel and add the totals.

If the building was originally agricultural, do not assume the gable walls are square. Measure both diagonals. A 1% twist over a 12 m run adds 30 mm to the verge timber and 12 extra cuts on the tile setting-out — small in cost but worth knowing before you order.

Step 2 — Decide the break height

The break height is the vertical distance from the eave to the kink where the lower steep section meets the upper shallow section. On a residential Dutch barn aiming for usable loft space, this is typically 1.8–2.4 m — that puts the kink above head height inside, giving full-headroom rooms in the loft.

On a true agricultural Dutch barn with a high ridge, the break can be 3 m or more. The taller the break, the more material the steep lower section consumes, but the more usable internal volume you gain.

Step 3 — Choose the two pitches

Typical UK pairings:

• 60° lower / 22° upper — classic Dutch barn profile, works with concrete interlocking tiles top to bottom
• 62° lower / 20° upper — common on equestrian buildings to help shed driven rain
• 55° lower / 25° upper — slightly less aggressive, cheaper to work on, but loses 8–10% loft volume
• 65° lower / 18° upper — tightest barn profile, only suitable for concrete interlocking or profiled metal on the upper because plain clay needs 35° minimum

BS 5534:2018 Annex A pitch and exposure tables decide the headlap on each section. On a 22° upper in exposure zone 2, you want a 100 mm headlap on Marley Modern; on a 60° lower you can drop to 75 mm. Two different gauges on the same roof — set the battens out carefully.

Step 4 — Calculate the horizontal runs

lower_run = break_height ÷ tan(lower_pitch°)
upper_run = (building_width ÷ 2) − lower_run

If lower_run exceeds half the building width, the inputs are inconsistent — the lower section physically cannot fit. Either lower the break height, steepen the lower pitch, or widen the building.

Example: a 9 m × 7 m Dutch barn with a 2.0 m break height and a 60° / 22° pair.

lower_run = 2.0 ÷ tan(60°) = 2.0 ÷ 1.732 = 1.155 m
upper_run = 3.5 − 1.155 = 2.345 m

Step 5 — Surface area

Slope factor for each section is 1 ÷ cos(pitch°):

lower_slope_factor = 1 ÷ cos(60°) = 2.000
upper_slope_factor = 1 ÷ cos(22°) = 1.079

lower_area_both_sides = 2 × lower_slope_factor × lower_run × length
upper_area_both_sides = 2 × upper_slope_factor × upper_run × length
total = lower + upper

Continuing the example with a 9 m length:

lower_area = 2 × 2.000 × 1.155 × 9 = 41.6 m²
upper_area = 2 × 1.079 × 2.345 × 9 = 45.5 m²
total     = 87.1 m² of on-slope surface

That is roughly 28% more roof area than the same building under a 30° gable (which would be 67.6 m²).

Step 6 — Apply a waste allowance

Use NFRC waste figures:

• 7–8% for concrete interlocking tiles on a Dutch barn — the transition flashing course increases cuts
• 10–12% for natural slate — slates cannot be re-cut and the lower steep section breaks more during loading-out
• 12–15% for plain clay tiles double-lap, where the top half-tile course at the transition is wasteful

For the worked example at 8% waste: 87.1 × 1.08 = 94.1 m² to order.

Step 7 — Materials and 2026 costs

Source: NFRC 2026 industry pricing guide; Marley, Sandtoft and Welsh Slate Company published trade lists; Tata Steel Colorcoat HPS200 pricing for steel sheeting.

The transition between the two pitches needs a flashing strip — typically a Code 4 lead apron 300 mm wide running the full length of the building. Allow 22 kg/m² × 0.3 m × length × 2 sides for the lead, then add 15% for laps. For the 9 m worked example: 22 × 0.3 × 9 × 2 × 1.15 ≈ 137 kg of Code 4 lead. At £18/kg that is £2,460 — a cost specific to gambrels that gable roofs do not have.

Step 8 — Building Regulations and BS 5534

A Dutch barn re-roof in England and Wales is a “controlled service” under Approved Document L 2026 and triggers a U-value upgrade once you touch more than 25% of the roof. Target: 0.16 W/m²K. On a Dutch barn, that usually means a warm-roof build-up over the rafters because the sloping ceilings on the steep lower section make between-rafter insulation hit a continuity break at the kink.

BS 5534:2018+A2 requires mechanical fixing on every perimeter detail. On a Dutch barn that means dry verge on the gable ends, dry ridge along the apex, and clip-fixed flashing along the transition course. Mortar bedding is no longer accepted for new or replacement work.

Step 9 — Specific risks on Dutch barns

Wind uplift on the upper shallow section. A 22° pitch in exposure zone 4 or 5 will lift tiles in a gale unless every tile is fixed. Use the manufacturer fixing schedule and do not skip courses.

Walking access to the steep lower section. Above 60°, the only safe access is a roof ladder hooked over the ridge — a flat-soled walk is unsafe. Allow for fall-arrest staging in the price.

Snow accumulation at the kink. Snow slides down the upper shallow section and stops at the kink. Snow guards on the transition flashing prevent it sliding off the steep section onto people below. Specify them as standard on residential Dutch barns over 50° lower pitch.

Gutter sizing. The catchment from the steep section dumps water at the kink and the shallow section dumps at the eave. Both flows arrive at the same gutter — size for the combined flow, not each section in isolation. BS EN 12056-3 sizing is mandatory for Building Control sign-off.

Worked total — 9 × 7 m Dutch barn at 60°/22°

On-slope area               =  87.1 m²
With 8% waste               =  94.1 m²
Marley Modern tiles 10/m²   =  941 tiles
BS 5534 battens 2.9 m/m²    =  273 m
Counter-battens 1.67 m/m²   =  157 m
Membrane (1.5 × 50 m rolls) =  2 rolls
Dry ridge 9 m               =  £288
Dry verge 14 m              =  £336
Code 4 lead transition 137 kg = £2,466
Snow guards transition 18 m =  £450

Material subtotal          ≈  £8,400
Installed price band       =  £220–£330/m² × 87.1
                           =  £19,160–£28,740 fully fitted

Related calculators

• Roof pitch calculator — find each pitch in degrees from a rise-over-run measurement
• Roof square footage calculator — convert plan area to on-slope area for any pitch
• Calculate roofing materials — full take-off of tiles, battens, membrane and lead

Sources: BS 5534:2018+A2 Slating and tiling for pitched roofs; BS 5250:2021 Management of moisture; Approved Document C 2026 (resistance to weather); Approved Document L 2026 (energy efficiency); HSE HSG33 Health and safety in roof work; Working at Height Regulations 2005; BS EN 12056-3 gravity drainage sizing; NFRC 2026 industry pricing guide; Checkatrade and MyBuilder 2026 UK quote data.