What is the standard depth for an internal window board?

Internal window boards (sills) are typically 150mm deep for standard cavity wall construction with uPVC windows. Deeper reveals (200-300mm) may be needed for thicker walls, bay windows or when adding internal wall insulation. The board should project 15-25mm beyond the plaster line.

How much do window boards cost in the UK?

uPVC window boards cost £10-£15 per linear metre for 150mm depth. MDF boards cost £7-£12/m. Softwood is £12-£20/m. Hardwood (oak) costs £30-£50/m. Stone options like marble or granite cost £80-£150/m. Labour to fit is typically £15-£25 per sill for timber/uPVC.

Should I use uPVC or timber window boards?

uPVC boards are maintenance-free, waterproof and match uPVC windows well. They suit kitchens and bathrooms. Timber boards (MDF, softwood or hardwood) can be painted to match decor and feel more premium. Hardwood boards are ideal for period properties. Stone sills suit high-end or listed buildings.

How much waste should I allow for window boards?

Allow 10% waste for cutting and fitting. If boards come in standard lengths (e.g. 3m or 5m), calculate how many pieces each length yields and round up. End caps (for uPVC) should be ordered in pairs for each window.

Do external stone sills need a drip groove?

Yes, external sills should have a drip groove (throating) on the underside to prevent water running back to the wall. This is a Building Regulations requirement. The sill should also have a slight fall (slope) away from the window of at least 5 degrees.

TradeCalculator

Window Board & Sill Calculator

Calculate the quantity, cost and ordering length for internal window boards and external sills in uPVC, timber, stone or composite materials.

Material

uPVC matches modern window frames

Number of Sills

Total window sills needed

Average Length

Average window opening width

Board Depth

Depth from window to front edge

Price per Linear Metre

Material cost per metre run

How We Calculate This

This calculator estimates the total length, cost and ordering quantities for window boards and sills.

The formula

Total cost = (Order length × Price/m) + End caps + Labour

Order length = total length × 1.1 (10% waste allowance)
End caps: 2 per window for uPVC boards (~£2.50 each)
Labour: £15-45 per sill depending on material

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.