How to Estimate a Deck or Pergola: A Line-by-Line Quoting Guide

Substructure quantities come from your chosen spacing centres, not from the deck area alone. Once joist spacing (commonly around 450mm for hardwood decking, sometimes tighter for composite boards) and bearer spacing are set, the lineal metres of each member and the post count follow as a straightforward calculation from the plan area and perimeter.

Post spacing and bearer spans are governed by span tables — AS 1684, or an engineer’s design for anything non-standard — so confirm spacing against the actual timber or steel size being quoted rather than copying spacing from a previous job. A different decking material or a raised deck height can change the substructure spec even when the footprint is identical.

• Joist spacing changes with decking material — check the manufacturer span table before pricing
• Bearer spans depend on post spacing and member size, not just deck area
• Count posts individually — each carries its own footing, hardware and labour line
• Confirm whether the substructure ties into an existing bearer or is entirely new

Footings are priced as a count, by type, not as a percentage of deck area. A raised timber deck typically needs one footing per post, and the type — bored concrete pier, screw pile, or a stump on a stirrup — carries its own labour and material rate, so the footing schedule should list each type separately with its own quantity.

Ground conditions and deck height both affect footing depth and cost. A low-set deck on flat, stable ground is a different footing job to a high-set deck on sloping or reactive soil, even at the same plan area, so note site conditions on the takeoff rather than relying on the plan dimensions alone.

The decking board you choose — hardwood timber or composite — is one of the largest swings in the whole quote, because material rate per square metre, waste allowance and labour time all move together. Hardwood is typically cut and fixed on site with more waste and slower fixing; composite comes pre-finished with hidden-fix systems that speed up laying but carry a higher material rate.

Because the gap between the two options is large, always price both against your own current supplier rates rather than a remembered figure — timber and composite pricing move independently through the year. The table below shows how the line items are structured; every rate is an example only, to be replaced with your own rates library before it goes anywhere near a client.

A pergola is quoted the same way as an open structure first — posts, beams and rafters sized to a span table or an engineer’s design — then, if it is roofed rather than open-batten, a roofing area is added using the same square-metre logic as the deck below it. Open-batten pergolas price mainly on lineal metres of rafter and batten; roofed pergolas add sheet or polycarbonate roofing priced per square metre, plus flashing and guttering as separate items.

Post and beam sizing depends on span and any load from an attached roof, so confirm sizing against the actual span table or engineering detail rather than reusing member sizes from a different job. Where a pergola roof ties into an existing house roofline, allow a separate line for flashing and any roof penetration work.

• Open-batten pergolas price mainly on rafter and batten lineal metres
• Roofed pergolas add roofing area in square metres plus flashing and guttering as separate items
• Post and beam sizes follow a span table or engineering — do not reuse sizes from a different job
• Attached pergolas need a flashing and tie-in allowance where they meet the existing roofline

Balustrades and stairs are priced separately from the deck itself — balustrade by lineal metre of run, stairs by flight or by tread — because both carry their own material and labour rates regardless of deck size. A small elevated deck with full balustrade and a stair flight can carry more balustrade and stair cost than a much larger low-set deck with neither.

Balustrade material — timber, glass panel, wire or aluminium — moves the lineal metre rate considerably, and stairs add stringers, treads, risers and a handrail as their own component. Measure the actual lineal run and the stair rise and going from the plan or site rather than estimating from the deck perimeter.

• Measure balustrade as a lineal run, not as a percentage of deck perimeter
• Price stairs by flight, or by tread for wider stairs, including stringers, treads and handrail
• Match balustrade height and gap requirements to the safety-barrier rules that apply at the deck height
• Keep balustrade and stair items on separate lines so a design change does not force a re-price of the whole job

Fixings and finish are small line items individually but add up across a whole deck, so they need their own allowance rather than being folded into the decking rate. This covers hidden-fix clips or screws, joist tape or protection, bolts and brackets for post-to-footing and post-to-bearer connections, and the final coating — oil, stain or paint — across the whole visible timber surface.

Coating is easy to under-quote because it is priced by area but takes real labour time for preparation, application and a second coat. Treat it as its own line item priced per square metre of exposed timber, separate from the decking supply-and-fix rate.

• Hidden-fix clips or screws — price per square metre of decking, matched to the board system used
• Joist tape or flashing — a small material cost that protects the substructure warranty
• Bolts, brackets and post connectors — priced per connection, not folded into the timber cost
• Oil, stain or paint finish — price per square metre of exposed timber, including preparation and a second coat