Deck Cost Calculator

The area you walk on, in plan, measured to the outside of the deck boards. Not the area of ground underneath, which on a sloping site is larger, and not the board area including waste — the rate below is per square metre of finished deck and the waste belongs inside that rate. An L-shaped deck is two rectangles; measuring the bounding box adds a corner nobody is building.

Take it from a quote, a supplier, or a comparable job you have priced. Composite decking typically runs two to three times the material cost of treated softwood, and the substructure is the same either way.

Only the open edges. A deck against two walls needs railing on two sides, not four — and railing is often the most expensive length on the whole job.

Your own supplied-and-fitted rate for the railing you are actually using, per metre run. Railing is the widest-ranging line on a deck: a simple timber balustrade and a glass-and-post system differ several-fold, and the fixings and posts are usually where the difference lives rather than the infill. Measure the railing length separately from the deck perimeter — a deck against a wall does not railing that side.

How many footings the frame needs, which comes from the joist and beam layout rather than from the deck's size — a deck with a longer beam span needs fewer, deeper footings, not more shallow ones. Count the ledger side too if the deck is freestanding. This is the line most often under-counted on a sloping site, where the low end needs taller posts and often larger pads.

Everything one footing costs in the ground: digging it, the concrete, the post base or anchor, and the labour to set it. Include the spoil — a footing produces more spoil than it takes concrete, and on a site with no access for a barrow that removal is a real cost. Frost depth decides the volume in cold climates and can double this rate against a mild-climate figure.

Count the RISERS, not the treads — a flight from deck to ground has one more riser than it has treads, and pricing by treads undercounts every flight by one. The number comes from the total drop divided by the riser height you can use, rounded so every riser is equal: unequal risers at the bottom of a flight are the commonest cause of a trip, and most codes limit how much they may vary.

One riser's share of the whole flight: stringer, tread, riser board if you are using one, and the fixings. Wide flights need more stringers, so a 1.5 m wide stair is not twice a 750 mm one in stringers alone. Handrails and the landing at the bottom are not in this rate and are frequently the difference between the estimate and the invoice.

Ten percent on a straightforward deck on level ground. Fifteen or more where the ground slopes, drains are in the way, or the substructure is unknown until you dig.