DeckCalc Tip

Composite deck calculator

Composite and PVC deck boards are usually 5½ inches wide and sold in 12, 16 and 20-foot lengths. Most require joists 16 inches on center for boards laid straight and 12 inches for diagonal layouts, and they list the gap to leave. This page starts with a 20 × 12 ft deck in 20-foot boards. DeckCalc counts boards, joists and fasteners and checks the joist spacing.

Feet and inches
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      Composite span ratings come from each manufacturer (tested to ASTM D7032), so follow the installation guide for your product.

      How the deck calculator works

      DeckCalc assumes the deck boards run parallel to the house and the joists run out from a ledger on the house to a beam near the outer edge. Rows of boards = depth × 12 ÷ (board width + gap), rounded up. Boards = rows × deck length × (1 + waste) ÷ board length, rounded up. Joists = length × 12 ÷ spacing, rounded up, plus one. Screws = rows × joists × screws per joist.

      Deck joist spans (IRC 2021 Table R507.6)

      SpeciesSize12″ o.c.16″ o.c.24″ o.c.
      Southern pine2×69′-11″9′-0″7′-7″
      2×813′-1″11′-10″9′-8″
      2×1016′-2″14′-0″11′-5″
      2×1218′-0″16′-6″13′-6″
      Douglas fir-larch, hem-fir, SPF2×69′-6″8′-4″6′-10″
      2×812′-6″11′-1″9′-1″
      2×1015′-8″13′-7″11′-1″
      2×1218′-0″15′-9″12′-10″
      Redwood, western cedars, ponderosa / red pine2×68′-10″8′-0″6′-10″
      2×811′-8″10′-7″8′-8″
      2×1014′-11″13′-0″10′-7″
      2×1217′-5″15′-1″12′-4″

      No. 2 grade, wet service factor, 40 psf live load, 10 psf dead load, deflection L/360, no cantilever. Higher snow loads need shorter spans.

      Maximum joist spacing for decking (IRC Table R507.7)

      DeckingPerpendicularDiagonal
      1¼″ (5/4) wood16″12″
      2″ wood24″16″
      Composite / plasticPer the manufacturer's span rating (ASTM D7032); 16″ and 12″ are typical

      Results are material estimates. Span values are from the 2021 International Residential Code; your jurisdiction may use another edition or local amendments, and beams, posts, footings, ledger attachment and guards need their own design. Check with your building department before you build.

      Frequently asked questions

      What joist spacing do composite deck boards need?

      Usually 16 inches on center for residential decks laid perpendicular to the joists, and 12 inches for diagonal layouts or stairs.

      How long are composite deck boards?

      Commonly 12, 16 and 20 feet.

      Are composite boards wider than wood?

      Most are 5½ inches wide, the same as nominal 6-inch wood decking.

      Is DeckCalc free?

      Yes, with no sign-up or ads.

      Are my measurements saved?

      Yes, in this browser only.

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