How to calculate deck boards needed

Written by david thompson
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How to calculate deck boards needed
Calculate the boards you'll need for a deck. (altrendo images/Stockbyte/Getty Images)

If you're planning a deck, you'll need to first check with your dealer to find out what lengths of deck boards you can purchase. If possible, plan the width of the deck so a common length will reach across it without any waste. If that's not possible, you may be able to use boards more efficiently by running them at a 45-degree angle to the joists, or you may just prefer the decorative look of angled boards. Once you've decided how to run the boards, calculate the number that you'll need.

Skill level:
Easy

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Things you need

  • Measuring tape
  • Calculator
  • Graph paper

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Instructions

    Boards Perpendicular to the Joists

  1. 1

    Measure the width of the deck across the joists in feet, using a tape measure, or note the length from a plan, if you haven't started building yet. Add 1 inch to 2 inches extra for overhang. Round up the result to the closest length of boards available, to calculate what length of boards to purchase.

  2. 2

    Measure the length of the deck along the joists in feet. Convert the result to a decimal by dividing the excess number of inches by 12 and adding the result to the feet. For example, a deck that's 12 feet, 6 inches would be 12.5 feet.

  3. 3

    Divide the result from Step 2 by .47 if you're using boards labelled 6 inches wide. Divide the result by .3 if you're using boards labelled 4 inches wide. The result will be the number of boards you need of the length you determined in Step 1. Add 10 to 15 per cent extra to allow for waste.

    Boards at a 45-Degree Angle to the Joists

  1. 1

    Measure the width of the deck in feet. Convert the result to a decimal by dividing the excess number of inches by 12 and adding the result to the feet.

  2. 2

    Measure the length of the deck in feet and convert the result to a decimal the same way.

  3. 3

    Multiply the length by the width. Divide the result by .47 if you're using boards labelled 6 inches wide. Divide the result by .3 if you're using boards labelled 4 inches wide. The result is the number of linear feet of boards you'll need.

  4. 4

    Draw out a plan for the deck on graph paper, or measure diagonally across the deck and choose the most economical length of boards to purchase in feet. For example, if distances across joists usually measure 6 feet to 8 feet, 8-foot boards would be the most economical.

  5. 5

    Divide the result from Step 3 by the length of the boards you want to purchase to calculate how many boards you'll need. Purchase at least 10 to 15 per cent extra to allow for waste.

Tips and warnings

  • If you're using a width other than standard 6-inch or 4-inch boards, measure the actual width of the board and add the spacing you want between the boards, in inches. Divide the result by 12. Use the result from Step 3, instead of .47 or .3.
  • If the deck includes several rectangular areas, calculate each area separately and then add the results together.
  • If your deck is too wide for one board to reach across it, plan to butt two or more boards together end to end on a joist. If you're laying boards perpendicular to the joists, the result in Step 3 will tell you how many of each size board to purchase. For example, if you're butting a 16-foot board and an 8-foot board to reach across a 24-foot deck and the result in Step 3 was 40, you'll need 40 16-foot boards and 40 8-foot boards. If you're laying boards at a 45-degree angle to the joists, purchase enough boards so their total length equals the linear feet in Step 3.

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