How to Calculate Deck Spans

By eHow Home & Garden Editor

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Building a deck onto your house is a job that requires a bit of planning to make sure it will support the weight of people and any furniture that you will be putting on it, as well as the weight of the deck itself. Choose the type of wood carefully as it will affect the joist spans of your deck and the cost.

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Instructions

Difficulty: Moderately Easy

Things You’ll Need:

Tape measure
Wooden stakes
Mallet to drive stakes

Step1

Measure out the size of the deck and place wooden stakes at the corners of the deck. This will let you visualize the exact size and placement of the finished deck. Make sure that the stakes are placed so that the deck is square at all corners.

Step2

Determine the distance from the corner closest to the house to the extent of that side away from the house. This measurement will form the beam for the outer edge of the deck. Also measure the length of the other sides of the deck.

Step3

Calculate the number of support posts that will be placed along each direction of the sides of the deck. Select an even number of posts to be placed along each side of the deck. Common deck support spacings are 12, 18 and 24 inches.

Step4

Space your joists either 12 or 18 inches apart. The smaller the spacing the stronger the deck floor will be. Once you have figured out the spacing of the joists you can calculate the deck span.

Step5

Go to the online span calculator at planhelp.com (See Resources). Enter the figures for the type of wood, spacing of joists, load limits and deflection limits. Select the Calculate Span Options button and read the span sizes that are recommended for your deck.