How to Calculate Deck Materials

Written by mark morris
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How to Calculate Deck Materials
Calculate materials accurately. (Deck de Madeira image by CarlosNeto from

Calculating materials is one of the toughest challenges for the do-it-yourself builder. A lot of time can be wasted making additional trips to the timber merchant for more material. Conversely a lot of money can be wasted on materials that may not be returnable if too much is ordered. In order to accurately calculate the needed materials you will need a plan including dimensions and a list of the various materials you will be using. Calculating materials by the 8-foot-square gives you a simple way to get an accurate estimate of how much you need.

Skill level:

Things you need

  • Paper
  • Pencil
  • Deck plan

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  1. 1

    Start with your posts. You will need one 4-by-4 inch treated post for each location. Typically posts are placed every 4 feet. You will need nine posts for each 8-foot-square section. Plan enough 3-inch lag bolts for attaching framing members. Calculate an average of three per post with fender washers. Plan for a half of an 36.3kg. bag of cement for each post. Add the listed quantity of each material for each post.

  2. 2

    Count your joists and hardware. You will need 72 linear feet of lumber for each 8-foot-square. This includes an outside frame and cross members every 16 inches. Calculate 14 joist hangers for each 8-foot-square. You will need six 1-1/4-inch deck screws for each hanger. Add the listed quantity of each material for every 8-foot-square of deck.

  3. 3

    Calculate your decking. Each typical 8 foot 5/4 -by-6 inch deck board will cover 4 square feet, 2 per 8 foot square. You will need 28, 1-5/8-inch treated deck screws for each 8 foot square. Add the listed quantity of each material for every 8 foot square of deck.

  4. 4

    Add up all raised edges and the sides of your stairs for railing. For each 8-foot section you will need 8 feet of top rail, three rail posts, 22 balusters and 8 feet of bottom rail.

  5. 5

    Calculate materials for your stairs. For each step of your staircase you will need 4 feet of 1-by-12 inch timber for the stringers and enough timber to cover the width of the stair. For a 4-foot-wide step you will need 8 feet of 1-by-12, four 2-inch L brackets and 16 treated deck screws, size 1-1/4 inch.

  6. 6

    Calculate the total square footage of your deck and divide by eight, round up to the nearest whole section. For example, a 326-square-foot deck has 40 full 8-foot squares and 6 square feet extra. Round up to 41. Calculate post, framing and decking materials by multiplying the quantities by the number of 8-foot squares needed to complete your deck. Add the materials for railings and steps.

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