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How to Calculate Deck Materials

Updated February 21, 2017

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.

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.

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.

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.

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.

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.

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.

Things You'll Need

  • Paper
  • Pencil
  • Deck plan
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About the Author

Mark Morris started writing professionally in 1995. He has published a novel and stage plays with SEEDS studio. Morris specializes in many topics and has 15 years of professional carpentry experience. He is a voice, acting and film teacher. He also teaches stage craft and lectures on playwriting for Oklahoma Christian University.