Deck joists are the framing of a deck. Just like floor joists, subflooring, studded walls, rafters and decking make up the frame of a house, deck joists are what the finish deck material attaches to. The strength and soundness of any deck is directly related to the strength of the joists. Material selection and proper installation give joists their strength and durability. Since decks are directly exposed to the elements, warping and rot are major concerns. Smart builders build decks with pressure treated wood, faux wood or metal to diminish and/or avoid these hazards.
Size the joists. Calculate the total square footage of the deck. Refer to the span table to find the live load for the deck. (The span table is a chart used to tell builders how far joists should be run from each post or pier/support, how many and which type joists will best handle the deck load/pounds per square foot. Live load is a variable that refers to the total weight on the deck at any given point.)
Check to find the best wood type for the deck layout from the span table also. Go up to the next size joist size any time you get close to the maximum acceptable span for any size joist per the span table. For instance, if the 2 by 8 is at or near the allowable span (per the span table) go to a 2 by 10 to better accommodate deck load.
Decide joist spacing. Choose to increase span by using smaller joists spaced closer together (16 inches) or take a more economical route by using larger joists spaced farther apart (24 inches). Consider the decking that will cover the joists. Use thicker decking (1 1/2 inches) if you are spacing the joists at 24 inches. Set joists at 16 inches if you use thinner than 1 1/2 inch decking. Use the recommended joist spacing for other composite decking material.
Measure and mark the ledger (the board to which the joists will be attached) at the chosen spacing interval (16 or 24 inches) of Step 2. Go to the other end of the deck and mark the rim joist to which the other end of the deck joist will be attached. Begin at the first joist and mark each succeeding joist in order until all are located and marked.
Mark the centre of the joist on the ledger. Move 3/4 of an inch to either side of this mark and make another mark. Use a square to continue this mark down the side of the ledger facing you to remind yourself which side the joist hanger will be attached. Mark the corresponding locations on the rim joist in the same order.
Set the joist hangers. Take a piece of joist scrap and drop it into the hanger. Align the hanger to the mark of Step 5 and nail it into place with 16 penny galvanised nails. Be sure to keep the joist scrap in the hanger while mounting it. Attach all the joist hangers before installing the joists. Drop the joists into the hangars and nail them into place.
Be sure to hold the joist hangers straight on the marks as you nail them in.
Having a helper for this project is strongly advised.