Deck Construction: Installing Joists

This is Part 8 of 9 in our Series on Restoring Wood Decks and Building New Wood Decks. Navigate to the first article: Restoring a Wood Deck

We've looked at materials and design tips. Now, assuming you have set your posts plumb and square with the beams level, it's time for due diligence on your joist installation.

(Tips on posts and beams: temporarily brace your posts on all four sides with one-by-fours staked into the ground, and criss-cross one-by-fours from post to post for stability while you install the joists. If possible, also temporarily tack a board from the house to the post for added stability while you install the beams.)

The Joy of Joists

Proper installation of joists is critical, as they are the surface on which your boards rest. Getting them as even as possible can be tricky. Here are some tips for setting your joists.

• Crown. Your joist boards will have a crown--if you look along the edge the board will swoop in or bow out. Always mount the joist (and beams) bowed upwards. The crown will not be the same for each board. Arrange the boards by the amount of crown they have--don't put a joist with minimal crown next to a joist with a high crown. If the board has an extreme rise, you may want to shave it down on the table saw (make sure to treat the cut face with a wood preservative if you do trim it).
• Width. The boards probably will not be all the same width--a two-by-eight joist will range from 7 3/8 inches wide to 7 5/8 inches or even more (it is supposed to be 7 ½ inches). You need to account for the differing widths of the joists when mounting them to the ledger. Always toenail the joist to the ledger with the top of the joist even with the top of the ledger. Then install the joist hanger snug against the joist. Don't attach the joist hanger to the joist first, assuming the board is 7 ½ inches, and then set the joist into it--you will end up with the joists looking like a Coney Island roller coaster. If the joists are resting atop a beam, shim them so that all are level, and then attach the fasteners. It is best to use a composite shim, not wood, which will decay. If you use wood shims, dip them in preservative.
• Cut Ends. If you have to cut the end of a pressure-treated joist, always treat the cut with preservative. Put that cut end where it is less likely to get wet--usually against the house, under the eaves.
• Green, Wet Wood. Pressure-treated wood that has dried out and aged can be almost unworkable. If it hasn't warped into a pretzel, it can be so brittle it will split upon sight of a nail. Always use green, newly treated wood for your joists. And don't install the joists and then let the deck sit in the hot sun for a week before attaching the boards--the joists could warp and split. The joists will keep their shape as they dry, if the boards have been nailed to them to hold them straight.

Once you have a firm joist platform, it is time to lay down the boards. The next and last part of this series explains how to finish your deck.

Next article: Installing Deck Boards. How to fasten a new deck onto your joists and apply finishing treatments.

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