Deck joist layout procedure:
How to mark the locations for connecting deck joists to the ledger board and supporting girder for decks - design/build deck project guide. This article explains how to mark joist spacings along the ledger board for consistent, accurate deck or other floor construction.
We continue with advice on attaching joist hangers along the ledger board to make later joist placement easy and fast. This article series describes critical safe-construction details for decks and porches, including avoiding deck or porch collapse and unsafe deck stairs and railings.
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Joists are the structural members that support the decking. The size of the joists, as well as their span and spacing, should be determined in the planning stage.
Joist spacing is commonly given as “on center” (o.c.), which means the spacing is measured from the center of one joist to the center of the next one.
The most common joist spacings are 12, 16, and 24 inches on center.
Watch out: nobody with any sense actually lays out joists by measuring to their center. We lay out floor joists by marking lines corresponding to one face of the joist. To avoid confusion we mark an X on the side of the line to which the joist will be placed.
So long as your lines are 16" apart your joists will be 16" O.C. as well.
Note that the distance from the end joist to the first intermediary joist is often less than 16 inches on center, to allow decking boards to reach or overhang the outside edge of the end joist.
Joists are attached at one end to the ledger with joist hangers. At the other end, they typically overhang the beam for a foot or two and are attached to it with toenails or metal fasteners known as hurricane or seismic ties. In some deck designs, joists are installed at the same level as beams, and are attached to both the ledger and the beam with joist hangers.
Choose joist stock carefully. Look for the straightest boards available, and always install them with the crown side up. Although you may have to square the ends where they meet the ledger, individual joists do not need to be cut to length at this stage; cut them only after all the joists are installed. If your design calls for double joists, do not disrupt the regular on-center spacing. Instead, treat the center of the gap between the double joists as a single joist center.
Hook a tape measure over the outside face of an end joist, then mark along the top of the ledger according to the planned on-center spacing. If your joists are to be installed 16 inches on center, mark the ledger every 16 inches.
You can reduce (but not extend) the spacing to the end joists if it better suits the length of the decking boards you are buying.
Then take a framing square and make a vertical line on the ledger at each mark. This line represents the edge of a joist, so you need to mark an X alongside it to indicate where to place the joist.
Framing tip: ALWAYS pull all of your deck measurements from the same spot. Consistency in measurement helps avoid errors and a crooked deck or floor structure. We pull our measurements from the left end of the ledger board.top corner at the building wall.
Well you could, but as you haven't squared-up the girder and end joists and ledger into a perfect rectangle (explained next at DECK JOIST INSTALLATION) there is a good chance that your marks corresponding to the girder-end of the joists won't be exactly in the right place with respect to the leger board markings.
For this reason we don't mark the joist positions on the girder until we've installed the end joists and squared the structure.
You might however want to lay out the rim joist - the joist that will be nailed against the ends of the floor joists and will be parallel to the ledger board. The rim joist (also called a header joist) covers the ends of the other joists.
The rim joist is not going to help you build the deck floor because you won't trim the deck joist outer ends to an exactly uniform length until all of them are in place.
Laying out the rim joist is an optional step, and one I have never seen a carpenter do but according to some, it does come in handy when you need to check to see that the joists are square.
On a basic rectangular deck, the rim joist is the same length as the width of the deck. Use the straightest board you can find and cut it to length. Hook your tape measure over an edge, and mark the joist spacing along the inside face. Mark vertical lines and Xs
To create a flat surface for your decking, the tops of all the joists must be level with the top of the ledger, and the only way you can achieve that is to install joist hangers in just the right position.
Carpenters sometimes use a short piece of joist stock as-a guide to help mount and nail joist hangers but you'll see in our warning below that you can get in trouble with this approach. Slide a joist hanger onto the 2x4, fasten one side to the ledger, then fasten the other side.
Or as we prefer, leave the right side un-nailed for now.
Framing tip: when attaching joist hangers make darn sure that they are going to hold the joist perpendicular, that is tha the hanger is not twisted to one side along the ledger board.
Joist hangers are available to fit almost any size joist imaginable. Always use joist hangers that are made specifically for the size joists you are using, and attach them with the type and quantity of fasteners specified by the manufacturer.
And no, that little bent over angled clip near the top of the joist hanger (red arrow) is NOT a structural fastener. It is there simply to help you mount the joist hanger in place. Position the hanger where you want it and give that bent-up pin a hammer-smack. That will hold the hanger in place until you've installed the appropriate construction nails or screws.
Watch out: use the proper sized joist hanger. Don't use a joist hanger designed for 2x4's on a 2x10 joist. It may be tempting to use those smaller hangers, especially if someone just gave them to you, but your deck joists will not be properly connected. Engineers figured out how many framing fasteners are needed to connect a 2x10 to the ledger board.
All is not lost if some fool already built your deck with 2x10 joists and just 2x4 joist hangers. You can add the missing fasteners as common nails or possibly structural screws above the hanger, working from both sides of each joist until the required number have been installed. If the proper sized joist hanger is used, it tells you how many nails to use - one per hole.
Joist Hanger Framing Tip: some carpenters have learned the hard way and don't nail both sides of the joist hanger in place until the joist has been set into it. They nail left side of the joist hanger so that the joist side itself will fall right on the joist line marked on the ledger board, but they leave the right side of the hanger open for now. Why? Well if you didn't take care to keep the joist hanger open adequately when nailing the second side, you'll find that some hangers won't accept their joist. In fact some of your joists may be thicker than the scrap lumber you used to align and keep the hangers open during the two-sided nailing approach.
Removing nails and then re-nailing all over again is not so great either. Pulling out a nail and then driving a new one into almost exactly the same location loses strength.
Continue reading at DECK JOIST INSTALLATION or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
Or see DECK & PORCH CONSTRUCTION - home
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