Nailing the joist hanger on the wrong side of the mark.
After your girder is in place, you are now ready to install your remaining joists. To do this you need to make marks, called layout marks, on the ledger and girder that show where each joist will be located. This process is called a layout.
Usually joists are located so their centers occur exactly every 16”, 24”, or 32”. This is called their "on-center" distance. The distance between joists depends on several important factors:
Size of the joists
Spacing of the joists
Length of span from ledger to girder
Type of wood used for joists
Any heavy loading, such as snow
This is one of the few crucial dimensions specified in deck building. If you under space or over space the joists, the decking can collapse or the building inspector can stop the project. Be sure you talk to a local retailer or building code office about the proper sizing for your joists and span. Joists placed 24” on center are usually adequate.
After you have determined their spacing, the layout is rather simple. Starting from a nail nailed at the center of one of the two outer joists already in- stalled, measure along the ledger, making a clear mark every 24” (or 16”). These marks indicate the center of each joist. However, it is a good idea to make a second mark 3/4” to either the left or right side of each of these first marks. This new mark locates one side of each joist rather than the middle; this makes it easier to locate the joist hangers on the ledger. Using a combination square, draw this new mark straight down across the ledger. Be sure it is a good, straight mark, since you will be nailing your joist hanger to this mark. Nail the hangers on flush with the edge of these new marks.
Do a similar layout on the girder, so you know exactly where each joist crosses the girder. Again, draw new lines 3/4" to one side of the center line (the same as you did for the ledger) and then make an “X” so that you will know on which side of this line the joists will finally sit.
After you have made all your marks, begin nailing the metal joist hangers on the ledger using the special stubby joist hanger nails provided with the hangers. Only nail on one side of the hanger, allowing the other side to float free until the joist is inserted. Nail on the one side so that the interior edge of the joist hanger is nailed along the line you drew to mark the side of the joist. Be sure as you go along that you are dear about which side of the mark the joist hanger and joist should be nailed to, to assure proper spacing. Even a seasoned professional can accidentally put the joist and joist hanger on the wrong side of the mark.