on page Inspecting a Deck, Illustrated - InterNACHI is this “rule of thumb”. “On-center spacing of ledger fasteners in inches = 100 ÷ joist length in feet.” This implies to me that for a deck that is 15’ x 15’ the formula would read 100/15=6.67 so for a 15’ ledger you’d need 15x12/6.67=27 ledger fasteners. Is this correct?
then it says there shouldn’t be any in the 5’ at the ends so for the end spacing you’d have to deduct 5" per side so actually the equation would be something like ((15x12)-10)/6.67= 25 ledger fasteners. If this is incorrect, can anyone clarify this for me?
This isn’t calculus. Keep it simple. Bolts alternating high and low 2" from the top and bottom of the ledger board with bolts spaced 16" on center. Done.
First rule of thumb. Never use the InterNACHI training as your basis for facts or figures. Methodology maybe, but not the actual numbers. Some of the training modules are old and have not kept up with multiple code cycles.
OP used a 15’x15’ deck as an example, so typically the deck would have been constructed with a joist span of 13’ (ledger to support beam) with a 2’ overhang. That would require lag screws/bolts at 13" OC or carriage bolts at 24" OC (assuming 2" nominal). So 16" OC is actually a more accurate measurement (generally speaking ;)) and has been accepted by every AHJ I’ve ever dealt with since the early 80’s.
Also, I don’t see 6" OC listed anywhere in the AWC DCA-6 2012 or AWC DCA-6 2015 ledger board fastening tables.
Close, but you’re forgetting that you can’t put fasteners within 5" of the end of the ledger. The 5" rule could reduce the number of fasteners by as many as 2 depending on the length of the ledger.
It’s basically a rule of thumb to allow home inspectors to point to a third-party (in this case, InterNACHI) Determining the exact required spacing for the ledger fasteners is based on many factors, including:
joist length;
type of fastener;
diameter of fastener;
sheathing thickness;
use of stacked washers;
type of wood species;
moisture content;
band joist integrity; and
deck loads.
In other words, it can’t be calculated. And a home inspector shouldn’t even try. Yet, the home inspector needs a way to note something in the report, if it appears there are too few. InterNACHI’s rule of thumb provides that way and is a good CYA tool.
