Deck inspection Ledger bolts spacing

Reviewing the deck inspection course and came across a formula for ledger boards. It says that in determining spacing of lag bolts (or other means of connections) the formula to be used reads:

InterNACHI’s ledger fastener spacing formula provides inspectors with this rule of thumb: On-center spacing of ledger fasteners in inches = 100 ÷ joist length in feet.

Does this suggest that if my ledger board is 10’ wide, that my fasteners should be 10" on center? 25’ wide board would be 4" on center? etc.

Sorry if a dumb question, just not sure how to decipher.


Correction: Does this suggest that if my ledger board is 10’ LONG, that my fasteners should be 10" on center? 25’ LONG board would be 4" on center? etc.

Note, there is more to it than that, such as “unsupported length”… (although you ever seeing a 25’ unsupported or supported continuous length joist is fairly improbable)!

I understand that there is more to a deck inspection. That is why I am taking the course. Appreciate the correction, however my question still stands regarding the intent of the original formula that was offered for consideration.

My correction answers your question.

I’ll assume that means Yes, my understanding is correct. I have seen your responses on this forum before and appreciate your knowledge and willingness to share, however I did not see how changing “wide” to “long” answered the question.

It was the only change necessary to make it a correct statement.
You asked a ‘Rule of Thumb’ question. I gave a ‘Rule of Thumb’ answer. :wink:

Note as well that it is the length of the JOIST, not the LEDGER that you use for that “rule of thumb” calculation.

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Thus my comment referencing “unsupported length”. A ledger should be fully supported!

OK, Thanks. your intent was unclear, but understand it now

As @tnicholson3 stated above, it is the joist length that determines ledger bolt spacing, not the ledger board length. Just wanted to make sure you understand that as you didn’t respond to Terry’s post.

Why rely on a rule of thumb when the proper fastener scheme is already prescribed in the building code.

This may help you, too, Dave.

Save it for future reference if your area uses these code standards:

And here is the 2015 guide:


Hey Dave -
That rule of thumb is based on the length of the joists, not the ledger or band board.
Second of all - IMHO - a good rule of thumb would be to ignore your thumb, and use your other available fingers to review the IRC Ledger fastener placement table.
There are variables to consider other than joist length. Such as type of fasteners used (thru-bolts vs. lag screws; 2x lumber vs. 1x engineered).
TABLE R507.9.1.4

Rock on!


As a rule of thumb, bolts can be spaced 16" on center for supporting joist span up to 12’ in length.