Ridge board fastening schedule

Hello - I’m studying for my upcoming state exam in NC. Can someone verify if I’m understanding this math correctly?

Say I’m building a deck 16’ L x 20’ W.
Is the spacing for the ridge board fasteners 6.5" OC? 100 divided by 16 = 6.5?

Say the the joists were 10’, 100 divided by 10 = 10" OC?

Here is the quote from the InterNACHI deck inspection section:
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…

…and so is beyond the scope of a visual inspection. However, the spacing of ledger fasteners required is primarily determined by the length of the joists.

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.

A deck with substantially fewer ledger fasteners than that recommended by InterNACHI’s formula may be unsafe.

Thank you for your responses.


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Jefferey I appreciate this. This is super informative.

However, the rule of thumb formula given by internachi does not seem to jive with the manual you just provided. Unless my understanding of their math for the rule of thumb is completely off, I come up with 6.5" OC for a 16’ joist span using the rule of thumb formula. Is there a better way to understand this if the state exam asks this question? The diagram on the manual you provided also factors in connections to LVL, and 1-1/2" lumber, how the heck can we figure this out on the fly without a code book?

I’ve never seen ledger bolts 6" on center. Something is wrong with your or NACHI formula.

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Me neither, it seems rather excessive in my opinion.

Don’t use the formula, it does not take into account many of the variables that determine the fastener spacing. Use the guide posted, and note that 18+foot long deck would need to be engineer designed because the guide does not cover it. Also know that in many, if not most, jurisdictions a deck, especially attached, requires C of O. Too many won’t have one because they are built without a permit. When built without a permit, anything goes. Decks for this reason are very dangerous and must be inspected carefully and closely.


Thank you Simon

I emailed Ben about this. Hopefully he will have that rule of thumb removed from the Deck Inspection teaching module. Other than that, I found this 120 hour pre-licensing course absolutely top notch!

The rule of thumb works like a rule of thumb for me. It provides a general idea of what a home inspector could be observing at a deck without looking up technical guides. I believe that’s the purpose of the “100 / joist span length”. So if I were inspecting a deck, and the 1-1/2" solid lumber joist span was a common 10 or 12 feet, as a home inspector, I’ll be looking for a 1/2-inch bolt fastener at just about every joist bay on the ledger (2 feet or so). If the joist is spanning 16 feet or more, then there’s something else needed there such as some engineering, as Simon mentioned.

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