Structural Bolt Challenge

The ledger board is a problem for about 90% of decks. If this gives way many can die.

Members are using a bolt equation (from Deck Course) to find out the # of bolts required.

My challenge is to PROVE that the # of bolts illustrated by the equation is actually correct. So let’s check it!!

Can someone come up with a proof (using other means) to check this. Lets assume we can place a 1200 US gal Hot Tub anywhere on the deck. Please post your PDF solution so all can see. This must be a valid proof to be acceptable.

Really??? Must be a slow day for you.

Frankly, I rarely see problems with the ledger strip…I do see problems with steps, balusters, improper footings, planks needing replaced, etc…but ledger strips…not really.

Placing a 1200 gallon hot tub which equates to over 10,000 pounds, I think I am more concerned with the structural members and design themselves…not the spacing of bolts.

Yes the rest of the deck will have problems, but are the # of bolts generated by this equation correct? This is the problem. Check out deck failures and you will see the problem.

Here check this out:

How does this prove the formula used? This is based on 40psf LL, 10 psf DL? If someone decides to put a Hot Tub on a deck that is inspected, it stands to reason one must know the # of bolts used. The equation in the deck course offers a solution. Is it correct? and Why??

Not sure what you are thinking of here but here is a tip…

A deck must have a very substantial sub-deck built underneath it that is designed or built by someone who knows about structural loading and construction techniques whenever a hot tub or spa is to be installed that is supported by the deck. In other words, the deck ledger should not be holding up the load from a hot tub.

InterNACHI’s Rule of Thumb would not apply for decks with unusual loads.

Is putting a Hot Tub on a deck an “unusual load”? The formula is used to determine the # of bolts to check…as it states “a deck with fewer bolts may be unsafe”…which can mean failure. So how does this explaination show “why” this equation should be used?

A deck should be built to carry the same load as the home it is attached to. Total load (not psf) is directly related to deck size. Deck size is determined by length of joists X length of ledger. Therefore the spacing between ledger fasteners is inversely related to joist length.

Many decks are not built by the people you refer to here, and inspectors are checking these decks. If the deck is a “standard” deck (connected to the house), what is holding it up? How do we use the recommended formula to ensure a minimum # of bolts? If we do that as an inspector, how can we show that this “rule of thumb” produces a # of bolts that is not unsafe (as the course material states)??

The rule of thumb does not determine the minimum number of bolts (although you could acquire that number using multiplication). The rule of thumb produces a spacing between ledger fasteners that, if exceeded, may be unsafe.

So this means no Hot Tubs allowed? Are inspectors stating this in the reports? ie: Hot Tubs may be hazardous.

What do you mean “not psf”? (lxw) is ft^2 and the LL + DL is in pounds…So is this not psf? Also please explain how this is “inversely proportional”. The equation has only “one” variable, doesn’t it?

What is it then, a maximum??
Yes, since there is only one variable it determines a spacing…so what if the spacing on the deck inspected is > the spacing recommended? Can it then be unsafe? If so, please show “why” this is true.

Also, since bolts are “generally” evenly spaced (based on the diagram in the deck notes), it only stands to reason one can obtain the # of bolts.

Nope. It does not mean that.

As joist length increases, the load the ledger fasteners must carry increases, and so the number of fasteners must increase, and so the space between those fasteners must decrease.

So you are saying as one increases, the other increases?? Is this not “directly proportional”? Please verify?

What if the joist length stays the same and various sizes of Hot tubs are placed on the deck?? Will this not affect the load, therefore the spacing? Please show “why” this equation is correct.

Nope. If the joist length increases the spacing between ledger fasteners has to decrease.

I am lucky if I even see bolts. Its usually nails.

OK, so what if the joist length remains the same as stated?

Yes, many are nailed. How would you prove that the bolt equation gives the correct spacing for you to use, if there were bolts?

Sorry, I don’t understand your question. If you give me a hypothetical joist length I can tell you (using the Rule of Thumb) at what distance the fastener spacing might be unsafe.