# Beefing up deck joists: Decrease spacing or increase joist width?

I'm looking for an expert opinion from builders that have been there done that:

I am laying out a 5' x 90' deck that is 40" above the ground. I'd like to meet 100 psf live + 10 psf dead load. This span card says that I should be able to run the 2x6 at 24 o.c. up to 5'2". That's just over my 5' run for the joists (actually 4' 7 1/2" after you subtract the width of the beams and ledgers they will be hanging off of).

The question is: I know that's a minimum spec, and I don't want this deck to feel bouncy at all, so what's the most economical way to build this so that it's super solid feeling but can meet 110psf load?

I could go to 2x6 at 16 o.c. to make it beefier, or I could go with 2x8 at 24 o.c. (or even 16 o.c. if necessary).

For you guys with experience, if you wanted to beef it up more than minimum spec to make sure it's not bouncy, what direction would you go?

Thanks!

First of all, those load tables are for L/360 deflection, which is already pretty stiff. That means that fully loaded, the beam will only deflect 1/360 of the length of the beam, or in this case about 1/8". If you can find a 480 deflection table that would give you even stiffer numbers.

Having said that, both adding depth and increasing the number of joists are valid ways of increasing the strength and stiffness of a floor. Upsizing to 2x8 would be a lot easier than adding more joist, and in general it is a more efficient use of materials to make a beam deeper than wider (and adding more joists is effective adding to the "width" of the joists). If you want to compare your two options, you can see that 2x8 @ 24" will give you a stronger floor than 2x6 @ 16" by just comparing the permitted span lengths. E.g. if you are using No. 2 southern pine with your loading conditions, 2x6 @ 16" has a max span of 6'-3" and 2x8 @ 24" has a max span of 6'-6". That means that the 2x8 is stronger.

(By the way, 100 psf live load?? Do you live in an area that gets a ton of snow? Or is this a public/commercial building? Typically residential decks are designed for 40 psf. Note that if you are serious about building a 100psf deck you will need to make sure that all parts of it are built with that load in mind... decking material, rim joists, columns, foundations, etc.)

• I didn't think to look at the deflection, which seems obvious now (but I don't really have practical experience with what L/360 "feels like"). Commercial property. This is a dilapidated loading dock that was scheduled for demo. We are going to reno it into an office space, and we thought connecting all the loading bays with a very long and skinny deck would be a great way to get some outdoor use and add some interest to the loading bay side of the building. Our company anticipates having gatherings with lots of family and employees coming and going on the deck. – alfreema Nov 30 '15 at 22:07
• Just found this on the same site: The live load deflection limit for the floor joist span charts used in this document is L/360. A stricter deflection limit may be obtained by multiplying the tabulated span by the appropriate factor shown in the table to the right. And then it shows L/480 = 0.91 adjustment factor, and L/600 = 0.84 adjustment factor. So it's just simple multiplication from chart. Thanks for the tip! southernpine.com/app/uploads/MAX-Spans_USINGtheseTABLES_L.pdf – alfreema Nov 30 '15 at 22:15
• @alfreema I'm skeptical about it being a wide enough space to be practical for major gatherings. A table or two chairs facing each other are enough to crowd my parents 8 foot porch. 5' will max out at a porch chair and enough room to pass comfortably (assuming a railing). – Dan Is Fiddling By Firelight Nov 30 '15 at 22:28
• @Dan Neely, I agree, it's going to be tight. We don't plan on having any furniture out there. We are thinking of it as a walk way more than a deck. Mainly we see it as a space where people can step out of their office and grab some air during a break. Each loading bay will be converted into an office, so you can step out from the office onto the deck to take a break. We envision gatherings are similar ... employees coming and going from each office, but able to walk down the "walk way". I'd love to make it wider but we are restricted by the property line. – alfreema Nov 30 '15 at 22:37
• @alfreema: OK it sounds like this is more of a "commercial corridor" than a deck, so 100 psf is probably the correct live load. I want to reiterate that this is substantially more weigh than a typical residential deck so you need to make sure the entire structure is able to support it, including proper foundations and columns. Do you have an engineer working on this project? – Hank Nov 30 '15 at 23:19

You're smart to upgrade, having some extra is a good idea in case your lumber isn't as good as it should be (safe bet.)

I'd go to 2x8's at 24" - that's my preference. It's fewer joist hangers to hang, and fewer nails to attach the deck to the joists.

With that said, there's going to be a little more give with 24" spacing, it won't be the joists that are sagging, it will be the decking between joists. If you want a really solid feel, go with 2x6's 16"OC. You get a couple more inches headroom underneath if that matters.

Whatever way you go, I believe that a real tight attachment of the decking to the joists helps stiffen the structure. Deck screws hold tighter than spiral shank nails, but more money and time. If you want to be a fanatic, re-tighten them when the wood shrinks, pressure treated shrinks like crazy.

If you want to upgrade a little without bumping up to 2x8s at 16"OC, you could shop for better lumber for the 2x6's. This is not how it's usually done these days so you may have to special order.

• I have indeed ended up at 2x8 at 24"oc. I will heed your advice and pay extra attention to attachment of the decking – alfreema Dec 1 '15 at 4:36