Beam Span Question

novicepro

I am building a deck over an incline driveway. The joists are gonna span
21 ft between the girders. Thought I'd ask here first before I look it up.
If I use 2x12 pt for the joists, is framing 12 o/c sufficient or should I double the joists. The deck (plywood) will be waterproofed with some kind of membrane with a drain , and the decking boards (2x6 redwood) will be on top. Your expert advice would be greatly appreciated. Thanks . Or if there is a better way to do this???

cleveman

Let's see what your span chart says.

I'm surprised you haven't been bombarded yet by people telling you to hire an engineer.

Offhand, I think you can span about 18'-6" with doug fir 2x12's 16" OC. I think you'll just make the 21' with 12" OC spacing. I'm not sure about the southern yellow pine. It may be better than doug fir now, who knows.

If you put two sets of blocking in over the 21', that would be ducky.

Oh, by the way, hire an engineer to look in his span chart and tell you what you can do.

novicepro

thanks, brother in-law is an engineer, I'll ask him tomorrow.
I hate to admit it, but this deck isn't getting permitted and no plans.
I'm just removing the crappy deck that's there that sags in the middle ( framed with regular doug fir 24 o/c) -by previous homeowner and his cousin.The inspector actually said don't worry about it he'll consider it a "repair".

Bone Saw

your joist specs meet l/180 @ 50 so as long as your deckings specific gravity doesnt work out to exceed 10 psf you're ok, pushing it but ok. far as your beam, don't know what you're looking to span (distance wise) but your going to prolly need a w835 steel beam and and footings about 38" round x 16" thick (not accounting for frost line depth or soil bearing cap.) hope this helps

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Kgmz

IRC code joist spans for decks.

Assumes 40 psf live load, 10 psf dead load, L/180 cantilever
deflection with 230 lb point load, No. 2 grade, and wet service

conditions.

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BuiltByMAC

What's the other dimension of your deck - 21' x ?' and would it make more sense to run two big ass beams across the 21' span and position your joists on those beams? If your other dimension is smaller, you could get away with smaller joists and not kill yourself moving 2x12x22' joists around.

And yeah, get the engineer to sign off on beam and joist sizes for that kinda span. I'm too tired today to give you sh!t about asking that question here...(had a good time last night! Maybe too good!)

Mac

novicepro

Thanks for your response guys. Mac there's a height restriction ( I can't go up any higher- door- and I can't lower the beams- suv has to park below)
The original was joist hanged from the beam. One end is tied to the house and the other end has a post. I just wasn't sure how stable it would be running the beams parallel to the house instead. I guess when I put the skirt and the boards on it will stiffen up. Thanks alot. And I will talk to an engineer.

BuiltByMAC

Shearwall nail pattern on the plywood (along with glue on joist tops) will help stiffen structure up nicely...

Mac