View Full Version : Sizing footers
05-19-2011, 10:47 PM
Building a deck on a pretty decent slope - 12" or so off the ground at the "top" and about 14 feet off the ground at the other end, 45' away - this is in ledger board length (not a 45' span).
I had planned on using 12" footers. The deck is not square and wraps around one side of the house with a 4' wide catwalk, to the front porch. The catwalk is about 8' high and due to a stone patio under it, will have a 17'6" beam holding it up.
Frost depth for me is 42". I'm assuming the two posts holding up the catwalk are going to be the biggest concern - will 12" diameter footers be enough? The entire deck plan calls for 14 total footers - but due to conforming to the house etc is a royal pain for me to describe where all the beams and footers will go, so you can't just picture a rectangle with 14 posts.
I know I'm not giving enough info here but am I at least in the ballpark with the 12" footers or is the code inspector going to laugh at me when I show him the plan?
05-20-2011, 01:02 AM
So your catwalk is 4' wide and almost 18' long!
You don't have any additional support because of the stone patio?
Your footing is for sure to small no matter where you are.
My bigger concern would be the beam. What is the size and material for the beam?
05-20-2011, 02:02 AM
2x12 doubled beam. Should I triple up the 2x12s? I should note I ran all this by the inspector last year and he was fine with it, footers, beam etc. but after reading a little more, maybe not? In fact when I ran it by the inspector I told him it's be an 18' span and asked him if I should have a beam engineered...the doubled 2x12s was his idea.
So 12" is too small for the 18' span...what's recommended?
05-20-2011, 03:47 AM
Where I am at, we are required to use column bases of some sort. For a deck that height, we would use 6x6 posts. The bases require at least 3 " of concrete, all around. This means the minimum diam. footing is 14".
Adding another 2x12, adds very little to the span strength. Increased depth of the beam is where you get added strength. That said, for a 4' wide catwalk, it's probably not the load that matters, but you might have some deflection (sagging) with that span, using 2x12's.
05-20-2011, 11:06 AM
OK, now I'm kinda confused.
I am using 6x6 posts. Redwood's using 14" footers. So, I'm not real far off base here.
What exactly are the concerns and recommendations for my beam, though? Are you saying I need to use larger than 2x12s?
05-20-2011, 12:50 PM
Part of the problem is regional differences and local code office differences. Soil conditions will dictate the size of footings, Do you know your soil bearing capacity? Usually your building department will use a standard (usually the lowest in the area, you could get a soils test to determine yours, although thats usually a waste of money). The 2x12's are questionable in that large of a span, steel or a treated glulam is probably the way to go, while the 2x12's might carry the load mathematically it's almost guaranteed to sag in the long term. With that being said you have a 4'x18' catwalk that is attached to the house and the rest is split between 2 footings but is there any other load on those footings? Using 60PSF you will have 2160 LBS on the ledger and 2160 LBS on the beam split between 2 piers 1080 lbs ea, this should tell you the size of you pier based on surface area and soil bearing. Of course keep in mind that the piers are critical portion of your deck (although in my opinion it's all critical because a failure at any part could mean injury or death) so even if 12" is fine, upsizing will not hurt, and will only help. Post bases are definitely the way to go, simpson makes quite a variety for various load and uplift requirements.
05-20-2011, 03:56 PM
You definitely need to think about your footers. It's entirely possible that your soil can handle 2160 lbs with a 12 inch footer, but the default soil bearing capacity in my area is 1500psf. If you've got good, gravelly soil without a lot of clay or organic material, you're probably alright, but you can't know for sure w/o testing or a good soils map. And a 12 inch footer isn't a 12 inch tube - that only provides about 3/4 of a square foot of bearing. You need a spread footing below that. Or, you could add another footer, which would help with both the ground load and the beam sizing.
DCA6-09, a deck design guide published by the AFPA provides some good beam sizing info. It's free online if your google skills are up to date.
05-20-2011, 05:14 PM
Thanks, guys. What prompted this question in the first place was the DCA.
I live on top of a granite mountain (in fact my town is Mountain Top, PA) - I have to assume the presence of all that rock is a good thing for load bearing, and was the #1 reason I hired someone out to auger the footer holes. Either way, within the next week I'll have all my footers laid out and ready to dig; before I do I'll call out the inspector for his first look-see. I did show him the plans, but maybe once he sees it in the flesh he'll have some better answers.
Question about the beam - I once built a porch with my FIL, who wanted as few columns as possible on it. For the longest span he doubled up 2x lumber (not sure what, I'll have to look next time I'm down there) but laminated PT 3/4" plywood between the 2xs, glued all over and screwed together. Would a lamination like that help in the case of this beam - 2x12s with 3/4 ply between?
05-24-2011, 01:52 PM
Adding plywood to a beam does practically nothing. You can't think of the vertical plies as adding strength, only the horizontal ones, so a 3/4 inch thick piece of plywood would only net a gain of 3/8 inch in beam thickness, and then only if the joints in the plywood were supported by posts.
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