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# how many [Decking] posts?

#1
05-17-05, 04:34 AM
Member
Join Date: Dec 2001
Posts: 158
how many [Decking] posts?

Building a 12' x 20' deck. Attaching to the house by a ledger.
How many posts do I need? What size of wood should the beams be?
What is my space between the beams?

thanks!

#2
05-17-05, 08:15 AM
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Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
Which way are the deck boards running?? If they are 20' long and parallel to the house, then your projection is 12'. Your joists are 12', and the beam is 20'.

How far will your joists overhang the beam? (That gives you the span of the joists, which determines their size and spacing.) What size beam are you thinking of using? That will determine the post size and spacing. (If you were using a 4X6 beam, the posts would be less than 6' apart. If you used a 4X12, you could place them 10 to 12 feet apart.

Need to know what you are planning on using for framing before we can answer your questions.

#3
05-18-05, 04:54 AM
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Join Date: Dec 2001
Posts: 158
thanks Lefty....not sure, guess that's why I've come to the experts for recommendations....if you were building this size of deck what would you do?

#4
05-18-05, 09:08 AM
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Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
Let's start at the beginning.

Where would I be building the deck? Need to know snow load, wind load, siesmic zone, and what the frost line is. You can go ask your local bldg. dept. for those numbers.

Where I'm at, frost generally isn't an issue, I have to build to meet siesmic zone 3, wind load is 80 mph, and snow load varies from none to 60 lbs. Most of the time I'm building in a 30 lb. snow load. So I make the deck free-standing, 16" square by 12" deep footings, PB44's embedded in 8" columns on top of the footings, use two 4X6 beams parallel to the house (assuming a 12' projection)with the posts 6' apart, cross-brace all of that, with 2X8 joists cantalevering 2' past each beam. The deck boards run parallel to the house, in the 20' direction.

But all of those things would have to be changed to meet different snow and wind loads, and the footings would have to be deeper than 12" if frost is an issue.

Exactly what would have to change and how it would change depends on local conditions where the deck is being built.

#5
05-18-05, 10:21 AM
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Join Date: Apr 2005
Location: Royal Oak, MI
Posts: 222
Also, how high from the ground is the deck? Is this something like a couple feet?

#6
05-18-05, 11:08 AM
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Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
Good point WillK.

I was assuming that the deck was at least 2' and less than 10' above grade.

#7
05-19-05, 04:56 AM
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Join Date: Dec 2001
Posts: 158
whoa...hadn't considered snow load....last year we got 4-5 feet, I live in Barrie Ontario....I'm helping build a deck for a neighbour...my deck heaved last winter the frost literally squeezed the footings out of the ground (like toothpaste)...so this is a serious consideration....the deck will only be a foot off the ground, a step down from the door

I'm planning on installing a ledger board and running 2 x 8 joists to a 2 x 8 beam, then the deack boards paralel to the house

#8
05-19-05, 08:52 AM
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Join Date: Apr 2005
Posts: 33
Originally Posted by jvirtue
Building a 12' x 20' deck. Attaching to the house by a ledger.
How many posts do I need? What size of wood should the beams be?
What is my space between the beams?

thanks!
I'm building a deck now, and I'm no expert, so I can only pass on this one tidbit I've learned. I planned my deck using span tables for a 40 lb/sq ft active load (my local code requirements). Even so, the building inspector came back and told me I needed more posts. The reason was the local soil bearing capacity would be exceeded with the design I had (approx 18' x 18' deck, ledger + two beams, 3 piers/posts per beam, 10" diam concete piers). I ended up having to use 4 piers/posts per beam and up the concrete pier diameter to 14" to appease the inspector.

So, just saying that before you draw up your plans it probably would be a good idea to check into that. Also, check out your local library. It probably has a good selection of how-to books on building decks. Most should include span tables in them.

#9
05-19-05, 12:58 PM
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Join Date: Dec 2001
Posts: 158
thanks JCS....ya may have to rethink the posts, double up, only \$30.00/ to dig

#10
05-19-05, 07:38 PM
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Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
At 12" above grade, you won't have room under the deck for a beam to support the joists, so each joist will have to have footings under it. How far apart the footings are is determined by the size of joists you use. And each footing is going to have to be AT LEAST 42" deep to prevent frost heave. Ask your local bldg. dept. what your frost line is, and go 6" deeper with the footings.

#11
05-20-05, 08:45 AM
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Join Date: Dec 2001
Posts: 158
thanks Lefty! I'm learning more everyday, planned on 36" holes....appreciate this

#12
05-20-05, 05:42 PM
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Join Date: Mar 2000
Location: Arlington, WA
Posts: 9,238
Using 2X8's for the joists, depending on the species of wood you use, you will the footings to be 8' apart. You'll need 2 footings per joists with or without the ledger board, so why the ledger board?? How far you space the joists apart depnds on what you are using for decking. Most composites and all vinyls need joists at 16" O.C. maximum. If it were me, I would dig 2 TRENCHES 16" to 18" wide and 42" deep, and use that for my footings. That's what you are going to wind up with anyway. So get a backhoe or Bobcat in to do the digging.

#13
05-20-05, 06:15 PM
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Join Date: Dec 2001
Posts: 158
great stuff Lefty!...will do....I work for Caterpillar...so no Bob who on my jobsite. yes going to double up my posts....you've been great...thanks so much!

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