help - deck beams, posts and joists sizing

Old 06-10-03, 06:16 PM
raggie man
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help - deck beams, posts and joists sizing

I am planning a deck of 16' x 16'. It will not be attached to the house although it will be attached to a cinder block structure on one end. Planned on using a ledger on the cinder block end then at around 8' out running a beam and one more beam at around 15'. I was hoping to set each beam on 4, 4 x 4 posts with a 4' span between posts and about 2' cantilever on each end. The max deck height will be around 30". Also planned on using 8' joists (2 x 6) at 16" center on hangers between the beams and then using 16' decking, either 2 x 6 or 5/4.
What I need help with is the beam sizing, attachment to the posts (on top or bolted to the side.... trying to keep below 30"), joist sizing and any other general ideas/tips.

Oh yeah, I will be using pressure treated Southern Pine.

Hope someone can help.
Raggie Man
Old 06-11-03, 06:00 AM
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Raggie Man,

Because of your post spacing, your beam spans do not have to handle that much of a load. Your center beam is the critical one and even that one isn't expected to carry much of a load.

The center beam will have to bear the weight halfway to the ledger (4') and halfway to the outer beam (about 4'), for a total of 8' across 4' post spacing. This gives you a bearing area of 32 sq. ft. If you figure 40 lbs/sf live load and 15 lbs/sf dead load (common requirements), any beam span will be expected to handle 1760 lbs., or 440 lbs. per linear foot. Even with the 2' cantilever, doubled up 2x6's should handle 500 lbs per linear foot. If you want to go a little further (safe side never hurts), there is no harm in doubled up 2x8's.

As for the floor joists, the 8 foot span (@ 16" o.c.) is probably coming close to the limit on a 2x6. While okay, you could make the deck more solid by going to 12" o.c. or using 2x8's.

Finally, with respect to the beam/post connection, put the beam on top of the post and let the wood bear the weight. Bolting to the side of the post puts all the weight on the bolts. Your posts will be short, but that shouldn't be a problem.

John P.
Old 06-11-03, 07:12 AM
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Rather than using 8' joists, I would use 16' joists and let the sit ON both beams rather than being attached ti the sides of the beams. And. like petrsjo said, put the posts under the beams, rather than bolt the beams to the side(s) of the posts. Use a Simpson BC4 (or similar) on top of each post to make this connection.
Old 06-11-03, 05:41 PM
raggie man
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Unhappy deck beams and joists sizing... again

Hi guys,

Thanks for the recommendations. I have reworked my deck plans and have now come up with the following.

Deck to remain 16' x 16', with ledger attached to cinder block structure. I have moved the beams, which will now be doubled up 2x8's, to 7' and 14' from the ledger, leaving 2' canted joists. I still plan on using 4 posts per beam but have increased the spacing from 4' to 4' 8" leaving 1' canted beams. I guess this will give me a total of 7' across 4' 8" post spacing and a bearing area of 33sq. ft. I will now also be setting the joists (16') on top of the beams as suggested.

My questions are:

a) What size ledger should I use. b) Will doubled up 2x8's be sufficient for the beams. c) Do you still recommend 16' joists (spanning 7' between beams) and if so what size should they be, or should I still use hangers and 8' joists on either side of the beam to make up the span moving back to the old beam spacing. d) Can joist hangers be hung with screws instead of nails. e) What is the best way to sandwich joists to make a beam ie screw/bolt pattern, glue ?

Appreciate your help,
Raggie Man
Old 06-12-03, 07:36 AM
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Raggie Man,

Here's my suggestions regarding your remaining issues:

Ledger - 2 x 8 should suffice.

Beams - Doubled up 2 x 8's are fine for your design. Your post spacing is still fairly short, so no need to go to doubled up 2 x 10's.

Joists - Definitely use 16 foot lengths, hung at the ledger and sitting on the beams. It's the most efficient and effective way to do it. At 7', 2x6 should be good, but 2x8 wouldn't hurt .... your call.

Joist hangers - There are special screws for joist hangers that begin to match the shear strength of hanger nails. Simpson makes them. Normal screws are not acceptable. For the amount of hangers you are puting in (12 to 14), it won't take very long to nail it.

Built Up Beam - Staggered top to bottom nail pattern every 12 to 16 inches on both sides with 12d nails will suffice.

Looks like your just about ready to go.

Old 06-12-03, 01:16 PM
raggie man
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Thanks for your advice guys and John, I hope to get started this weekend.

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