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Old 11-27-2006, 05:51 PM   #1
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deck girder beam span?


Hi all
I will be building a deck soon and am trying to determine what kind of girder beam I need. The deck will be 25' wide, 15' deep, joists are 2x10's spaced 12" OC. There will be two girder beams, one in front with joists cantilevered over the beam 12". I am hoping to space the posts every 8'; can I use (3x)2x12 for this girder?

In the back, near the house, I plan on using a second girder about 2' from the house to accomodate a bay window that juts into the deck at one end. I need this girder because the second story of this house is cantilevered about 2' over the first floor wall framing. And the second floor joists are strong but the rim joist is pretty weak, so I can't use it alone to bolt my ledger to without the posts.

I am hoping to use only three posts underneath this beam, to accomodate a sliding door unit to be put in later. These would be spaced about 11' apart. Can this beam also be (3x)2x12's? My ledger will be bolted to the rim joist, so there will be some stabilty there. The height of this deck is about 9' 3". That would be about 7'6 to the top of the posts- 6x6 posts out front and hopefully 4x4 posts under the back girder. Does that sound adequete? I am struggling with beam span charts that vary and I am new to this. Thanks for any advice
Fenton in CT

Last edited by zmusashi; 11-27-2006 at 05:53 PM. Reason: clarity
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Old 11-27-2006, 06:10 PM   #2
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deck girder beam span?


You should be ok with three... But you can probably drop it down to 2 if you want!!!
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Old 11-27-2006, 09:04 PM   #3
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deck girder beam span?


You should be good with 3-2x10, Around here you wouldn't be allowed to use 4x4 posts without corbells, but the internet is an iffy place to get structural advice on something your entire family is going to be standing on. If you are getting a permit for the work you're going to have to submit plans and then it will all be figured out.

Code span tables here don't consider two of anything to be a beam. It has to be three or four.
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Old 11-28-2006, 06:51 AM   #4
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deck girder beam span?


The joists can be 16" o.c.

The outer beam (8-0 span) can be two 2x12's or three 2x10's easily

The inner beam (11-0 span) can be three 2x12's but it is close to the limit for 60 psf live load which is required in MA

4x4 posts are OK at the double member beam but the triple member beam should be supported by 6x6's. All should have proper metal base and top connections.

I would add blocking between the joists at the beam lines even though the cantilevers are short. A brace at the post tops would stabilize the beam to post connection and prevent rotation.

Pay close attention to the connections, use heavy duty galvanized hangers and nails and consider Deckprotector from WR Grace http://www.na.graceconstruction.com/...d=5&id=41#3456

Be sure the deck is well anchored horizontally to the house with lag screws or thru-bolts. I recommend brackets instead of a ledger board http://deckbracket.com/

Don't use any sheet aluminum in contact with pressure treated wood

Flash at the hoiuse sheathing level with Ice & Water Shield

If in doubt have it engineered.
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Old 11-28-2006, 06:58 PM   #5
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deck girder beam span?


Mighty is right on the money... But if you are planning on putting a hot tub on the deck you should let us know. As this will require a stronger frame.
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Old 11-28-2006, 07:58 PM   #6
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deck girder beam span?


Quote:
Originally Posted by zmusashi View Post
Hi all
I will be building a deck soon and am trying to determine what kind of girder beam I need. The deck will be 25' wide, 15' deep, joists are 2x10's spaced 12" OC. There will be two girder beams, one in front with joists cantilevered over the beam 12". I am hoping to space the posts every 8'; can I use (3x)2x12 for this girder?

In the back, near the house, I plan on using a second girder about 2' from the house to accomodate a bay window that juts into the deck at one end. I need this girder because the second story of this house is cantilevered about 2' over the first floor wall framing. And the second floor joists are strong but the rim joist is pretty weak, so I can't use it alone to bolt my ledger to without the posts.

I am hoping to use only three posts underneath this beam, to accomodate a sliding door unit to be put in later. These would be spaced about 11' apart. Can this beam also be (3x)2x12's? My ledger will be bolted to the rim joist, so there will be some stabilty there. The height of this deck is about 9' 3". That would be about 7'6 to the top of the posts- 6x6 posts out front and hopefully 4x4 posts under the back girder. Does that sound adequete? I am struggling with beam span charts that vary and I am new to this. Thanks for any advice
Fenton in CT
Your second floor joist are cantilevered over the first floor walls and you don't want to bolt the deck ledger to that. Your putting a girder there to support the deck. I don't understand what that girder has to do with the second floor bay window and sliding door.

The second floor cantilever holds that up, not the girder. If your second floor box is rotted out or something, then it should be fixed. Cantilevers hold up the whole second floor windows, doors, ceiling, rafters and everything else.Not the deck girder.

What am I missing?
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Old 11-29-2006, 08:44 PM   #7
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deck girder beam span?


Hey thanks for the advice, everyone.
Hi Joe- There is a bay window in the second story wall framed out as a full bay, down to and including the band joist of the cantilevered second story framing. I plan to bolt a ledger to this band joist as it runs straight along the house fifteen feet and then around to create this bay. The 6' bay extends four feet- or two feet further- from the first floor wall framing than the rest of the house, which is only 2' away.
I plan on placing my inside girder just under the ledger where it meets the bay at its outmost point, then lay the perpendicular deck joists on top of that, using joist hangers to attach to the ledger. This girder will thus be 2' away from the ledger for the rest of its length- that is, 2' in towards the center of the deck.

The sliding door unit I briefly mentioned so casually (in retrospect) is going to be UNDER the deck, and there are limited locations to install that. We want to create a dry patio beneath, and hopefully screen it in some time in the future. I would like to use three posts under that inside girder so the basement mother-in-law's bedroom that looks out into that underdeck area won't be too obstructed. I hope to use RainEscape vinyl water drainage system that directs rainwater from each joist bay into a gutter mounted onto on the inside of the front girder beam.

I am planning on using 12" OC because I have heard a few stories about composite decking that has sagged in just two years. Has anyone had similar or other experiences or advice?

PS Does anyone know what happens with aluminum (flashing) and PT lumber?
Thanks a lot!
Fenton in CT

Last edited by zmusashi; 11-29-2006 at 08:53 PM. Reason: clarity
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