size of joists for open spans


Old 01-27-07, 01:56 PM
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Join Date: Mar 2006
Location: USA
Posts: 94
size of joists for open spans

We are planning a two story 18 x 16 addition to the back of our house.

What is the longest span that can be unsupported for the 2nd floor (attic space above)? I'm wondering if an open 18x16 space will create problems or will have to have support posts or walls.

What about the longest unsupported span on the first floor (in the existing house it seems to be 8'.

I realize a structural engineer may be required before construction, but I'm trying to get a ballpark so we can develop an initial plan.
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Old 01-27-07, 04:03 PM
Join Date: Sep 2004
Posts: 155
2 x 12's will span the 16' between floors. 2 x 6's will hold the ceiling in the attic. Go bigger if you want to store a load in the attic. 16" centers for both.
Old 01-28-07, 09:15 AM
Join Date: Dec 2005
Location: USA
Posts: 3,188
Walkman - A lot depends on the structural design and what type of materials you choose. Trussed roofs will allow greater spans than conventional 2X rafter framiing. Laminated beams supporting traditional joists (or I joists) can be hidden in ceilings to provide greater spans.

If you visit a lumber yard, they can size an engineered beam for your needs. Just tell them the application and span.

In any case, IMO 2X6 attic joists are a bit on the small side. They cannot span 18' without some sort of support.

Google around on the internet or peruse some of the framing carpentry books at a big box store to get an idea about what's available.
Old 01-28-07, 12:20 PM
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Join Date: Jul 2002
Location: USA
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You can span 12' for a floor based on minimum loading and 16" oc with a 2x8, but a 2x10/2x12 (or laminated beams) would be better and have less bouce. If you are using a pitched roof and rafters/trusses, you can span the 12' with 2x4's, but you couldn't use the space to store anything, you'll need at least 2x6's for that.

ref: southern pine span tables for joists and rafters.

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