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We have an attic I'd like to use as an extra room. The frame of the house is solid 2x8" beams and i'm told that the weight is not a problem there. The attic framing is just a typical framing though.
Can I reinforce it with joists and have it hold up a typical load of furniture and people? I'm not going to put the family safe up there, but i would like to make the attic useful without tearing the roof off.

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What do you mean by "typical framing"? (Makes me think 2x6 joists on 16" centers, and 2x4 trusses). –  Niall C. Aug 20 '10 at 3:29
    
that's about it. the house is 100 years old and the frame is made of pretty sturdy timber. The attic and roof have been redone more recently with more typical spec. –  shigeta Aug 20 '10 at 16:22
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3 Answers

up vote 8 down vote accepted

In Massachusetts, attic floors must support at least 30 lbs/ft² live load (the same as bedroom floors) if the attic is accessed by means of a fixed stairway (780 CMR 5502.3.1). Use a span table to determine the necessary joist dimensions, given their span length, spacing, wood species, and grade. For example, if your floor joists are 2x6 spaced 16 inches o.c., they can then be between 8'5" and 10'11" depending on the wood species and grade (780 CMR Table 5502.3.1(1)).

Uninhabitable attics with limited storage need only support a live load of 20 lbs/ft² (780 CMR Table 5802.4(2)).

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This is a great answer. In my 80+ year old house the attic had 2x6 floor joists (about) 16 oc. I went to get a permit from the city to finish the attic and they said I needed to reinforce the floor since for a "live load" it was not strong enough for our span below (about 12'). I ended up getting an structural engineer to write a report saying if we sistered all the joist with second 2x6 it would up to code. –  auujay Aug 20 '10 at 15:30
    
Thanks this helps a lot - the table is helpful - i want to be safe! –  shigeta Aug 20 '10 at 16:20
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Don't forget to check code regarding egress and other requirements. –  Dennis Williamson Aug 20 '10 at 23:39
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How would one determine wood species and grade value for existing construction? –  Troy DeMonbreun Sep 24 '10 at 15:50
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@Troy: Anything since at least 1970 (when the standard PS 20-70 came out) is stamped with species and grade. For older construction you have guess and leave plenty of room for error. –  Vebjorn Ljosa Sep 24 '10 at 22:59
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This is one of those questions that is tough to answer without actually seeing the attic, there may be more issues here with turning this in to a living space than just will it hold the load. There are codes to what can and cannot be a livable space, because of this I would consult a contractor and/or engineer.

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True, usually your local building department can give you this info. For an attic a few of the major things are: ceiling height (there is likely a lower limit a certain percentage of the room must have), light and ventilation (a certain area of windows relative to room size), alternate egress route (if there is a fire in the stairs, is there another way out of the room?). –  auujay Aug 20 '10 at 16:21
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You should consult an architect or engineer.

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