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We started remodeling the 2nd floor this last fall, starting with a spare bedroom, but between our day jobs and the holidays, we ran out of time, stopping short of dry-walling - the room is sealed and insulated but it's still "open".

After we had gutted the room, we noticed that there was a pretty decent amount of space in the attic. It's the style that's all open (no wood supports in the middle). I'd like to use this space to make a small office and provide some additional storage (clothes, toys, etc...) - the only problem is, the ceiling joists / floor of the attic is made of 2x4's.

I know this is wrong; it may be ok for storage with some plywood covering it, but I don't think I want to trust it to support a desk and me working up there.

Would anyone know the minimum floor joists we would need to install? 2x6? 2x8?

3/4 plywood - I know that - and later we could do a dormer if we feel we need it.

Also, any links to how to finish the attic? It's just rafters, plywood and the actual roof up there. Can I insulate the actual rafters and drywall over or should I use that foam channel to allow air to pass through from the soffits that I've seen.

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The size of the joists will depend on the wood and the span length, which you didn't include. –  Jay Bazuzi Feb 8 '11 at 17:45
    
Don't forget to factor in local regulations. –  John Gardeniers Feb 10 '11 at 1:54
    
The minimum floor joist size is what the engineer specs for the structure. –  Matthew Dec 10 '12 at 18:58
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5 Answers

up vote 7 down vote accepted

The 2X4's you are looking at are collar ties attached to the end of the rafters and span wall to wall. They are really only strong enough to support ceiling materials, not a live load. The new floor joists will need to rest on the upper wall plate (load bearing) adjacent to a rafter, and cross supported.

The items you need to determine are span, joist spacing and material to be used as floor joists. This link shows a chart in feet and inches for you application: http://www.the-house-plans-guide.com/joist-span-tables.html. Even on short spans, I would never go any smaller than 2X8's to assure a flex free floor.

The second issue you will face is insulating the roof. The size of your rafters are going to dictate how much insulation you can install. If they are less that say, 2X10, you will not be able to get a good R-value. You may have to consider maxing out the cavities with blanket insulation, then also adding some 2 inch rigid foam sheets over the entire field to increase the total R value. You could then install strapping (purlins), with long screws, to mount drywall to. In this kind of roof design, there would be no venting required. This is a very common type of construction on cathedral type ceilings. Good luck.

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thanks for the edit jay. got a problem here. Can't format. It truncates all paragraph breaks etc. –  shirlock homes Feb 8 '11 at 21:45
    
Lots of good information here - thanks - no single 2nd floor room is greater than 12x12 - i think the price is around $9 per 2x8x10 joist at home depot, which is perfectly acceptable to me - i dont think i need more than 60 or so if i my count is right. there are headers above the windows and doors; and we installed new windows (not replacements) so we reframed the window why we did this. this is a top plate where the roof meets the 2nd floor. will put the new joists on top of that. 1 joist per roof rafter. –  lsiunsuex Feb 10 '11 at 12:07
    
the goal for this round is just to get the joists and plywood up there while we have access to it. the attic doesn't currently account for a means of getting up there other than a 3foot x 3foot hole in a closet, which is barely enough to climb through, let alone put a plastic bin up there or a desk, so a stair case of some sort is in order. as for weight on the foundation - i'm removing x of plaster walls throughout the house :) and that stuff is heavy. all that'll be up there is clothes and a desk. no parties, no bedroom set, etc... me and desk and some storage –  lsiunsuex Feb 10 '11 at 12:10
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The regs on this will be different in all countries – it will be covered by lots of regs in most countries. In the UK you would need to submit your plans to “building reg” to get them approved then pay building regs to make a few visits to check the quality of work etc. (Any planning permission for new windows etc is a separate item)

So you need to find a local structure engineer that can draw up a set of plans and specs for you and tell you what approval system you need to go though. Then once you know what is needed you can decide how much to DIY.

You can save yourself money by understanding you local regs on stairs cases etc first and working out were you want the stairs,wall and doors – their time is money!

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I think that maybe something else you need to consider is the ability of your house to take the additional load of the extra material you are proposing. Whatever you put the joists onto will need to be able to support the extra load from the new floor, floor coverings, furniture, people, new dryall etc. Also if you add more load to the roof (such as additional insulation, more drywall, etc) you need to know that whatever it is resting on (probably the same walls as the 2x4s) can take the extra load.

You might need to consider things like how strong any weak parts of the wall are (e.g above the window openings), what materials the walls are made of and how solid the top of the wall is (any damage) and whether the foundation is sufficient for the extra load. I am not a structural engineer.

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Too many "ifs" in your answer. Any standard well constructed wall will take the small distributed load of an attic room. Unless a moron built it, never had a building permit, the doors and windows will have headers. Non issues. Even a 2x4 wall will support the story above it. The foundation should be fine unless several additional stories are added. I've never had a permit denied to finish an attic on structural issues that you brought up. I don't foresee any structural problems. –  shirlock homes Feb 8 '11 at 22:08
    
@shirlock homes - There are lots of ifs because there are lots of things to check. I agree, they should not prevent you from doing it, but they are important. Sometime people do miss the lintels out from above windows on the upper floors. And there are many types of wall construction, and your wall could be damaged in places if water has got in, etc. And I am told that in very old buildings you may find that the foundation is not up to modern standards. I think that all of these things can be probably overcome, and in a modern (ish) building are probably not likely to bite you. –  flamingpenguin Feb 9 '11 at 10:29
    
BTW the term lintel is sometimes used in place or header, but in the building trades it is usually a steel plate used over a fireplace to support a mantle, a masonry feature or a beam over an extended bay window. If the second story windows etc will support he roof, it will support the new attic floor. Trust me on this one. –  shirlock homes Feb 14 '11 at 23:47
    
@shirlock homes - OK. That is not what is says in my book on loft conversions (haynes.co.uk/webapp/wcs/stores/servlet/…), and it doesn't match the experiences I've had where new steel supported by party walls supporting the new floor/walls/furniture is specified (I have plans in front of me for such an installation). I'm in the UK and lintel seems to me to be a commonly used term for steel or concrete above a window. But as I said, I am not a structural engineer, and it is quite likely that I am missing something. –  flamingpenguin Feb 22 '11 at 18:14
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You need to make sure the floor is strong enough (or it could break) and rigid enough (the drywall underneath it could crack). This is basic structural engineering. I've seen good descriptions of it in Rob Roy's Timber-Framing for the Rest of Us and Sam Clark's Independent Builder: Designing & Building a House Your Own Way. A funky thing about span loads is that as they get longer, the load increases but the beam strength decreases. A slightly longer span might require doubling the size of a beam.

A friend was showing me around a cottage he built. He made a second floor by running heavy timbers as joists, and then laying 2x6 TG planking over. He used construction adhesive between the planks, to stop dirt from working its way through. He said this was the cheapest way to make a second floor. The open spaces between the joists gives you a sense of greater height, and a space to hang small items. It is also quite beautiful.

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most of that is true as long as the 2X planking runs full span. If the planking is jointed, it becomes part of the load. I have built several post and beam houses, with exposed beams and plank floors. I live in one I built in 1973. Been doing this a long time. Gotta send you a pic sometime –  shirlock homes Feb 14 '11 at 23:54
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I've found this page of Common Domestic Floor Joists Sizes.

The dimensions of the joist depend on the span as well as the load it's intended to bear.

From the table the minimum is 50x97mm which is approximately 2x4 in, but it doesn't give any indication of the load that that will bear.

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