What size of ceiling joists should I use

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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!