Ceiling spans

[Bryan_Lund]

Building 14' x 24' on a slab. 2 x 4 walls, 8:12 roof pitch. Who has covered this span with 2x6 ceiling joists for the purpose of overhead storage space? I'm concerned about sag without columns or beam support? Any pitfalls to avoid here?

 

[readhead]

A 2x6 will be okay for a rafter tie but to small for any load.

 

[oldironfarmer]

You need at least a 2x8 and a 2x10 for decent loads. In any event, blocking at mid-span is necessary to let the joists hold a load without buckling sideways.

 

[bczygan]

It depends on the load you intend to impose and the grade and species of lumber and it's size and spacing as well as the ceiling you want below it.

All ceiling joists deflect or sag (Sag is not the right term). The question is how much. For a finished ceiling you want L/360 (Or better), or a deflection of 1/360th of the length of the span. For a situation with no ceiling, L/240 is OK.

The load is a combination of the things that will be stored, any human occupation and the weight of the structure itself. People and stuff are called the live load and the structure is called the dead load.

Typical LL for residential floors is 40#/SF (Square foot). Structure like this is usually 10#/SF. This is what I would use if the space is tall enough for you to walk around up there and fill the space with typical residential stuff. If the space is shallow and only light storage, you have to adjust the LL.

So let's look at yours. The 8/12 pitch means you can get up there and store stuff up there. You will need a 3/4" T&G plywood floor deck, screwed and glued to the joists. You can insulate between the ceiling joists, but that may not be adequate and you will need additional insulation at the rafters. Don't forget an airspace in each rafter bay for ventilation.
You may install a pull down stair. I'll also assume a ceiling will be installed.

Using a joist calculator and choosing readily available #2 and better hem-fir dimensional lumber, you can span 13'10". Your span is 13'5".

Adjust species or grade or spacing or loads or allowable deflection to get other results.

Span calculator:
http://www.awc.org/codes-standards/calculators-software/spancalc

Personally, I don't like to space floor or ceiling joists with a floor above them, more than 16"o.c. So if there is no ceiling and storage up there is going to be relatively light, with a ceiling that could flex without cracking, I would use a #30 live load and L/240 so I could use 2x8's at 16"o.c. Also, if zoning allowed the height, I would go 12/12 roof pitch to give me 7' inside at the peak.

Understand?

Bill

 

[Bryan_Lund]

It depends on the load you intend to impose and the grade and species of lumber and it's size and spacing as well as the ceiling you want below it.

All ceiling joists deflect or sag (Sag is not the right term). The question is how much. For a finished ceiling you want L/360 (Or better), or a deflection of 1/360th of the length of the span. For a situation with no ceiling, L/240 is OK.

The load is a combination of the things that will be stored, any human occupation and the weight of the structure itself. People and stuff are called the live load and the structure is called the dead load.

Typical LL for residential floors is 40#/SF (Square foot). Structure like this is usually 10#/SF. This is what I would use if the space is tall enough for you to walk around up there and fill the space with typical residential stuff. If the space is shallow and only light storage, you have to adjust the LL.

So let's look at yours. The 8/12 pitch means you can get up there and store stuff up there. You will need a 3/4" T&G plywood floor deck, screwed and glued to the joists. You can insulate between the ceiling joists, but that may not be adequate and you will need additional insulation at the rafters. Don't forget an airspace in each rafter bay for ventilation.
You may install a pull down stair. I'll also assume a ceiling will be installed.

Using a joist calculator and choosing readily available #2 and better hem-fir dimensional lumber, you can span 13'10". Your span is 13'5".

Adjust species or grade or spacing or loads or allowable deflection to get other results.

Span calculator:
http://www.awc.org/codes-standards/calculators-software/spancalc

Personally, I don't like to space floor or ceiling joists with a floor above them, more than 16"o.c. So if there is no ceiling and storage up there is going to be relatively light, with a ceiling that could flex without cracking, I would use a #30 live load and L/240 so I could use 2x8's at 16"o.c. Also, if zoning allowed the height, I would go 12/12 roof pitch to give me 7' inside at the peak.

Understand?

Bill

Bill, yes I understand perfectly. Your detailed response is very much appreciated and I will play with the calculator tonight.

I wanted to go 12/12, but I'm getting some female aesthetic pushback on the height (wife) and some fatherly safety pushback from my primary contractor (Dad). I choose to pick my battles.

Storage is going to be the usual Christmas decorations, lawn furniture, sports equipment, etc. No intent to make it livable or playable space, just want lots of it.

Thanks to all for the input and experience.

 

[2oolhound]

You could also make a short pony wall above the ceiling to get more height for storage and maintain the "look" of the 8:12.

 

[bczygan]

Bill, yes I understand perfectly. Your detailed response is very much appreciated and I will play with the calculator tonight.

I wanted to go 12/12, but I'm getting some female aesthetic pushback on the height (wife) and some fatherly safety pushback from my primary contractor (Dad). I choose to pick my battles.

Storage is going to be the usual Christmas decorations, lawn furniture, sports equipment, etc. No intent to make it livable or playable space, just want lots of it.

Thanks to all for the input and experience.

As mentioned above, you could have a pony wall, or you could have a taller wall and hang ledgers on both inside faces of the long walls (Lagged to the studs), and then use joist hangers for the ceiling/attic floor joists. Depends on height restrictions and look of the building.

To keep the height of the building to a minimum and attic to a maximum, you can also play with spacing and species/strength of the members. And you can also limit the load bearing capacity, based on the intended use.

Closer spacing and lighter loads could mean you could go to 5/8 ply, or even 1/2", thus cutting weight, but there is a limit to optimizing member size and plywood thickness. Frankly speaking, I think about old single car garages where there are just a few 2x4 cross members tying the side walls together, no ceiling or insulation, and people throw light storage items up there. The next step up might be 2x6's at 24" or 16"o.c., still no insulation or ceiling, but some 1/2" ply in the middle, so you could put small items up there, still light storage. You will have 4'8" clear at the peak up there, so you're in a middle ground somewhere, and need to make value judgements about capacity balanced against cost and complexity. Deciding is there will be insulation and a drywall ceiling, and what the loads will be, are the biggest decisions, and will determine the size, spacing, species and deflection limits.

An example would be to use Doug Fir - Larch #1 and better at 16"o.c, set the L/180 and 30#LL and 5 DL. You get 13'11" span.

If you're pulling a permit, and wanted to do that 2x6, bare minimum solution for very light and mostly open storage, you would probably have to build it as open framework and throw some sheets of ply up later.

One other thing. Depending on the other architecture you are trying to match or complement, you could add an overhang and fascia depth that would all the roof to come down enough to minimize the look of the height of the wall.

And concerning matching or complementing, you can do either, or completely depart and do a barn type structure with a Gambrel roof.

Engineered joists are also a possible choice. As always, using extraordinary measures to stretch a design, will almost always cost more for materials and methods.

And what is the factor limiting you to the size you specified? We always say build bigger here on GJ!!!

Bill

 

[Bryan_Lund]

Ok, so I'm going with 2x8, 16"o.c., with 3/4 OSB up on top of this...

Progress...