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Old 04-07-2009, 02:51 PM   #1
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Rafter question

Hi, I have a question about rafters for the shop I am about to build.

I am doing a 30'x80' shop. The first 40' of it, I am planning on finishing, so I am going to space the rafters at 2 feet. This building is going to have perlins running across the rafters and then steel on top.

My question is this; For the part that is not going to be finished inside, will basically be like a pole building, but with a concrete foundation. For that part, do you think I could space the rafters at 4 feet instead of 2? They are 2x4 residential rafters rated at 2' OC. But they are going to have steel on them instead of sheating and shingles like a house. Also, there will be nothing hanging on the inside of the rafters, like in a house you typically have 5/8 drywall. So you wont have as much weight on them as you would in a house.
Just curious if that would work. Would cut down on the ammount of rafters needed, and if needed would be able to easily store long boards up in the rafters.
Thanks Mark


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Old 04-07-2009, 07:36 PM   #2
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I think you are talking about trusses. They have a maker's stamp on them, call them to verify the new design. Anyone else could give you info, but then - who do you sue, if it fails? Be safe, GBR


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Old 04-08-2009, 06:17 AM   #3
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As GBAR said, call the manufacturer or a structural engineer. There are a lot of other things to consider besides the weight of a shingled roof verses metal.
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Old 04-08-2009, 07:05 PM   #4
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Does it snow in your area? If so, the snow load will have to be considered!
In my area roofs must be designed to carry 30 psf!
I would doubt that 4' centers would be adequate, if snow is to be considered!
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Old 04-08-2009, 09:19 PM   #5
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You need to contact a local structural engineer. They can determine what design loads need to be considered, what load combination will govern the design, and what material you should use. The material specification will have to be made on what is available to you, specifically what species of wood, and especially the grade of the lumber (Select Structural, No. 1 or better, No. 1, No. 2, or Stud Grade). This will all impact the spacing and sizing of the roof members to carry gravity loads (weight of structure, snow load, etc.).

Also they will specify a connection of the roof members to the wall system. They will need to determine if your walls are adequate to carry shear of wind or seismic loads. They can also specify what type of roof materials need to be used to construct the roof diaphragm including how the roofing material is attached to the roof members, which is extremely important for the lateral force resisting system (this is how wind or seismic load is transferred through the structure to the foundation and ultimately into the ground). Just changing the spacing of rafters will have an impact on all of these details.

Bottom line is you need to contact a design or building professional. I can tell you right now that a 2x4 spaced at 4' o.c. is not going to be adequate for gravity loads, and with only sheet metal roofing it isn't going handle lateral forces well at all. Rafters are never rated in such a manner as "Residential @ 2' o.c."

Last edited by wildcat; 04-08-2009 at 09:25 PM.
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