Lumber size needed?

Question

I want to frame a regular roof without the use of trusses because I need the space inside and trusses wouldn't allow me enough room, plus I'd like to leave some of the wood exposed. Also, I don't want to use post and beam type construction. The house is 32x40 and the span will be 32 ft. with a 12/12 pitch and will covered with a metal roof. What size lumber will I need for the ridge beam and rafters to properly hold the roof up? Location is Kentucky, so Very little heavy snows.it's a 1- 1/2 story with bedroom upstairs on one side and on the other side of the house there is a loft overlooking the great room. With trusses, that would limit the bedroom space and loft space. Plus it wouldn't be very nice looking exposed.

Photos of the plans can be seen at:
http://photos.yahoo.com/hillbilly_jester

Answer 1

I'm not sure there is a simple answer - you need to know the wind load for both download and uplift requirements. And whether the ridge beam is supported only at the ends of the 40' span, or additionally at midpoints. As well as the 'dead' load of the roofing material - what is the weight of the plywood or OSB sheathing, tar paper/underlayment, and metal roof on a per-sq-ft basis. And finally, what kind of rafter-to-rafter spacing do you want to look at (12", 16", or 24")?

A static roof design supports a dead load of the materials plus a live load of a wind, which applies a downforce to the roof. (Steep roofs will block more wind, and increase the downforce. 12-12 is steep.) If you are exposed to hurricanes or tornados, you also need to design for uplift forces. Check with your local building dept for their requirements.

The Western Wood Products Association (WWPA) has an extensive website, where you can download rafter, joist and beam tables (Publication 572.) Depending on the species and type of wood available, you can go with smaller beams using the highest grade of lumber (knot free) or larger beams of less-expensive average quality wood.
For example:
Rafter span: 16 ft (actual length 24 ft + overhang for eaves)
Rafter spacing: 24" c-c
Load: 10# live + 10# dead. Deflection L/240
Then using Douglas Fir, you could use
a 2x8 in Select Structural grade,
or 2x10 in No. 1 grade.
(This data should not be used because it does not include uplift loading.)

At a minimum, consult with an experienced general contractor. Or an architect or registered engineer.
You don't want the roof to collapse, so the investment in technical expertise is worth the extra cost.

Answer 2

My seat-of-the-pants guess is 2x10s for the rafters and three 2x12s for the ridge beam. You will still need to support the ridge beam mid-span, and I recommend two supports evenly spaced.

The 2x10 rafters will give you enough space for R25-30 insulation, an important consideration. A skylight would look great, too!

Answer 3

Use a scissor truss.
It's cheaper then all of that conventional framing, plus youcan put insulation in.