Tag Archives: design roof loads

Understanding How to Size Rafters

Understanding How to Size Rafters

Reader ANDY in SHELBYVILLE writes:

Pole barn garage“I’m looking for an explanation on how to size a rafter on the side of an existing pole barn. The lean-to will be 12′ wide, 80′ long, ~10 height on the inside, pole barn post are 8′ spacing. I was planning on using 2×4 on end for the purling on top of the rafters. I am looking to understand how to size the rafters not just an answer like use 2×8 or 2×10. I’m finding it hard to find any explanations on how the purling affect the rafters sizing. I see lots of rafters spacing at 18″ and 24″ but nothing when using 8′ spacing. Thanks for any assistance.”

As you are in Shelbyville (where my oldest daughter Bailey lives and trains Tennessee Walking Horses), here is how to calculate rafters yourself.

Purlin dimensions and orientation have no affect upon rafter sizing.

First, check for adequacy in bending:

[(Plive + Pdead)] x rafter spacing in inches x rafter span (in feet) squared / (8 x Sm x Fb x Cd x Cr) must be less than or equal to one (1)
Plive = live load where Code minimum is 20 psf
Pdead = roof dead load, for steel roofing, purlins, weight of rafter and a condensation barrier could be as little as 3.3 psf (we typically use 5 psf)
Sm = section modulus of board chosen and is calculated by taking board width (1.5″) x board depth (5.5″ for 2×6 or 7.25″ for 2×8) squared and divide by 8. 2×6 = 7.5625 2×8 = 13.1406 2×10 = 21.3906 2×12 = 31.6406
Fb = Fiberstress in bending (Can be looked up here: https://awc.org/wp-content/uploads/2021/10/AWC_NDS2018-Supplement_20200827_AWCWebsite_Chapter4.pdf)  NOTE: Lumber values are specific to lumber species and grade, this is why they must be looked up.
Cd = Is Duration of Load, if little or no snow = 1.25, if Pg (ground snow load) is 20 or more = 1.15, in areas where snow remains on roofs for extended periods = 1.0
Cr = 1.15 if rafters are 24″ or less on center, else = 1

Second, check for deflection:

Δmax = (5 × w × l4) / (384 × E′ × I)

Where

w = pounds per linear inch of beam from snow (or live load) only 20 psf /12″ (converting feet to inches) x spacing of rafters (in inches)/12″

l – span of rafter in inches

E = modulus of elasticity (Again. can be looked up here: https://awc.org/wp-content/uploads/2021/10/AWC_NDS2018-Supplement_20200827_AWCWebsite_Chapter4.pdf)

I = width of member divided by depth of member^3 / 12 for 2×6 = 20.79; 2×8 = 47.63; 2×10 = 98.93; 2×12 = 177.97

Δallow = l / 150 IBC Table 1604.3 Footnote a

Δmax = (5 × w × l4) / (384 × E′ × I)

Δmax must be less than or equal to Δallow

And now you know why we hire engineers to perform structural calculations.

I would look to place a rafter on each side of columns, then joist hang purlins between rafters. Purlins on edge, going over top of rafters, being a questionable attachment and purlins should then have solid blocking between them at rafter to prevent rotation.

Bracing Site-Built Trusses for Lateral Loads

Bracing Site-Built Trusses For Lateral Loads

Reader in SHINER writes:

I am building a gambrel style barn, 30×80, in two directions, in plan view, it looks like a cross. I am building the trusses based on an LSU publication, giving sizes of structural members, etc… I have built several structures before, not a gambrel style barn though. My Question, Guru, how do I brace the trusses for lateral/wind/racking/diagonal loads? I want to do an exposed ceiling of 2×10’s T&G, and am worried about how to brace the trusses so they don’t fold over like a deck of cards during and after construction. Surely the nails don’t provide all the lateral bracing? I’ve seen too many leaning barns over the years to know that nails alone don’t provide sufficient wind load protections. How are the trusses braced, one to the next, so they don’t fold/roll over? Your help is so appreciated, Thank you.”

Mike the Pole Barn Guru writes:
Even with my decades of truss industry experience, I wouldn’t begin to think of field constructing 30-foot span gambrel trusses. 

I did review LSU’s “truss” design (these are actually rafters not trusses), it is from 1988 and includes this disclaimer:

“This site makes available conceptual plans that can be helpful in developing building layouts and selecting equipment for various agricultural applications. These plans do not necessarily represent the most current technology or construction codes. They are not construction plans and do not replace the need for competent design assistance in developing safe, legal and well-functioning agricultural building system. The LSU Agriculture Center, the Mid-West Plan Service, the United States Department of Agriculture and none of the cooperating land-grant universities warranty these plans.”

Several problems – no design roof or wind loads are incorporated in their design and design values for Southern Pine lumber were down graded as much as 30% in 2012 https://www.hansenpolebuildings.com/2012/06/southern-pine/

Any permanent roof truss/rafter bracing system should be developed only by a Registered Professional Engineer. Given LSU’s information is highly out dated, should you wish to continue upon this path, I strongly urge you to reach out to a competent Registered Professional Engineer to design a structurally adequate system, including a permanent bracing system you can rely upon.