Tag Archives: Rafter

Rafter Size, Lean-to on Slab, and “Barndominium?”

This Wednesday the Pole Barn Guru tackles questions regarding rafter size for a lean-to addition, adding a Lean-to to an existing building on a monolithic slab, and “the difference between a pole barn home and a barndominium?”

DEAR POLE BARN GURU: I am building a lean-to on an existing pole barn. It is 32′ long and it will be 14′ wide on a 3/12 pitch The posts are 8 ft on center. My question is what size rafter do I need to span 14′ at 8′ on center with 2×4 purlins on edge? Thanks. MICHAEL in LIZTON

DEAR MICHAEL: A caution – if your shed roof ties in at any height other than exactly at eave, or is not at same slope as existing roof, you have a snow slide off/drift load to contend with and are best to engage an engineer to account for this extra loading. An exception would be if you have a snow retention system on your existing roof. Assuming above is not an issue, please read on. As I do not know your loads, you can fill in blanks in this formula to find out: (roof live load + roof dead load) x spacing (in your instance 96″) x span in feet squared (14′ squared for you). Divide this answer by: 8 x Rafter Section Modulus x Fb (fiberstress in bending of lumber proposed to be used) x 1.15 (duration of load for snow) If your result is 1.0 or less, then you are golden. Section Modulus is depth of member squared x width of member divided by 6 Example : 2×12 = 11.25″^2 x 1.5″ / 6 = 31.64 Fb for #2 grade Southern Pine will be 2×8 = 925; 2×10 = 800; 2×12 = 750


DEAR POLE BARN GURU: Can I build a pole lean-to and attach it to my shop that is built on a monolithic slab? I did not intent to put a concrete floor in the lean-to. Can I do this without pouring a foundation for the lean-to? The Lean-To will be 50′ by 16′ with metal sides and roof. Thank you. GLEN in HYSHAM

DEAR GLEN: Maybe, provided your existing building footings are adequate to support weight you will be adding. Easiest and safest way is to set a row of columns directly alongside existing building wall, so you can treat new structure as being self-supporting. You will not have to pour a foundation, you can auger holes, place UC-4B pressure treated columns in holes, then backfill bottom 16-18″ with premix concrete to create a bottom collar. If your new lean-to has a pitch break, or is lower than main roof on high side, you do need to account for weight of slide off/drifting snow onto it. This can be avoided, by installing a snow retention system on your existing roof.


DEAR POLE BARN GURU: What is the difference between a pole barn home and a barndominium? SHARON in WESTCLIFFE

DEAR SHARON: Barndominium is pretty much a made-up term, with no recognized or official description. For sake of discussion, any pole barn (technically post frame) home would be a barndominium, while barndominiums could also be other structural systems with a ‘barn like’ look and most often steel roofing and siding.

Why Self Engineering is Risky

Reader MICHAEL in EAGLE POINT writes:

“I want to span 18 feet on a shed roof with any pitch necessary should I nail two 2x 6 rafters together and place this in the center and use 9 foot 2×6 purlins. The shed will be 18 X 18. Do I need to nail two 2×6 together for the two outside rafters?

All spans are clear span no center post The high side of shed is up against an existing building the low side will be new post and beam or posts with the rafters bolted to the post

This is a shed with only two walls. The existing high side as explained and the new post and beam side as the low side The two other sides will be open to drive under.

I plan on three or four posts on the low side depending on whether I use beams post to post underneath the rafters Or Bolt the rafters to either side of the post and eliminate the beams.

Since I’m using 2 x 6 purlins between the rafters I wondered if I could span the 18 foot length using two 2 by sixes (nailed together )For rafters. Since I’m using purlins I thought I could only have the rafters at each end and one in the center keeping in mind it is only a metal roof.”

Mike the Pole Barn Guru responds:
This sort of armchair ‘engineering’ is far too typical of what I read in social media groups.  When designs such as what is proposed are utilized, and buildings fail, folks are quick to point fingers as pole buildings being responsible, rather than lack of proper engineering design being our true culprit.

Please confirm this with your Registered Professional Engineer who will be sealing your building plans. Most of Oregon (your part included) has a minimum roof live load of 25 psf (pounds per square foot).  For sake of discussion, we will use 3 columns spaced nine foot on center along low and high sides and a five psf dead load (just in case someone decides to add plywood or OSB under a reroof some day).

Moment force = (25 + 5) psf x (9′ distance to next rafter / 2 [1/2 distance to next rafter] x 12″) x 18’^2 [ span of rafter] / ( 8 x 1.15 [Cd = duration of load for wood]) = 57,052.17 in-lbs

57,052.17 / (2 x 31.6406 [Section Modulus of a 2×12]) = 901.57 Fb [fiberstress in bending] required

2×12 #2 DougFir has a Fb of 900, so given bearing width at each end would most likely be approved by your engineer.

For rafters I would recommend a 2 ply 2×12 #2 DougFir on each end, and at center use two rafters on each side of the column. Connection at ends must be capable of withstanding 1215# so a single bolt will be nowhere near adequate (again, your engineer will properly design and detail this connection). Your thought of nailing two 2×6 together would be woefully inadequate (and would be over 300% over stressed probably failing during construction).

Hiring a Registered Professional Engineer is not an expense, it is an investment.