Tag Archives: monolithic slab

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.

New Build Features, Concrete Costs, and a Monolithic Slab

This week the Pole Barn Guru answers reader questions about specific features for a new build, possible costs for concrete for a 60x150x12 red iron building, and installing a monolithic slab in an area with rocky soils.

DEAR POLE BARN GURU: I like a 35′ wide by 45′ deep 18′ high. Ridge running the depth of the barn. I’d like 20′ lean-to’s on each side. 2 14’x14′ doors, one in front and one in back. Want to drive through. I’d like to know if we can have an open floor plan? I’d like all the inside walls to open into the lean-to’s. I was also wondering if we can use roofing panels that allow light through on 1/3 of left lean-to? That area would be used for plants. Let me know if it can be done and we can add some more details to get it done. Thanks. SHANE in BRIDGETON

About Hansen BuildingsDEAR SHANE: Provided you have available space, you might want to consider a 36′ x 48′ main building, as it is more efficient in materials usage and will likely be roughly same in investment. Yes, you can have an open floor plan, with no inside walls between main building and lean-to areas. While it is possible to use opaque white translucent polycarbonate panels for portions of roofs, they can prove to be problematic. From a structural aspect, polycarbonate will not adequately transfer wind shear loads, so those area will require steel strapping between roof purlins and sheathing. Below this area, everything will seriously bake – including supporting lumber. All of this lumber (below polycarbonate roofed areas) should be pressure preservative treated to reduce potential incidence of decay.

Can you estimate concrete cost for a 60x150x12 building? Red iron.

DEAR WALTER: For a red iron building, I would need to see engineer designed foundation plans in order to estimate required concrete for foundation, it will take a significantly greater amount of concrete than would a fully engineered post frame building. For a similar size post frame building, expect to use roughly 11 yards of concrete for footings and bottom collars for columns – this is based upon a 2000 psf (pounds per square foot) soil bearing capacity, Vult (Ultimate design wind speed) of 150 mph (miles per hour) and a Wind Exposure C. https://www.hansenpolebuildings.com/2022/06/wind-exposure-and-confusion-part-iii/


DEAR POLE BARN GURU: I am working with Jackson on a quote for a 40x60x16 pole barn. The plan was to install the barn on a monolithic slab as our soil up here in Rhode Island is rather rocky. My town building inspector informed me that I would need to submit an engineering/stamped plan for my concrete work. Jackson suggested I reach out to you to discuss if there are any alternatives to a monolithic slab and what your recommendations would be. ALEX in WEST GREENWICH

DEAR ALEX: If a monolithic slab on grade ends up being your best alternative, we can provide engineer seals for it within our drawings. Anticipate having to dig a trench 24″ deep around perimeter and having to insulate slab edge.

There are alternatives – least invasive and most affordable will be to excavate square piers at column locations, slab itself will pour on grade.

Either of these would require wet set brackets (included in quote Jackson provided) and insulation requirements would depend upon if building will be heated or not, as well as if radiant heat will be used in slab.

Fun With a Cheap Steel Truss Pole Building

Most of our country is unfamiliar with low budget steel truss pole barns produced and sold primarily in Southeastern states. There is a reason these are prevalent where there is no snow – just in case you were wondering.

Disclaimer, I have no issues at all with prefabricated light gauge steel trusses, provided they have been manufactured to engineer sealed drawings, specific to loading conditions where they will be put into use. AND if they are fabricated by a certified welder (rarely occurs).

Reader KEVIN in COLUMBIA writes:

“Hi Guru,


I recently purchased a home and the property included a 32×84 Pole barn. The barn was never dried in and was barely completed by the previous owner. However, everything is solid, square and plum; with the exception of a few pieces of steel on the roof that were never installed, it is a solid structure. Immediately after purchasing the home I had a monolithic slab poured under half of the structure that will become the foundation for the home of my new shop. I am quite comfortable with carpentry, but not so much with insulation. How do I go about condensation proofing the roof? I really do not want to pull the existing roof panels off to lay a vapor barrier over the purlins. My intention is to have a well ventilated attic, insulate the walls, and blow in insulation over the ceiling. I have heard of some people installing foam board to the exposed steel. Spray foam is an option, but one that is out of my price range for the moment. I have attached some images for your inspection. The 3D CAD models should provide a better illustration of what is under the roof. I look forward to your response.”

Kevin is now experiencing joys associated with buildings sold ‘on the cheap’ – with barely enough materials to get a roof on with a minimal budget. It would have been so simple for this building to have been originally sold and erected with provisions to control roof condensation. Either a Reflective Radiant Barrier (RRB) or an Integral Condensation Control (ICC) would have easily avoided your current situation.

This style of building does not lend itself well to installation of a ceiling (there are no clips along truss bottom chords to accept ceiling joists). My educated guess is these trusses are not designed to support weight of a ceiling. With 2×6 purlins spanning 12 feet, they are sadly not stiff enough to keep drywall joints from cracking – so you are going to be faced with lots of limitations.

Foam board might be a solution, however you would need to have each panel 100% air sealed between purlins in order to do so. Chances of success range close to zero. You are left with two choices – remove roof steel, install a RRB and reinstall roofing or two inches of closed cell spray foam (roughly $5700). Hopefully you have poured your slab on grade over a well-sealed vapor barrier, if not – use a sealant on top of it (not as effective, but better than nothing).