This week the Pole barn Guru answers reader questions regarding a 6ft attic space over a 30×44 pole barn, the most “cost effective” method to build, and the point at which a post increased from a 6×6 to a 6×8.
DEAR POLE BARN GURU: We want a decent-height (a bit over 6ft) attic space in our 30×44 pole barn (on a concrete slab foundation), can we use steel trusses or would wood trusses be a better option for this? The ground floor ceiling height is 8ft. Thnx for the help. RON in TONEY
DEAR RON: Lots of possible design solutions available. To begin with, I would recommend wood trusses – should you ever want to finish a ceiling, or if you want to have a product fabricated under strict quality control standards, then wood trusses are your best choice. On to options…. #1 My favorite. Make your building tall enough to have a full second floor. This will give you greatest usable space and best resale value. By using prefabricated wood floor trusses, you can still have a clearspan floor (no posts below) and it provides an unencumbered space below without columns to dodge. Yes, it will be a greater investment, but one you will probably never regret. #2 Prefab wood ‘bonus room’ trusses. With a steep enough slope (roughly 8/12) you can end up with an eight foot ceiling height at center and a room roughly 10-12 feet in width. For amount of space being gained, this is a fairly costly design solution. #3 Increase sidewall height and use scissor trusses to allow for a central mezzanine supported by columns. While likely to be your least expensive design solution, you will be faced with columns below (unless opting to again add in floor trusses.
DEAR POLE BARN GURU: What is the most cost efficient size to build (Can I save money by buying a specific width/ length/ height?) What are the pros and cons of choosing metal frame or post frame? Is it still true that you can add square footage more affordably by building up (adding a second floor)? Can you put a basement under a Barn House? ANNIE in FORT LUPTON
DEAR ANNIE: As you get closest to square, your costs per square foot will decrease slightly. This is due to having less exterior wall surface, although it does not necessarily lend itself best to layout of rooms. For post frame construction, your most efficient use of materials typically comes from multiples of 12 feet in width and length. Work from inside out – do not try to fit your wants and needs within a pre-ordained box just because someone said using a “standard” size might be cheaper. Differences in dimensions from “standard” are pennies per square foot, not dollars. Post frame will always be your most cost effective structural design solution: https://hansenpolebuildings.com/2022/01/why-your-new-barndominium-should-be-post-frame/ Two story is not necessarily your least expensive design solution: https://www.hansenpolebuildings.com/2020/02/barndominium-one-story-or-two/ And yes, fully engineered post frame buildings can include a full, partial or walk-out basement: https://www.hansenpolebuildings.com/2020/02/barndominium-on-a-daylight-basement/
DEAR POLE BARN GURU: At what point do the post size change from 6×6 to 8×8, thanks. JEFF in SOUTH HAVEN
DEAR JEFF: If erecting a roof only “pavilion” style post frame building, column dimensions are often dictated by L/d ratio as there is no wind load on totally open sidewalls. “L” is unsupported length of column (grade or top of concrete slab to bottom of truss connection), divided by least dimension of column. Playing a part in this is a factor known as Ke. Ke is determined by fixity of columns. On a roof only structure, columns act as cantilevers (think of a diving board), so Ke = 2.1. Looking at your 5-1/2″ square 6×6, 5.5″ x 50 / 2.1 = 130.95″ (or just under 10’11”). This means at an unsupported length of 11 feet, a 6×6 would fail. Obviously, truss span, spacing, and loads from dead, wind and any snow must be properly factored into equations to be verified by your building’s engineer. Adding an eave sidewall (or sidewalls) with open endwalls will cause bending forces to fail most 6×6 columns at lesser heights.
Most pole barn trusses are designed for a one psf (pounds per square foot) bottom chord dead load (BDCL) – inadequate for any type of ceiling. On truss spans of up to and including 40 feet, we include a five psf BCDL as a matter of practice (too many clients decide later on they want a ceiling) and if we know in advance a ceiling is to be installed, we use 10 psf BCDL. You have been fortunate to have at least a five psf BCDL. Your 10 psf bottom chord live load is a non-concurrent (assumes no other live loads are being applied such as snow) one and basically is there to provide a minimum degree of structural integrity allowing for occasional access to an attic space for maintenance purposes.
Controlled mechanical ventilation should be used to control and limit interior moisture levels in severe cold, cold and mixed climates, as well as to limit other interior contaminants in all climates. In other words, all conditioned post frame buildings require controlled mechanical ventilation in order to “breathe”. This necessary air change should not happen via construction of a leaky attic ceiling (or leaky walls) and installation of attic vents. Hence requirement for an air barrier and controlled mechanical ventilation in all conditioned post frame buildings regardless of climate.
No wonder the writer can’t believe it actually worked is because it doesn’t!
Now the fun parts! How wide and tall will this room be? The room width will be approximately ½ of the span of the truss. As the truss span increases, the room width will become slightly less than ½. How tall will it be? Plan on a 7’6” tall finished ceiling. Allowing for ¾” oriented strand board (osb) on the floor and 5/8” drywall, the framed height is usually 7’8”.
I am personally experiencing this the hard way. One of the heating units for my pole barn is in the attic. It needs to be serviced next week, and guess what? Yep – the drywaller neglected to leave an access!
