Tag Archives: pole barn truss

Roof Truss Costs, Moisture Barriers, and Integrated Condensation Control

This Wednesday the Pole Barn Guru addresses reader questions about “the most cost effective length to procure, transport and install of a Post Frame Roof Truss 4/12 Pitch; 36′, 40′ or 50′?” a suitable moisture barrier for a shed, and Integrated condensation controls.

DEAR POLE BARN GURU: Typically, what is the most cost effective length to procure, transport and install of a Post Frame Roof Truss 4/12 Pitch; 36′, 40′ or 50′? Question is relevant to a mixed-use agriculture building. Thank you for your assistance. LYNNE in EDINBURG

DEAR LYNNE: Transportation can often be a limiting factor. In order to avoid pilot cars and over width permits, maximum truss height is limited to 102″ (8’6″). A 50′ span, 4/12 slope truss will normally be roughly 108″ tall, plus any overhang “tails”. So, this would entail an over width permit. Now most truss companies purchase year-long over width permits for their trucks, so this cost is negligible. Most states do not require pilot cars, unless loads are 12 foot or greater in width. This allows for 60 foot span trusses to be hauled without expensive pilot vehicles. As to procurement, while you will pay more per lineal foot of truss as spans increase, you will also need fewer trusses. For an agricultural building, I always encourage clients to build as large as they can economically justify and have space to build on, as it will never be too big. Keeping your building length to three times building width will also help with your budget, as these ratios are typically within shear load carrying capabilities of properly fastened steel roofing and siding.


DEAR POLE BARN GURU: I have a 12′ x18′ unheated shed framed and ready for the metal roof and siding. The roof will anchor to untreated 2″x4″ purlins. Is it ok to use Tyvek or a similar wrap to cover the seams from the top of my purlin ends and hang over the rake rafters that will be covered by the metal siding???? Is this an acceptable secondary water barrier between my roof and wall seam? My shed will most likely have a crushed limestone floor. It will be unheated and “unfinished” on the inside. No windows and 2 barn doors across from each other. Thank you so much. MACK in BATON ROUGE

DEAR MACK: Before placing your limestone floor materials, install a well-sealed vapor barrier (I recommend 15mil in order to minimize potential for punctures). This will help to minimize, or even eliminate condensation challenges. Order your roof steel with an Integral Condensation Control (Condestop, Dripstop or similar) factory applied. Cover walls with Tyvek or similar before installing wall steel.

If purlins cross rake rafters on endwalls, install 2×4 blocking between them to seal any open spaces.

Order steel Rake/Corner trims to cover last rib of roof steel and top of endwall steel (similar to below):


DEAR POLE BARN GURU: Building a 50x36x12 pole barn with 4:12 roof pitch. Attic space will be unconditioned with blown in cellulose insulation in ceiling and batten on walls. Attic will be vented with soffit and ridge vent. Walls will have exterior house wrap under metal panels. Question is, what to put under the roof panels? Just a vapor barrier or a dual purpose vapor/radiant barrier? I hear different opinions on placing radiant barrier under roof. STEVEN in SUGAR LAND

DEAR STEVEN: I would order your roof steel with an Integral Condensation Control (Dripstop, Condenstop or similar) factory applied. You can read more about these products here: https://www.hansenpolebuildings.com/2020/09/integral-condensation-control-2/



Changing Bottom Chord Height

At least in my generation we were raised to believe the experts – whether it be doctors, attorneys, or building contractors really knew what they were doing. I know I have the expectation and I believe our clients expect the same from us – as they should.

Truss Drawing - Bottom ChordI recently wrote about an article my Uncle Neil had written for the Journal of Light Construction (JLC). Just out of being curious I did a search under “pole barns” and was amazed when I found the thread below (keep in mind these are JLC-Online Expert Forums).

Here is the question posed:

I would like to raise the bottom chord on two trusses in my pole barn. 
I have set many a floor truss and roof truss and know that this kind of modification is based on approval from an engineer, I was just wondering if any of you have done this in the past. The trusses are 42′ long with a 2X10 bottom chord, 3/12 pitch. I would like to raise it 12 to 16″ inches from the bearing point out 12 feet.

And the responses from the “experts”, and my take on each one:

Should be possible. We have done similar things using LVL and plywood gussets. Contact your engineer and have him draw you a repair.

At least this one recommends having an engineer involved. What the Original Poster (OP) is asking for is a major repair, and is very likely economically impossible (I always tell clients we can design nearly anything, as long as they have enough money to pay for it).

In the few cases where I’ve needed a truss repair I’ve always gotten them from the truss manufacturer… lo and behold they’ve been asked before and have a filing cabinet full of ready-to-go solutions.

I owned two prefabricated metal plated wood truss manufacturing plants over 17 years. There is no such thing as a filing cabinet full of ready-to-go solutions.

If you are only modifying two trusses you could sister the top cords with 2/12 rafters, (I get 21’ 7 ¾”, full rafter length, 3/12 pitch at 21’ run, btw, I also get a lateral change distance of 5’ 4” for a 16” rise, not 12’, so perhaps I’m not understanding you completely, the maximum new bottom cord length would be approximately 30’ 9”, if the seat cut of the new rafters are full bearing @3 ½”, not likely.) Nail them solid from both sides, pop your new ceiling line on the trusses to cut, cut on the line and cut out of the 2/10 bottom cord that is being raised to fit directly under the new rafters, nail well to the cut webbing from both sides. Add substantial gusset scabs over both sides of the new connections at the 2/10 to 2/12 rafters. Finish cutting the bottom cord as needed. Modify each bottom cord one at a time.

Without seeing it, this sounds like the modification I would entertain. Proceed at your own risk. Also, call the Truss Co and speak to their engineer.

I am guessing this expert is NOT a registered design professional (RDP – architect or engineer), but they are passing out dangerous advice as if they knew what they were talking about. No RDP in their right mind would give instructions like these (or any others) without, at the very least, a copy of the original sealed truss drawings.

And in the end, from the OP:

Thanks for the replies. I am going to call Cleary the builder and see if they have any stock
drawings for this application.

I would like to have been the fly on the wall at Cleary Buildings when this call came in, especially after the expert advice given by the experts. Pole building trusses are typically engineered so the design loads are very close to capacity (to do otherwise would be bad business on the part of the builder/supplier/fabricator).

My best guess as to a solution – would be to order scissor trusses strong enough to carry the loads and place them alongside the existing trusses, then remove the portions of the old trusses which fell below the scissors truss bottom chord. In any case – a truss should never be cut or modified in any way, without involving a RDP. Keeping your roof up, could be a lifesaver. Maybe yours!