Tag Archives: purlin spacing

Condensation Control, Load Requirements, and A Sloped Site

This week the Pole Barn Guru answers reader questions about condensation control in Spokane, WA, the availability of a hipped roof design to meet wind and snow loads, and planning for a post frame build on a sloped site.

DEAR POLE BARN GURU: I’m in Spokane, WA– a semi-arid region, and I had a question about using metal roof panels with prefabbed integral condensation control, such as Condenstop, along with a double bubble reflective barrier. I have a few left over rolls of the reflective barrier that I can use and would only need to buy an extra roll or two for our prospective 36×36 post frame building. Would it be ok to use both without trapping moisture between the 2? Or, should I only use 1? Building will have continuous soffit venting on both eaves and ridge venting as well. The building will be used in-part to store food-crop and will be temperature-controlled during all seasons, and has drywalled ceilings. Therefore, I’d like to insulate to the max. Was thinking spray foam between the purlins and also fiberglass batts? MATT in SPOKANE

DEAR MATT: I was born and raised in Spokane, owned a house on Newman Lake until just a couple of years ago. In the 1990’s I was the area’s most prolific post frame builder – one year we erected over 200 post frame buildings in Spokane county alone!

Let’s look at doing this right, and not spend money just to spend money.

Spokane is Climate Zone 5A. 2021 International Energy Conservation Code requirements (IECC) are R-60 roof, R-30 walls

Roof: 16″ raised heel trusses, vent overhangs and ridge in correct proportion, roof steel with Integral Condensation Control, blow in R-60 of granulated Rockwool.

Sell your reflective barrier on Craigslist or Facebook Marketplace.

Walls: Steel siding, Weather Resistant Barrier (Tyvek or similar), 2×8 commercial bookshelf girts, R-30 Rockwool batts, well-sealed vapor barrier.


DEAR POLE BARN GURU: In your blog you state that hip roof style the strongest against high winds. as I live in tornado alley wind and snow drifting are concerns of mine and would like my building design to be highly resistant to these forces. Why can I not find examples of this roof style on your web site? Is the gambrel style the closest you come to this design? CARY in RAYMOND

building-plansDEAR CARY: Very few clients have been willing to make an extra investment into full hip roofs, explaining why our website has no photos of them (we do rely upon our clients to provide photos). We can engineer traditional (and most cost effective) gable roof designs with wind speeds in excess of 200 mph. Our Building Designers can incrementally adjust design wind speeds to allow you to make decisions to best meet your concerns and budget.

While most roof truss manufacturers meet Code requirements for unbalanced (drift) snow loads, we are one of few (if not only) building providers who also design roof purlins appropriately to resist these same loads. This typically results in purlins closest to ridge to be either more closely spaced and/or larger in dimensions.


DEAR POLE BARN GURU: We purchased property in north central Tennessee and are planning a post frame barn for a RV and SUV storage and then later adding a post frame home as our forever home. Building site has about a 5 – 10 degree slope. Can you recommend some reading material, books, articles, how-to’s that I can learn and start making some educated research and decisions? thanks. JEFFREY in PRAIRIEVILLE

DEAR JEFFREY: A plethora of options are available for sloped sites. They can be excavated to create a “walk-out” or “daylight” situation. I was faced with this situation on one of my personal buildings (albeit with a more extreme slope): https://www.hansenpolebuildings.com/2019/05/solving-massive-pole-building-grade-changes/
Sites can also be built up: https://www.hansenpolebuildings.com/2020/01/supporting-fill-when-considerable-grade-change-exists/
And there is always an option of “stilts” https://www.hansenpolebuildings.com/2020/03/stilt-home-barndominium/
For research, a great source of information is always to navigate to www.HansenPoleBuildings.com, go to SEARCH in upper right corner, type in whatever topic you are looking for information on (e.g. BARNDOMINIUM) and hit ENTER. Over 2000 articles are available, covering a broad myriad of subject matter.

Whatever route you do ultimately pick, fully engineered post frame is likely to be your most cost effective and energy efficient structural design solution.



Condensation Prevention, Sound Proofing, and Snow Loads

This Wednesday the Pole Barn Guru tackles reader questions about condensation prevention by placing foam board over the top of 2×4 purlins, options for sound proofing a post frame home, and if 2×4 flat purlins can hold a specific snow load.

DEAR POLE BARN GURU: I am planning a pole barn; the roof will have 2×4 purlins spaced 24″. I plan on putting 1/2″ foam board (faced on both sides) over the purlins with the seams taped. Then putting metal panels over the foam board. Will this be sufficient to prevent condensation from dripping within from the roof? JIM in MIDLAND

DEAR JIM: Good news and bad news…. while your proposed solution will likely take care of possible condensation issues, it is structurally unsound. Post frame (pole barn) buildings rely upon shear strength of steel roofing and siding in order to remain stable. When you add in even a half-inch gap of non-rigid material (foam board) between framing and steel skin, you greatly reduce (or eliminate) shear strength of your steel panels. Even if your building were to remain standing, shifting of steel panels with small diameter (#9 or #10) screws will cause slots to form around screw shanks, eventually causing leakage.

You would be best served to order roof steel with an Integral Condensation Control (read more on I.C.C. here https://www.hansenpolebuildings.com/2020/09/integral-condensation-control-2/) factory applied. It will be less costly, easier to install and not cause negative structural issues.


DEAR POLE BARN GURU: Afternoon! Thanks for all the great info in your blog. It has really helped me plan my new home. I have one question: I’m having a post frame home built and am wondering about soundproofing. The roof(s) will be steel directly on purlins with a radiant barrier in between. The conditioned space will have R-60 blown-in fiberglass above it so I’m not worried about that. What I am concerned with are the front and rear porches. The front will have a, 8″ shed roof porch and the rear will have a 16×16 gable-roof screened porch. Both will have finished ceilings, steel and knotty pine respectively. I’d like to be able to sit under either when it’s raining and it not be loud. What would you suggest I do to mitigate the drum effect in an uninsulated porch? I’ve thought about using rockwool batts, maybe mass-loaded vinyl, or even painting the underside of the steel with liquid rubber before the panels are screwed down. Any advice? Thanks much. PETER in HUDSON

I am pleased my information has proven helpful to you. I would consider one of two options – installing steel in these two areas over solid sheathing (OSB or plywood) with 30# felt or a synthetic underlayment, or using two inches of closed cell spray foam applied directly to underside of roof steel. Either of these should assist in mitigating sound.


DEAR POLE BARN GURU: In an area with a 50 p.s.i. snow load ground rating and trusses 4′ o.c., can 2×4’s 2ft. o.c. be laid flat for roof purlins? STEVE in WYOMING

DEAR STEVE: Maybe – this will depend upon numerous factors including (but not limited to):

Risk Category of building
Roof slope
Wind Exposure
Roofing material
Snow retention systems
If building is heated or not
Available grade of lumber

Ultimately this decision should be made by whomever your engineer is who is sealing your building plans. Special care should also be taken to insure adequacy of purlins in drift zones (closest to ridge), where purlins may need to be closer spaced, higher grade and/or larger dimension material.

Also – ground snow loads (Pg) are typically expressed in p.s.f. (pounds per square foot), rather than p.s.i. A p.s.i. of 50 would equate to 7200 p.s.f

Roof Purlins for a Bar Joisted Lean-To

Roof Purlins for a Bar Joisted Lean-to

Reader JIM in MOORESBURG writes:

“First, I have spent the last 3 hours reading around your site and am impressed. I appreciate the manner you answer questions on your site”, respectful and factual with no hint of disrespect (even if deserved) in your replies. Professional and well done! Second…My question is about correct purlin sizing/spacing on a lean-to shed roof 48ft long 25ft deep with 12ft bar joist spacing spanned by these purlins. (this is an open front machine shed). I wish to size for the snow load that occurs here (mountains of East Tennessee!) about once every 15 yrs which is 2 ft of snow. Purlins will be bolted into “U” shaped brackets welded to the tops of the bar joist similar to the design used on many kit pole barn metal trusses. My attempt to calculate purlin size/spacing results in using 2x8x12′ purlins at 24″ spacing but too many assumptions on my part to be comfortable yet other detail…Bar joists attached to post (in ground) adjacent to an existing wood barn. Roof Metal is Type “R” deep rib. Yes…this is DIY with materials from my surplus pile:) thanks.”

Thank you for your kind words. Our eldest son and his lovely bride lived in Maryville for many years, so I know where you are and have driven in your area.

Two feet of snow should give roughly a 25 psf ground snow load (https://www.hansenpolebuildings.com/calculating-loads/). Without an engineer’s review, your attempt at calculations look as though they should be plenty adequate – PROVIDED:

Roof slope of existing wood barn and lean-to are the same and in the same roof plane.

Otherwise you will have either an accumulation of snow at a steeper-to-flatter roof plane, a slide off load, or drifting to account for. If any of these are your case, then you should contract a Registered Professional Engineer for an accurate structural design. Any of these situations could easily cause an overload of your bar joists as well and should be validated.

Carport Attachment: Part II

We Don’t Always Do Things Perfect, But We Do Listen Part II

Last summer Hansen Pole Buildings Supplied a pole building kit package to a client who experienced a few challenges and took the time to address them.

Here is a portion of the email I was responding to:
“On the design flaws, and other issues, here is what I have experienced.

If you just joined in this blog – read yesterday’s blog for Part I in a 4 part series from a client who experienced some construction challenges…

“2-Carport attachment.

My plans have a 12 foot carport section on the front.  The engineering on this section is very confusing.  The plans state that the double truss system is supposed to be used but, the outside truss is supposed to lowered by roughly 6 inches to make room for the purlins to travel over the top of it.
With this system there is no double truss.  There is one truss that is supporting the load of two sections of roofing.  To further complicate the matter, the trusses that were engineered and supplied were constructed with 2 X 6’s.  What does this do, well, when I lowered the outside truss 6 inches there is absolutely NOTHING to attach that truss to the other truss.  A few spots of webbing is it.  This seems like a totally weak link in the system and I cannot figure out why this was done this way.
It seems to me that you would have run a normal double truss and then nailed a siding backer onto the outside truss for tin placement.  This dropped truss created other issues as well.
When placing the X bracing for the carport as well as the first bay in, the X is placed at 2 different locations on the bottom attachment.  On the carport section the X bracing is attached to the outside dropped truss, 6 inches below the other truss.  On first interior bay the X is attached to the upper truss.  The two braces are not pushing on each other.  And, I have no idea how I am going to place the tin on this section because I have a 2X4 10 inches above my garage door.  Am I supposed to cut around the 2X4 and try and figure out how to make it work?”

carportMy response: Actually in the hundreds of buildings we have provided with carports, you are quite honestly the first person who has ever brought forth any of these issues as being challenges. Your bringing this to our attention is greatly appreciated.

With Hansen Buildings knowing the carport to enclosed portion the trusses were not going to be at the same height (per the plans), the trusses for your building were ordered and designed to work with a single truss placed every six feet (the worst case scenario).

One truss does not support two sections of roofing, as the end of the purlins in the direction of the carport rests on top of the top chord of the lowered truss. Each truss at this juncture is supporting six feet of roof, just as it was designed to be.

Your recommendation of keeping both of these trusses at the same height and having a siding backing nailed to the face will be taken under serious advisement. The challenge will be, in many instances, the wall columns in this area are 4×6, oriented with the wide face against the wind. In those circumstances, it would be impossible to notch three inches into a 3-1/2” thick column. We are in the process of discussing with our engineers turning the corner and endwall columns 90 degrees to be able to notch in the two trusses, as you suggest. Because we do so many different applications, we need to see if this will cause other challenges.

The idea of the X braces is not to push against each other – it is to create a rigid brace frame which is restrained against buckling in both directions and transfers load into the roof diaphragm. There is no structural reason for them to be at the same height.

Install the steel siding on this wall first, then the X brace. A small slot can be created, using a punch, to slide the Simpson LSTA12 bracket through for attachment.

“3-Girt Spacing

The girt spacing is an interesting way to save a few hundred bucks, probably not mine.  The plans call for the girts to be about 40 inches on center.  I know this is not exact but close enough.  This is the absolute maximum allowed by the tin specs, I know because I called and spoke with the engineer.  Common practice is girts places 2 feet on center.  It does not take a rocket scientist to know that when those girts are placed in a funky location, like 40″ OC, you cannot hang insulation, drywall or anything else without a bunch of waste or owner supplied materials.  I chose to purchase my own 2X6’s to build the thing at 24″OC.  It cost me a whopping $250 for the lumber and a bunch of headache every place else.  I now need to order more screws because I do not have enough for attachment.  You would think that for $25,000 it could be done right without cutting corners.”

My response: There exists no “common practice” for the spacing of wall girts, other than what is needed to support the given load conditions.  Your building, with the loads imposed on it – came out to what we designed, 37-5/8″ spacing.

This is not about “cutting corners”, as with any structural member of a post frame building, the wall girts are located and spaced to carry the loads (in this case wind) being placed upon them, without added pieces being placed “just because”.

The information provided by you, in your request for your initial quote, indicated: “insulation_options: cold”, which precluded us from knowing your intention was to insulate the walls of your building. This is just one of the many reasons why we have each of our clients review and approve their building plans online prior to materials being ordered. At time of review you could have asked why the girts were spaced at 37-5/8” on the plans, or requested any spacing you desired, which would have gotten you not only the extra lumber, but also the screws. There is no indication in the notes in our records your intentions of insulating the building at a future date, or even of it having been discussed.

As for the spanning capabilities of the steel – your building’s steel roofing and siding is Imperial Rib® manufactured by American Building Components. In looking at the span tables for this product, for 29 gauge over three spans (crossing three or more framing members) and spanning 3.5 feet (42 inches), the allowable minimum loads in pounds per square foot (psf) are 54 for positive wind force and 53 for negative wind force. The formula to convert wind speed in mph (miles per hour) to force is Speed^2 X .00256 = psf, so force to speed is the square root of psf divided by .00256, or 143 miles per hour. AS the design load for your building is 100 mph, it doesn’t appear “This is the absolute maximum allowed by the tin specs”. I’d be happy to supply a copy of the chart, should you desire.

Tomorrow’s part III of a 4 part answer from Mike the Pole Barn Guru to a challenged client deals with ceiling loaded trusses. See you then

Dear Pole Barn Guru: How to Replace a Sliding Door with an Overhead

New!  The Pole Barn Guru’s mailbox is overflowing with questions.  Due to high demand, he is answering questions on Saturdays as well as Mondays.

Welcome to Ask the Pole Barn Guru – where you can ask questions about building topics, with answers posted on Mondays.  With many questions to answer, please be patient to watch for yours to come up on a future Monday or Saturday segment.  If you want a quick answer, please be sure to answer with a “reply-able” email address.

Email all questions to: PoleBarnGuru@HansenPoleBuildings.com


DEAR POLE BARN GURU: Have pole barn with sliding doors which are being wedged with weather changes. Looking for overhead door option for door that is 16′ wide and 12′ tall. Do you provide these and conversion labor to install? LOOKING IN LEBANON

DEAR LOOKING: Switching from sliding doors to an overhead door is going to pose a massive challenge to do correctly. This, in itself, is reason enough to spend the generally few dollars up front to use a sectional steel overhead door.

To begin with, the openings are not framed to the same size. It is easier to frame smaller than have to try to hack out and replace one or more columns. This will probably entail framing down to a finished hole 13’10” in width and 10’11” in height (measured from the top of the concrete floor) and installing a 14’ x 11’ residential overhead door. In order to get things looking right from the outside. All of the steel on this wall should be replaced, to give uniform color and no splices.

We can certainly provide a wall’s worth of steel siding, color matched powder coated screws, the appropriate steel trims, the overhead door and hardware to hang it. We are not contractors in any state, so we do not and cannot provide any labor to install.

You may want to look at what the real problem is – sounds like you have frost heaving, which is pushing the ground, or concrete, up at the location of the door. Just switching doors is not going to take away the problem.

If heave is the root cause of the problem, then remedial action can be taken by installing a French drain along the side of the building in front of the door. The sliding doors can also be taken off, and their overall height shortened enough to keep them from binding when the heave occurs.

DEAR POLE BARN GURU: How do I calculate what size of purlin I need based on my snow load, and the bay spacing of my pole barn? Thanks. CURIOUS IN CULDESAC


DEAR CURIOUS: From the ground, a roof purlin looks pretty simple – it is usually a piece of 2x material, fastened on top of or attached to the side of rafters or roof trusses. Roof sheathing (typically OSB – oriented strand board, plywood, or steel roofing) is then attached to the top of the purlins.

Purlins are not simple at all. They must carry all applied dead loads, live loads from snow as well as wind loads. They need to be checked for the ability to withstand bending forces (both compressive and from uplift), to not have too much deflection and be adequately attached at each end.

In snow country, purlins near the roof peak need to be checked for the added drift loads which are applied.

I could spend several thousand words and numerous pages to teach you how to be able to properly calculate the purlins for your individual case, however it is far more information than the average person wants to, or is able to, absorb.

The best recommendation – hire a registered design professional (RDP – architect or engineer) who has the ability to run the calculations to adequate design your purlins based upon the climactic (wind and snow) loads being imposed upon them at your building site. Or better yet, order a complete pole building kit package which has been designed by an RDP.