Tag Archives: flat purlins

Greyed Lumber, Insulation, and Flat Purlins over Trusses

This week the Pole Barn Guru answers reader questions about cleaning up rough cut lumber that has greyed from exposure to the elements, advice on house wrap and insulation, and the ability of flat purlins over trusses to carry a load in Kentucky.

DEAR POLE BARN GURU: My barn project has been a long drawn out process. The project stalled for 6 months but has picked back up again. I’m using rough cut lumber. Unfortunately, the wood has a grey color to it (probably from dirt, mold or algae on the surface).

What’s the best way to clean it to make it look fresh/revived again? Any products that you recommend?

Thank you again for all your help and advice. JAMES in MILTON

DEAR JAMES: Clean with sodium percarbonate or hydrogen peroxide, then apply oxalic or citralic adid (second step restores wood to its natural pH and neutralizes sodium percarbonate cleaner).


DEAR POLE BARN GURU: I added a lean-to to my 60×90 pole barn. The builder put house wrap around exterior walls. When the tin guys put up the metal they put bubble wrap over the house wrap then the metal. I spray foamed with closed cell inside 2 inches. The interior will be knotty pine so do I need to put another barrier before the wood? Also on the roof they put the bubble wrap under the steel I will have blow in on top of the knotty pine. I plan on putting plastic sheathing before the knotty pine. Is this the correct way of doing or should we change something? SCOTT in KOUTS

DEAR SCOTT: It was bad enough when your tin guys put bubble wrap over your housewrap. Compounding your having spent your hard earned money on both, is closed cell spray foam should have been applied directly to inside of steel siding. Water under a bridge at this point. You should fill balance of wall cavity with unfaced rock wool and no interior vapor barrier. Wall will now dry to inside (meaning you may have to mechanically dehumidify). You did not say if your added lean to has an attic space or not. If your intent is to insulate with plane of roof (purlins) here is some guidance: https://www.hansenpolebuildings.com/2023/10/properly-insulating-between-roof-purlins/

If you are insulating above a lower ceiling height (as you say blow in – I will guess this is your case), in your Climate Zone 5A a vapor retarder should be on warm in winter side of insulation (not a vapor barrier, like plastic sheeting). A vapor retarder could be as simple as kraft facing from batt insulation, or latex ceiling paint. Make sure to adequately vent any non-conditioned attic space.


DEAR POLE BARN GURU: I love reading your posts. I’ve learned much about your ideas and it’s changing the way I think. Thank you. Iowa and SD have different wind and snow loads than central KY. I routinely see farmers building barns with 2×4 purlins laid flat on trusses spaced every 8 feet. Your designs call for 12 spacing, which I love, and 2×6 purlins on edge in joist hangers. Would 2×4 purlins in joist hangers work in central KY?


Welcome to Kentucky road sign at the state borderDEAR CHRISTOPHER: Thank you for your kind words.

We have provided fully engineered post frame buildings in places with no snow, to places where snow load is over 400 pounds per square foot – so we have pretty much seen it all!

Those farmers laying 2×4 purlins flat (wide face to sky) spanning eight feet are risking not only their buildings, but their lives. I am amazed they can even apply roof steel to them without failures.

For 12′ spans, without snow, purlins on edge, 2×4 2400 msr roof purlins 24 inches on center would carry loads, however would overly deflect. You could probably use 2×4 #2 Southern Pine at 12 inches on center, however 2×6 #2 at twice spacing would be more economical both in materials and labor.

Read more about msr lumber here: https://www.hansenpolebuildings.com/2012/12/machine-graded-lumber/

Dear Pole Barn Guru: Can I Lay Purlins Flat?

DEAR POLE BARN GURU: With trusses spaced 6 feet on center can you lay 2×4 purlins flat? Curious in Columbus


DEAR CURIOUS: In order to answer your question properly, I will have to make a whole bunch of assumptions. I will assume this is a single truss, code minimum snow load, 4/12 roof slope and 29 gauge steel over purlins 24 inches on center (o.c.).

The purlins have to be able to support their own weight of about 0.5 psf (pounds per square foot) plus the steel 0.63 psf.

As the roof live load of 20 psf, is being carried by members 24” o.c. with the slope of the roof, the live load can be reduced using the cosine of the roof slope (0.949) to get the applied load of 18.98 psf. Adding the dead loads, the total load to be carried is 20.11 psf.

To calculate whether a 2×4 will work or not, we use this formula:

(Roof live load + roof dead load) X spacing  X (span of purlin)^2

Divided by:

8 X (Section modulus of a 4×2 of 1.3125) X Fb (fiberstress in bending) X 1.15 (duration of load) X 1.15 (Cr, repetitive member factor as the purlins are spaced 24” o.c. or less) X 1.1 (Cfu, the flat use factor for applying the load to the wide face of a member)

If the resultant is less than 1, then the 2×4 works in bending.

Let’s try 2×4 std&btr (standard and better) SPF (Spruce-Pine-Fir), which is the most common framing material found at lumber yards.

20.11 psf X 24” X 6’^2 / 8 X 1.3125 X 550 X 1.15 X 1.15 X 1.1 = 2.068

Upgrading to #2 SPF (not available in most lumber yards) with a Fb value of 1312.5 which plugged into the above equation yields a result of 0.87, so it works in bending.

However – it must also be checked for deflection (while a member might carry the load without breaking, if it bends too far, the roof will have “waves” in it.

In Table 1604.3 Footnote a of the IBC (International Building Code), “For secondary roof structural members supporting formed metal roofing, the live load deflection shall not exceed l/150”. The “l” being the span of the structural member (roof purlin in this case).

Being as liberal as possible, using the distance between the trusses every 6’, gives us a “l” value of 70.5”. This means the maximum allowable deflection would be 0.47”.

To calculate the actual deflection, this formula is used:

5 X Ps (roof snow load adjusted for slope factors) X purlin spacing X purlin span^4

Divided by:

384 X E (modulus of elasticity for the lumber being used) X I (moment of inertia for the lumber size)

Plugging in the variables:

5 X 18.98 psf/144 (has to be divided by 144 to give psi) X 24” X 70.5”^4 / 384 X 1,400,000 X 0.984 = 0.795”

Bottom line: As the deflection is far greater than what is allowable, a 2×4 #2 roof purlin laid flat would not meet the requirements of the Code, or sound engineering design.