Tag Archives: roof peak

How to Best Use Roof Steel Already Owned

How it is people end up owning steel roofing (or siding) when they do not have a structure to put it on has always been somewhat of a mystery to me.

Reader TIM in IRON RIVER writes:

“Hoping to build 40’x56′ post frame structure with 2′ overhang and 4/12 pitch using steel roofing I have on hand. That said it looks like I need roof lengths of 23′-1 1/4″ long to get that 2′ overhang. I have plenty of 20′-8″ and 2′ steel roofing and when combined with overlap I’ll be short (20′-8″ + 2′ minus 4″ overlap = 22′-4″) approximately a 9″ overhang. We do get plenty of rain here so the more overhang the better. For a 4/12 pitch how much overlap should I have for two adjoining roof pieces? Is there a vent gap at peak and if so how big of gap? Would 9″ overhang be sufficient for a rainy area to maintain building longevity? I guess I could go shorter on the 40′ width in order to use one single roof piece and to attain 2′ overhang. Your thoughts please.”

Mike the Pole Barn Guru responds:

If at all possible I try to avoid steel panels overlapping steel panels along their run.


Because all panels have been run through an identical set of dies, they are not designed with ‘outer’ panels having slightly larger ribs in order to accommodate a smooth lap transition. From experience, this overlap will attempt to “grow” on you as you go along your roof.

Most steel manufacturers recommend a minimum end lap of 12 inches. All overlaps should be made so as to center on a roof purlin.  A 3/32″ x 1/2″ butyl endlap sealant should be applied on the bottom panel just below the centerline of the purlin where lap will occur. Due to the probability of panels not lying smoothly, sidelap sealants may also be needed.

Roof panels from each side should be held down from the peak of the roof by several inches (generally no greater than four with standard ridge caps) to allow for ventilation.

With all of this said, if it fits on your property, you should consider going to say 36′ x 60′ (or 72′), with 18″ overhangs to best utilize material you have and eliminate any overlapping splices. With 18″ overhangs on ends also, three foot width panels will lay out nicely without having to rip a panel when getting to building ends. Either of these building lengths will work out well for framing material usage with double trusses aligned with columns every 12 feet.

Steel Ridge Cap to Roofing Overlap

Hopefully no one wants to create a roof with leaks. Reader MIKE in HARBOR CREEK wants to make sure he is doing things correctly. He writes:

“How much overlap do you have to have with roofing and ridge cap? Is 2.5″ enough and then you use metal to metal screw you do not have to penetrate the purlins?

I cannot vouch for how other building providers assemble their buildings, so I will go with how we do it.

To calculate a building’s roof steel length we take one-half of the building’s span (or horizontal measure from peak/ridge to the outside of columns) and multiply this times a factor for roof slope. 

For slope factor – multiply slope by itself and add 144. Take the square root (use a calculator) of this number and divide by 12.

Example to calculate slope factor for 3.67/12:  [3.67 X 3.67] + 144 = 157.47. Square root of 157.47 = 12.549. Divided by 12 = 1.0457.

For a 40 foot width gabled building with a 4/12 slope this length would be 21.082 feet (call it 21’1”).

Outside of columns at eave we have a 2x of some sort as an eave strut, with a width of 1-1/2 inches and roof steel must overhang this by 2-1/4 to 2-1/2 inches. Using 21’1” for our roof steel length, this means the top edge of roof steel will now be four inches from the peak/ridge.

Standard steel ridge caps are generally very close to 14 inches in overall width, giving somewhere around three inches of overlap on each side. Placed in this overlap will be either a form fitted outside closure strip or a vented closure (https://www.hansenpolebuildings.com/2015/11/ridge-cap-foam-closure-strips/). Either of these products properly installed will prevent weather (rain and/or snow) from being driven beneath the ridge cap into your building. You can read a little more on correct placements of closures here: https://www.hansenpolebuildings.com/2015/11/outside-closure-and-vented-closure-installation/.

By using metal-to-metal stitch screws to attach the ridge cap to high ribs of roof steel, there is no need to have to miraculously hit any ridge purlins with screws. Here is a brief tale involving a builder who went off on his own tangent https://www.hansenpolebuildings.com/2011/12/stitch-screws/.

In summary Mike, provided you have a 2-1/2 inch overlap, have used proper ridge closures and stitch screws your life will be good and you will have a happy end result!