Tag Archives: roof pitch

Mono-Truss Pitch, Moisture Issues, and Steel Replacement Panels

This week the Pole Barn Guru answers questions about roof pitch on a mono-truss, issues with moisture in Florida, and a reader in need of steel replacement panels.

DEAR POLE BARN GURU: Designing 40 foot MONO trusses for a 200 foot long building and want to know what the pitch should be in North East Kansas. ROBERT in LINWOOD

DEAR ROBERT: My answer is going to depend upon what your proposed roofing material is. If you are using colored steel roofing then minimum roof slope to keep paint warranty intact would be 3/12, this also allows trusses to be built with need for a cap. If you are considering a shingled roof, then minimum slope should be 4/12 – and you are going to have a two part truss as trusses will be around 14 feet tall.

Whatever your building happens to be, it will probably prove more affordable to use gabled trusses with a centered peak. Wind load design considers full truss height in structural calculations and this extra height from monoslope trusses could very well have implications upon other members (especially columns).

In any case, run this by your engineer who is providing sealed plans for your building.

 

DEAR POLE BARN GURU: I have an existing pole barn with moisture issues. Most questions I see deal with heating issues. My situation is different. I live in the very humid central Florida region. The barn has no insulation, the roof had been covered with deep leaves for a long time. The roof clogged with organic material. The outside had had leaves and such built up enough around the perimeter enough to start rusting the lower in many areas.
I have cleaned around the building and want to repair the lower portions of the steel. I am trying to figure out my best plan to moisture proof and insulate the building from the Florida sun. The moisture is the biggest issue the place is like a sauna at times. JAMES in APOPKA

DEAR JAMES: It may behoove you to entirely replace your existing roofing and siding. As you mention you already have rust on sidewalls panels and organic materials on your roof for a long time has probably ruined any paint finish. Once this is done, use a good sealant on any concrete floors. Spray foam interior of all roof and wall panels with no less than two inches of closed cell foam. Your local installers may recommend a greater thickness. Grade around exterior of building to provide at least a 5% slope, from building walls out. Put gutters on eaves with downspouts ending five or more feet away from building. It may be necessary to have a dehumidifier inside your building, once it is all sealed up.

 

DEAR POLE BARN GURU: I need to repair some damaged panels on an existing barn. Do you sell individual panels? JULIE in LA CANADA

 

DEAR JULIE: While we can provide just a few panels, for small quantities you are best to go to the Pro Desk at your local The Home Depot, as they do not get charged inbound freight from steel roll formers. You might pay a little more for panels, however freight savings will more than offset it.

 

 

 

 

 

Roof Slope

Roof Slope

Dang, up jumped an altogether conspicuous term included as a part of every building, and I totally neglected – roof slope.

Growing up in low-middle income level suburbia, I had never given much thought to slope of roofs. This oversight existed even though my father and his five brothers (as well as their Dad – my grandpa Pete) were building contractors. There wasn’t a huge variety to pitch of roofs where we lived, no architecturally designed steep slope roofs.

Once I entered the prefabricated wood truss industry, my perspectives had to change and change fast. Building designers and architects were starting to embrace possibilities afforded to their designs by trusses.

Roof pitch or slope is angle of roof surface above a “flat” or horizontal plane.

Most often roof slope expressions are as “rise” or “pitch”, measured in inches of vertical rise per foot of horizontal distance or “run”. So a four-inch rise roof, also described as a 4 in 12 roof, means for every 12 inches (or foot) of horizontal distance, height of roof increases by four inches.

While roof slopes are typically expressed in “rise”, they can also be expressed in degrees (on occasion) or in percent of slope (rarely). A 4-in-12 roof rises four inches for every 12 inches of run. Same as a 18.4 degree slope, or a 33-1/3 % slope. Four inches of rise per 12 inches of run equals one inch of rise per three inches of run or 1/3 = 33-1/3%.

Definition of Rise: vertical change in height per unit of horizontal movement or run.

Definition of Run: horizontal change in distance, typically in construction we use 12 inches or one foot and express roof slopes as units of rise per single unit of run, such as four inches of rise per 12 inches of run or four inches per foot.

Definition of Slope: angle of change in elevation, expressed in one of several forms

Slope as rise / run, e.g. 4 in 12 or 4/12

Slope as an angle: e.g. 18.4 degrees

Slope expressed as a percent or “grade”: Slope calculated as rise / run or 4 / 12 = .333 x 100 to convert to a percent or 33-1/3% grade

Our AutoCAD drafters happen to prefer pitches given as an angle, rather than slope. It speeds up their ability to create, by not having to calculate angles.

Prefabricated roof truss drawings specify slope as rise / run, although cutting lists sent to manufacturing plant for component cutting are in degrees.

 

The Roof, The Roof… and Sheds without Sidewalls

Mike answers questions about a Roof Line, Roofing with OSB layer, and Endwall Only Sheds:

DEAR POLE BARN GURU: What is the roof line style in the attached picture called? BRYAN in MARYSVILLE

DEAR BRYAN: The prefabricated roof truss folks would refer to this as being a Polynesian roof – one in which the outer portion is at a lower slope than the center portion. I tend to steer clear of steeper-to-flatter roof slope changes as they just increase the possibility of a future leak, however it is certainly doable. Your photo also features a widow’s peak at the center.

 

DEAR POLE BARN GURU: Hello Guru, I am Putting 1/2″ Plywood On the trusses first before the metal roof, what do U recommend to cover the plywood, for vapor barrier protection? I was just gonna use felt paper! any recommendations? Thanx MIKE in ROCHESTER

DEAR MIKE: If applying the plywood directly over trusses spaced every two feet, you will need to add purlins in order to screw the roofing down (screws into plywood or OSB are inadequate for wind uplift). Steel manufacturers recommend the use of 30# asphalt impregnated paper (roofing felt) between sheathing and roofing.

DEAR POLE BARN GURU: What is it called if I only want a structure that has a wall in the front and wall in the back but open on the sides for animal pens? I can’t seem to find ANY images of this type of building. CHRISTINE in BERTRAM

DEAR CHRISTINE: I would call it a loafing shed with both eave sides open. It could be either single sloped, or have peaked endwalls.

Perfect Pitch, Added Expense, and Endwall Roof Leak!

DEAR POLE BARN GURU: Could you tell me the pitch of the roof…for small pole barn…..front wall is 10 foot high…back wall is 8 foot high…rafter span is 13 feet. HERBERT in HAMPTON

DEAR HERBERT: Roof slopes are expressed as the number of inches of rise per 12 inches of run. 24 inches of rise in 13 feet of run would be a slope of 1.846/12 or 8.745 degrees. Some cautions – if you intend to use steel roofing, chances are excellent the paint warranty will be voided due to a roof slope of under 3/12. The side laps of each panel should also have strip mastic placed on top of the under laps to assist in preventing leaks. This slope is also too flat for shingles, unless special considerations have been made for underlayments (most Building Departments require 90# felt paper or an equivalent). I’d recommend you increase the roof slope to 3/12 or greater.

 

DEAR POLE BARN GURU: Can I order specific species with my kit? I’d prefer Doug fir for all the wood, including PT.

Can I choose heavier gauge steel panel for roofing and siding than 29 gauge?

Thanks. JEFF in GREENVILLE

changeDEAR JEFF: You can specify a specific species, however be prepared to pay a premium for doing so, especially in the case of the pressure treated wood. Douglas Fir does not take pressure treating well, so only a few limited chemical formulations are available which will work with it – some of which happen to be highly corrosive to steel. This results in having to place a barrier between the treated lumber and the steel siding, as well as the use of heavily galvanized or stainless steel fasteners.

Most roof truss manufacturers will not guarantee a specific species of lumber will be used in their trusses – so this could become problematic, in your case.

Please read more about lumber species choices here: https://www.hansenpolebuildings.com/2012/03/lumber-species/ (please note this article was written prior to the design values for Southern Pine being reduced).

You may also choose heavier gauge steel panels, however you should read this article first: https://www.hansenpolebuildings.com/2012/01/steel-thickness/

 

DEAR POLE BARN GURU: We built a 30 x50 pole barn with 2 10 ft attached sheds about 1 1/2 years ago. It has sprayed foam insulation and finished inside with sheet-rock. Our problem is every time it rains hard it leaks at the corners on one end. Any suggestions of what we could do to try and find the leak? Thank you so very much for any information you could give us. MARK in SASAKWA

DEAR MARK: I am going to take a WAG (Wild Arse Guess) here. The guess is actually probably not overly wild, since you are telling me the leaks only occur at one end of the building and happen on both sides of the roof.

Your building is 50 feet in length, which means it took 16-2/3 sheets of three foot wide roof steel to cover each side of the roof. This is with the assumption your building has no endwall overhangs. This also means the last sheet of roof steel on one end had to be ripped lengthwise to remove the extra foot of steel. When the rake trim was then installed, there was not a high rib of steel under the trim. When it rains (or snow falls on the roof and melts) water seeps under the edge of the rake trim and into your building. If indeed this is the case, the rake trims on the offending end can be unscrewed, and strip mastic can be used between the edge of the rake trim and the roof steel to keep the water from going where it is not intended to be – screw the rake trim back into place and all should be good once again!

Let me know if this is not the case and we can look for other solutions.