Truss Spacing and load question?

[City Farmer]

Hi all,
I'd appreciate a little help. I'm fixing up a barn on my new property.

The Bad: The metal roof is leaking around the nailed on rubber washers, theres a few (bullet?)holes the roof, the exterior is dented and it's been painted a few times.....so I'd like to replace it all.

The Good: The structure is in great shape and all the poles are solid. When I shoot it with the laser, it's only out 1/4" in 40'. I want this to be my primary heated workshop when I'm finished.

The barn is 24'x40', engineered trusses 4' OC, Purlins 2' OC laid flat, 3/12 pitch, 2x10 truss carrier, and 8' pole spacing. .
Can I sheet it with 1/2" OSB, shingle it and still meet the snow loads in SE Michigan? I've been looking and I cannot find the info. I'm looking for.

 

[1stDeuce]

You're asking questions that can only be answered by a structural engineer. What can be said is that sheeting with OSB and shingling the roof will be quite a bit heavier than the current steel roof. That weight WILL reduce the snow load capacity of the roof by the amount that the roof weight increases.

What you don't know is what snow load it was designed to hold in the first place... It may be that the trusses are a bit over-designed, or they may be right to the limit with design snow load. Depending on your required snow load numbers, it may also be that the 2x10 truss carriers are actually the weak point.

My suggestion is to rip off the nasty steel roof and replace it with a new steel roof. Use foil backed insulation when you do so, as it will prevent condensation, and add a good amount of insulating effect. If you plan to heat it, you may still want some additional insulation, but you may also find that it doesn't pay to add much more if you don't plan to spend a lot of time out there in the winter. A few ceiling fans to send the heat back down might be money better spent.

FWIW, finishing the ceiling and insulating it will also reduce the snow load capacity of the roof... Any weight you add reduces the carrying capacity. The only way to know how much is to get a structural engineer to do an analysis.

Good Luck!

 

[RedNeckRacin]

Are you saying you want to overlay the existing purlins with 1/2" osb and still meet the snow loading?

 

[City Farmer]

Are you saying you want to overlay the existing purlins with 1/2" osb and still meet the snow loading?

Yes. Ideally, I'd like to shingle it.

 

[City Farmer]

You're asking questions that can only be answered by a structural engineer. What can be said is that sheeting with OSB and shingling the roof will be quite a bit heavier than the current steel roof. That weight WILL reduce the snow load capacity of the roof by the amount that the roof weight increases.

What you don't know is what snow load it was designed to hold in the first place... It may be that the trusses are a bit over-designed, or they may be right to the limit with design snow load. Depending on your required snow load numbers, it may also be that the 2x10 truss carriers are actually the weak point.

My suggestion is to rip off the nasty steel roof and replace it with a new steel roof. Use foil backed insulation when you do so, as it will prevent condensation, and add a good amount of insulating effect. If you plan to heat it, you may still want some additional insulation, but you may also find that it doesn't pay to add much more if you don't plan to spend a lot of time out there in the winter. A few ceiling fans to send the heat back down might be money better spent.

FWIW, finishing the ceiling and insulating it will also reduce the snow load capacity of the roof... Any weight you add reduces the carrying capacity. The only way to know how much is to get a structural engineer to do an analysis.

Good Luck!

I didn't know if there was a standard on the pitch, span and truss spacing for shingled and metal roofs. I'll check later this afternoon on the truss carrier size, they might be 12".

 

[1stDeuce]

There is not. It has to be engineered based on wind, snow, and dead loads. Or rather, it should be... Bit many are put up based on guesses and luck...

 

[RNeumann]

The building was engineered and approved as it stands now. Having said that I haven't seen 1/2" sheathing for a roof in snow country-yes in coastal California though. Typically folks sheet with 5/8" ply or more typically OSB. They also use the Simpson 5/8" plywood clips between the 2' span- this prevents uneven movement between sheets.

This is the first time I've ever posted about NOT getting an engineer. However, over sheeting an existing roof is common practice for various reasons- including yours.

Simpson Strong-Tie 5/8 in. 2-Gauge Plywood Sheathing Clip (25-Pack)-PSCL 5/8 - The Home Depot

 

[CADplans]

I am not a carpenter, but, I know a LOT about stress,,,
I can only guess as to all your abbreviations, but, if I am correct,,,
it sounds like the roof you are trying to build is far worse (structurally) than my house in Virginia.

Switch from metal, to shingles, the snow will never slide off.
If the roof is to flat, the snow will never slide off (a 3/12 pitch is flat,, IMHO)

I built my 45X57 shed,, my metal roof covers the steel supports
I can not use words like "purlins, etc" because I did not use conventional techniques.
My metal roof spans 4 feet between the supports.
The "local carpenters" that helped me thought the roof would not survive the first snow.

Actually, my steel is so strong, you could park a car on it,,, literally.

Be careful about replacing a metal roof with wood.

This pic shows the 4 foot gaps between the roof metal support beams

The tan roll up door in the pic is 18 feet wide.

I use the word beams, the roof is supported by actual I-beams

This saw horse is made out of some of the beam material,

 

[KennyG]

As others have said, trusses are specifically engineered and it's pretty hard to say what they were designed for in an old building. That said, here in Michigan, most trusses are designed for either steel or shingle roofs. This is especially likely with a building only 24 feet wide.

If it was mine (and I had a similar situation a few years ago with my 40 x 48 pole barn), I wouldn't consider a shingle roof. Put on a new metal roof with good eave and ridge venting. Then add a metal ceiling under the trusses and blow in insulation. That will give you the best performing and longest lasting upgrade you can get at a reasonable price.

 

[LD1]

No way to know without knowing what the trusses were designed for.

And even knowing the dimensions and design of the truss is no help. The lumber used....be it 2x4 or 2x6 or whatever is only half the info. Need to know what grade the lumber is. Is it #2, #1, DSS, MSR (and what grade MSR) etc. What is the species also?

Given a 24' span an 4' centers, I am assuming its an all 2x4 W type truss. But there is a HUGE strength difference between #2 grade and a MSR rated lumber. And the size of the metal plates is also critical.

When I was having trusses quoted/designed for my building, I wasnt sure if I wanted 4,6,8,10,or 12' span.

Sometimes when upping one size, the only thing that changed was larger heavier plates. Sometimes it was simply going from a #1 grade board to a MSR board.

So final question, why shingle it? Why not new metal and screws? IT will likely last alot longer than shingles and will be easier and faster to put up