Underlayment for metal roof

I am attempting to build a timber frame barn and have questions regarding how the roof should be constructed. The barn will be 30' x 48'. The construction is using 5 bents spaced 12' apart. The roof trusses for the bents are 8"x8". The bents will be connected by 4"x6" purlins connected inside the trusses. The purlins are 4' on center. Here is my problem. I planned on using 3/4" shiplap boards to create a deck or subfloor for my roof and then use a vapor barrier and then a metal roof over it all. I am worried that the 3/4" boards do not have enough support under them due to the purlin spacing and they will sag over time.

Am I fine with my plan as is?
Do I need more purlins?
Do I need more trusses?
Do I need thicker underlayment boards?

Rookie

Whats the pitch?

Rookie,
Is this your first frame? If it is, you may be attempting to run before you've learned to walk. (Based on the questions you asked)

You could use 1 1/2"x 6 T/G. Not sure why you need vapor barrier.
Most barns I have seen are Common Rafter with planks spaced for air circulation and cedar shingles. Eventually recovered with steel at some point.
Another solution would be to double the purlins(24" centers) and apply the steel directly.
Also, depending on the roof pitch, the 4x6x12' purlins @ 48" centers seem a bit light. (A gut feeling) At 24" centers, they would be better. To be safe, crunch all the numbers for load and wood species before you finalize your design. If you have any doubts, ask an Engineer to approve you design.

Just my two cents worth...
Smart remarks are not required guys.

And type of wood, hard or soft, species? I have seen full length purlins, let into the tops of the rafters, daps, with 5' spacings. But someone added 3"x4" purlins in between later.

Tim

ps I see species has already been asked for.

Hi Mr. Rookie;

The metal installation specs may specify 2' spacing for supports.

The width of the boards matters for wide spacings between purlins or rafters, the wider the better. If you are trying to span 4' with 3/4" x 6" boards it will be weak. Even so, it may work, unless someone in the future wants a different type of roofing than metal.

I have heard it is good to put felt paper under metal roofs in the event of a leak or condensation forming on the metal, if this is what you ment by a vapor barrier.

Can you add some purlins to make the spacing 24" or use 1"x 12" and wider boards?

Jim

In our climate (the Southeast), it's wise to put felt under a metal roof on a barn, otherwise it rains inside whenever the metal hits the dew point.

Depending on where you are (snow loads?), the pitch (since the boards are running up the pitch) and the species of the decking, 3/4 shiplap may be strong enough. Or it may not. Generally speaking, I like to use 1x up until about 36" span, and 2x above that.

Your climate is critical. If there's no insulation, that steel will get cold in the winter, and the resipration from the animals will condense on the steel and drip back onto the T&G where you can't see it. Adequate spans can be calucated if you know:

a) roof pitch
b) load
c) wood species

Yes, if you have stock inside then insulation is agood idea. Normaly we had a loft full of hay which provided for that but we can't presume you even have a loft in your design.

T&G lumber is very expensive and if your covering with steel anyway, propably simple rough planks at 5/4 x 10 or 12" wide with tar paper over them would likely be fine with the 48" spans.

I mentioned T&G in my last post but neglected to consider the costs involved.

A barn in which animals are living does not want/need to be insulated. Draft free is nice. Animals are designed to deal with the weather, they can over heat which could lead to unhealthy animals.

Is this barn intended for animals?

I splined the roof boards on such a span using full 5/4" boarding and it was nice.

Tim

Thanks for all the replies. Here is some more info.

I am in northern VA.
The barn is going to be a work shop, no animals.
It will be somewhat heated. Maybe a wood stove.
Pitch is 8/12.
Lumber is 80 percent white oak with some pine mixed in.

White Oak is a good choice. Very strong and works nice...when green. Will you have a loft for storage or maybe an office?

I am planing to build a shop for myself one day.

Send us some pictures when you are raising please.

I just finished the roof on my timberframed workshop. 4x6 common rafters 46" on center with 1x12" rough cut that I shiplapped. I decided to insulate with 2" polyiso then OSB, 15# felt and metalroofing. Went together well. Glad now I didnt cheap out and just put metal down. Winters are long here! What ever you do I would put felt under the metal.

BB

Yes, I will have a second story loft for both an office and storage.

I plan on taking a lot of pictures during the construction and raising, I will post some. It will be next spring though.

How did you cut your shiplap? I have heard suggestions from table saw to router.

Rookie I have misquoted myself. Those rafters are set at 33" on center.
The shiplap was cut with a 2hp tablesaw,feather board and dado blade. Make sure to use the cheaper adjustable dado with one blade. Learned this the hard way after buying the expensive big stack. The single blade works much better for this sort of thing.
Worked great with green pine and hemlock.
BB

R:

Not having done any of the math for you, I'd agree with your initial intuition: that the purlin spacing is too wide. Of course I live in snow country, and my intuition is biased. CB.

Don't know if this will work but I'll try.

http://www.davisframe.com/Images/Product/lg/Saw%20Mill-GR.jpg

If you can see this picture it is VERY close to what I have talked about. Note the large span between the roof joists and the large span of the purlins with the "subfloor" boards going up the roof slope. It seems like this is possible based on my original concept but the roof pitch in the piture is obviously much greater than mine would be and I do not know how thick his roof material is.

Thoughts?

That's right. And you don't know his snow load, or if he is using SIPs. SIPs are very stiff, and can span a dozen feet or more on their own if properly supported at the ends. CB.

The pitch of a roof makes a big difference. Even without snow on the roof you have the weight of the rafters, purlins, decking and steel or shingles. It will be the forces perpendicular to the beams that count. Gravity wants to pull everything straight down so if a beam is horizontal then you have 100 percent of the force acting against the beam. If a beam is straight up like a wall then there is (for this exercise)no force acting on the beams. The roof in the picture looks like a 12/12 pitch roof which will have 50 percent of the load acting against the beams.
You will have to brush up on your Trig. & Geometry to calculate the actual forces acting against your roof members.
There are likely some tables kicking around as well.

Fun with math

Cheers

The codebook doesn't alter sheathing, rafters or purlins based on pitch. The horizontal clear span from plumb to plumb would be the way the NDS measures the span. Roughly then, 4' centers on an 8/12 with 4" wide purlins is going to give a clear horizontal span of about 40".

Your building department can give you the snow load for your area, I'm guessing it will be 30psf. Add 10psf for dead load so 40psf total load.

White oak base design values for #2 are;
Fb-850
E-.9
Fv-220
I would bump up the Fb by 15% for short term loading(snow), 15% for repetitive member use, and you could take 15% more for flat use but I don't think I would. I'd call the adjusted Fb-1125psi

Assume a 6" wide board spanning the 40"=1.66 square feet X40psf=67lbs load

I looked at it as a simple beam in flatways bending;
http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I entered;
Load-67lbs
span-40"
width-6"
depth-1"
Fb-1125psi
E-.9
Fv-220psi
Click "enter"
the fail in deflection shows it would make for a bouncy floor but a passable roof board.

BUT,
Let's look at those purlins..
40square feet of roof bearing on a purlin X 40lbs per square foot= 1600lbs load bearing on each 4x6 purlin. I also changed the span in the calc to 140" then entered this info assuming the same species. It's too small, it might squeek in at a 6x6.

Don,
Thanks a bunch, lots of good help here. I think I have decided to add more purlins and have a 2' span instead of 4'. Whether I need that much or not.......I'll feel better working under it.

Thanks to everyone for all your help.

One other thought. A layer of 30# felt will prevent nailheads in the roof boards from contacting the underside of the metal roofing, and forestall any damage or corrosion to the paint coating.

Personally I'm comfortable with rough sawn I"x boards on about a 36" purlin spacing, for metal roofing here in NH. Especially if the roof has 1x4 strapped horizontally for screwing the metal to, and ventilation. I always want a water shield (felt etc) underneath due to condensation etc. Closer purlin spacing is going to look awfully 'busy" or heavy, aesthetically.

Once I reach the 2' spacing I am using 2x stock.

I just cut a bunch of step lapped rafter pockets at 4' spacing. It took me 2.5 songs on the radio for each pocket. I don't know yet if I am boarding or just strapping and using heavy shakes. 6x6 rafters or there about depending on what I find or logs.

Tim