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.