Timber Frame roof sheathing

TheEnglishLumberjack · March 27, 2014, 05:21:15 PM

We have a timber frame building to construct for a customer this summer. The building is approximately 20x40 and has no walls, basically it is a 12/12 roof on posts. The customer likes the look of common purlins as apposed to common rafters. The purlins will be approximately 3' OC, so my question is: what roof sheathing should we use?
The inside of the roof sheathing will be visible from below so we were going to use 1x6 T+G boards running vertically, perpendicular to the purlins. We were going to put 3/4 4x8 sheets of ply on top of the T+G with cedar shingles for the roofing. Is the T+G with 3/4 ply on top, a finished thickness of 1 1/2", enough to span the 3' OC between the purlins without sagging?
What is the norm for roof sheathing on timber frames with common purlins? There's lots of information out there for building a timber frame from floor to rafters, but not a lot of information from there up.
We'd appreciate any advice. Thanks

Brad_bb · March 27, 2014, 05:56:07 PM

I assume you want tongue and groove to hide the plywood? Just make sure you allow space for expansion and screw to nail it so it can expand and contract. If you have a purlin to divide the space in half, you could do a herringbone pattern which I like a lot and looks good from underneath. If you're building a TF, why not?

Can you use full one inch thick? If you can buy rough sawn and plane on one side(inside)... It will also additionally act to help resist racking. Will it ever be an insulated/heated building? If so, now would be the time to build an insulated roof, like wrap and strap.

Jim_Rogers · March 27, 2014, 06:26:37 PM

Quote from: TheEnglishLumberjack on March 27, 2014, 05:21:15 PMhas no walls, basically it is a 12/12 roof on posts.

Hey, Brad, you need to read the post more clearly. "No walls" it's a pavilion.....

No need for insulation....

Jim Rogers

Rooster · March 27, 2014, 07:54:59 PM

Go with 1 1/2" T&G instead of 3/4 + 3/4 plywood...

Cedar needs to breathe, so think about firing strips that you can nail the cedar shingles to...

Good Luck,

Rooster

TheEnglishLumberjack · March 27, 2014, 08:59:12 PM

Rooster, I like your idea of 1 1/2" T+G. Do you still think then that 1 1/2" is sufficient to span 3' without any sagging?
Can I spread the purlins to 4' OC instead? What's the normal distance between purlins?

Rooster · March 28, 2014, 03:40:41 AM

Quote from: TheEnglishLumberjack on March 27, 2014, 08:59:12 PM
Rooster, I like your idea of 1 1/2" T+G. Do you still think then that 1 1/2" is sufficient to span 3' without any sagging?
Can I spread the purlins to 4' OC instead? What's the normal distance between purlins?

Sorry, I'm a common rafter kind of guy...

Snow loads and such will come into play...I will defer to the engineers in the group...

Peace, Rooster

Jim_Rogers · March 28, 2014, 07:45:10 AM

Quote from: TheEnglishLumberjack on March 27, 2014, 08:59:12 PM
Rooster, I like your idea of 1 1/2" T+G. Do you still think then that 1 1/2" is sufficient to span 3' without any sagging?
Can I spread the purlins to 4' OC instead? What's the normal distance between purlins?

You need to figure the load on the purlin and based on their size, wood type and spacing will determine if you can move them from 3' to 4'. 1 1/2" t&g can normally span 3' or 4'.

Jim Rogers

giant splinter · March 28, 2014, 08:40:18 PM

I found a material known as 2X6 Select Decking, it serves both purposes ( the roof sheathing and the exposed ceiling ) looks like 2X6 v-grooved material on the ceiling.https://forestryforum.com/gallery/displayimage.php?album=4276&pid=141097#top_display_media

TheEnglishLumberjack · March 29, 2014, 08:46:24 PM

Thanks for all the help guys. When our local sawmill opens up I will have to check to make sure that I can get 1 1/2" T+G boards milled. That will be the deciding factor at the end of the day. Again I appreciate your replies.

witterbound · March 30, 2014, 08:20:48 AM

Quote from: Jim_Rogers on March 28, 2014, 07:45:10 AM
Quote from: TheEnglishLumberjack on March 27, 2014, 08:59:12 PM
Rooster, I like your idea of 1 1/2" T+G. Do you still think then that 1 1/2" is sufficient to span 3' without any sagging?
Can I spread the purlins to 4' OC instead? What's the normal distance between purlins?

You need to figure the load on the purlin and based on their size, wood type and spacing will determine if you can move them from 3' to 4'. 1 1/2" t&g can normally span 3' or 4'.

Jim Rogers

The few timberframes I worked on that had purlins spaced 3 to 4 feet apart, had sips above them to provide the necessary rigidity.

razor · March 30, 2014, 12:58:06 PM

As giant splinter says there is 2x6 DF decking that would easily span your purlins with no issues. I installed this t&g on a restaurant we did a couple of years ago. The 4x12 rafters were 4' apart if I remember correctly. This makes for a really solid, and heavy, roof deck.
The 2x6 nominal was actually 1 3/8"x5 1/2", again if I remember correctly.
It comes double tongue - double groove. It also requires 8" spikes driven horizontally every so often into the board beside it. At first we thought this part was added work that could be skipped but it soon became evident that you need those spikes to draw them together. They come with a pre-drilled pilot hole for the spikes.

razor · March 30, 2014, 01:07:39 PM

Getting my t&g's mixed up.
It was 3x6 DF t&g we were using. Not 2x6.

shinnlinger · March 30, 2014, 03:11:53 PM

You also need to figure a way to secure the frame to the ground because that roof will be a giant sail in a heavy wind. Lots of different ways to do it, but just make sure you do...