I'm in the process of planning the reconstruction of barn that will be used as a garage. The original barn was a common rafter frame with 12' 8” sidewalls, 30' tie beams connected at the top plate and supported by one post mid span. There were 7 bents. The rafters are 4-6" diameter poles placed 28" on center with a 12/12 pitch. There were rafter ties on every other rafter pair that are about 13' long. There was no ridge board.

I didn't see the original structure before it was disassembled, but it appeared to have a problem with the sidewalls bulging out. The pictures I have are poor, but it looks like steel tie rods were added along the tie beams to hold the walls together.

I want to reconstruct the building as a 30'x40' structure with the same common rafter design (with a 12/12 pitch). I plan to cutoff the original tenons and use timberlinx connectors with the tie beams lower on the posts (9'6"). I'm planning 4 bents, with the same H-design, post mid span.

My main concern and the reason for this posting is that I am concerned about the common rafter design. Should I add a ridge board to keep everything in line with these rough rafters? Should I add a ridge beam to carry some of the roof load?

Thanks

If you lower the tie beam it may effect the outward pressure on the walls called thrust at the plate. This may not be a good thing.

If you added a ridge beam directly supported by the tie beams and posts below it could reduce the outward thrust of the rafters.

A lot depends on your snow load and wind load for your area.

The entire design should be looked at the thought through thoroughly.
Good luck with your project.
Jim Rogers

I was hoping to keep a clear span on the second floor.

Another idea I was playing with was replacing the collar ties with a more complete truss system to resist the thrust created by the roof load. I also considered just lowering the collare ties.

Another factor I need include here is that I will be putting 30,000 pounds of slate on the roof. I will check the and wind load too.

Is it your experience that building inspectors require that lumber be graded?

Thanks,

-Dave

I can only tell you about my personal experiences.

Years ago, I had a customer who wanted to create a new barn from old salvage timbers. These timbers were to be re-cut to smaller sizes to make the timbers for the barn parts.
Their building inspector wanted these salvage timbers grade stamped.
The customer hired a traveling graded to come to my sawmill yard and grade stamp these timbers. He rejected several that he though weren't good enough.

Last year at the TTRAG conference in Southern NH, I met this traveling grader again. He told the group that his lumberman's association will NOT be grading salvage timbers any more.
It was his advice that before you attempt to re-construct a new building from salvage or recycled timbers you check with your building departments, as to whether or not they will allow it or not.

I would suggest you have an informal conversation with your building inspector.
Asking the wrong question will get you a conditioned response. What I mean is if you ask them "Do I need to have my timbers grade stamped?" They most likely will reply: "yes, of course."
But if you tell them what you intend to do and ask: "what will you need from me to allow me to do this?" You may hear that you need your timbers grade stamped, you may not.
My point is don't ask a leading question.

With the fact that you intend to place such a roof load on this recycled structure you should have your plans reviewed by an engineer who is familiar with timber framing, as that is a substantial load on old timbers.

Good luck with your project.

Jim Rogers

DAB,
I'm with Jim on this one. Lowering the Tie Beam will definately add some outward thrust to the wall posts. Could you convert the material to a Principal rafter/common purlin configuration to reduce the outthrust? And with the Slate material going on this roof, I'd definately get an Engineering opinion. Sometimes you just have to spend the money...
Also, the Guild has a book available that deals with the issues discussed here (the red one). If you haven't read it yet, there might be some info in there to help you out. Also, see the downloadable articles on the website here re: Trusses... regards

I don't think moving the tie beams down the posts will increase the outward thust, but it will increase the tension on joint between the tie beam and the post because there is small lever arm between the bottom of the post and the point of force application. That was one of reasons for going with the timberlinx connector, which, according to the companies test data are 5x stronger in tension then a mortise and tennon joint its replacing. 5x sounds like a pretty good safety margin. The other factor going in my favor is the 12/12 roof pitch which directs a good bit of the dead roof load down the post as opposed to the side. Along the same lines, I considered narrowing the building and increasing the pitch beyond 12/12 to take further advantage of this.

For those of you who have built common rafter roofs, do have any suggestions for incorporating wind bracing in the roof without it looking kind of awkward?

Any opinions on the timberlinx connectors. I've also looked at the Simpson Strong Tie Archetectural series connectors but would pre to use a completely hidden connector.

Thanks,

-Dave

On second reading of my last posting, and your subsequent reply, maybe I should not have said that moving the tie-beam down would increase the thrust. The amount of thrust is a given value, as determined by the roof slope, rafter length, roof area, presence of collar ties, etcetera ;however, the effect of the given amount of thrust at the tie beam location will change due to the increased length of post between the Tie Beam connection and the rafter foot. Sorry about that.

The ability of the timber-lynx fastener to handle the imparted thrust is a seperate issue from the posts ability to handle the bending moment created by moving the tie down.

Resovling thrust before it reaches the plate will reduce the moment and is in my opinion, always part of good design.

Only an analysis will tell you if the collars are helping with this. If they are near or above the bottom third, it is probable they are not.

The slate should only be considered after someone extremely knowlegdeable looks at your frame and the planned alterations.

Will, You are right that lowering the tie beam has placed a bending moment on the post centered at the tie beam location which did not exist in the original design, but this bending moment is much smaller than moment centered at the base of the post, which is resisted by the tie beam tension. I'm only plannin to lower the tie beam a couple feet.

That said, you guys have got me thinking about this and I'm now leaning toward adding a ridge beam and posts that extend from the middle of each tie beam up to ridge. This would also allow me to add bracing to resist racking of roof.

I will get someone to check my design, too.

I'm trying to do this with the materials I have on hand.

Keep the comments coming, this really helping me. I've also read all of the Ted Benson and Jack Sobon books my library had to offer.

-Dave

Dave –

The mere presence of a ridge born by posts does not alone resolve thrust.

This now common framing scheme is known as a “structural ridge” and the key ingredient without which this design will not work, is for some reason I do not understand, almost always not mentioned when this comes up here at AtE.

A Simpson type strap must be present to fasten the rafters either to the ridge or each other, essentially eliminating thrust by putting them in tension and now imparting something more than half of their dead and live load to the ridge and the load path that bears it.

That path, with this design, always going directly to ground.

And “a couple feet” translates into much moment.

Anything greater than say one, requires careful planning, in that the moment increases from every inch thereafter, almost exponentially.

Be your best.