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COMMON RAFTER INSTALL QUESTION

Started by stanwelch, April 03, 2014, 07:12:24 AM

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stanwelch

I am building my first timber frame barn that is 24' x 32' with a 5/12 pitch roof and eave height of 11'.

  
I plan to use a common rafter that is constructed from two 4"x8"x15' legs with a tongue and groove joint at the peak with no ridge beam.  I am using ash and each leg of the rafter weighs about 150#.  How do I get each rafter installed?
I have access to a scissor lift and a JD tractor with loader and forks.  Is it possible/pratical to lift each leg with the tractor to the ridge beam, rest one end on the beam and raise the other end to the scissor lift and assemble/peg to the other leg raised the same way?  Or, am I better to assemble on the ground and raise with a crane? Any other methods?  Please help.
Woodworker, Woodmizer LT15, Stihl 026, MS261CM and 460 chainsaws, John Deere 5410 Tractor 540 Loader,Forks & Grapple, Econoline 6 ton tilt bed trailer

Jim_Rogers

When we raise a frame we put up the bents then add the plates. After the plates are on the frame is some what stable. We then add planks over the tie beams to stand on. We also put a long plank over the plates to use as a table.
We slide each rafter up onto the tie beams and then pick them up and put them on the plate and plank.
Once on the plate and plank we connect the two together.

Like this:



 

With the feet of the rafter near where they will sit on the plate, we then rotate the pair from horizontal to vertical. This is fairly easy as there isn't much lifting, as we aren't lifting the entire weight of the rafters.
Once they are vertical we make sure that they are in the correct location, spacing from one to the other. And then plumb. To hold them in place we place a piece of strapping onto the underside of the rafters from on to the other to hold them from tipping until the roof sheathing or tin roof strapping is added to the top.

To make your final decision on whether or not if you can do this, place your two plates on the ground on some blocks so that there is enough room between the rafter tail and the ground and put a pair together laying down and try and stand them up. If you and one or two helpers can rotate a pair up to vertical then you can do it up in the air on the stagging planks.
If you and several helpers can't then you'll need to assemble them on the ground and fly them in with the crane.
If you fly them in, with a crane, then put a piece of 1x6 across from rafter to rafter like a collar beam to hold them apart as they are flow up and into place. Use duplex nails or screws so you can take them off after the rafters are secured and the roof decking or strapping is on.

Good luck and good question.

Jim Rogers

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

stanwelch

Thanks Jim.  Just what I was looking for.  I'll give it a try and post pictures of my progress.
I'm about two months behind schedule because of this lousy weather.  I was able to get into the woods Wednesday for the first time to get some logs I cut last December.  Still some snow and a lot of water/mud.  Fired up the sawmill yesterday for the first time and was able to get started cutting rafters. Raining today and tomorrow so working inside.
Woodworker, Woodmizer LT15, Stihl 026, MS261CM and 460 chainsaws, John Deere 5410 Tractor 540 Loader,Forks & Grapple, Econoline 6 ton tilt bed trailer

Jim_Rogers

When we raise the rafters we usually start at one gable end, and roll that pair up. We'll put some strapping on the end of the plate to prevent the pair from rolling off the end. A piece of wood like a 1x2x12 or something like that secured to the end of the plate will prevent it from rolling off.



 

Above is a shot of us doing it on one end of this frame.

After we have the first pair vertical we put a piece of strapping or something like that, usually a 1x4 down from the rafter to the plate to hold it plumb. And we do that on both sides of the frame to both plates:



 

Here you can see us doing that. And some how we forgot the end safety sticks on this raising. oh well some times that happens.

Then after the first pair are up and they are secured back to the plates and plumb we attach the next pieces of strapping to them and on and on down the line:



 

I think it was during one of these times one of my students suggested we put these pieces of strapping on the underside of the rafters so that it is easy to attach, and they can stay there until the roof decking is added.

The last few frames we have done we've done this and it has worked for us.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

stanwelch

Great pictures!! I can see now the way you describe would be much easier than the way I was thinking. Do you screw each rafter to the plate as you go or wait until they are all up ?
Woodworker, Woodmizer LT15, Stihl 026, MS261CM and 460 chainsaws, John Deere 5410 Tractor 540 Loader,Forks & Grapple, Econoline 6 ton tilt bed trailer

Remle

Pardon my interruption but a 300 lb, 24' rafter can become quite problematic in an instant, you might also consider a temporary collar tie to keep the  rafter from splaying out during the raising putting extra stress on the peak joint. A ratchet strap is a good solution and helps to pull the rafter together when it is vertical to the correct width to be fastened to the plates. A small rope block and tackle from the previously set rafter, to the peak also offers help in raising the rafters as well as a safety factor to those pushing up the rafter.

stanwelch

Remie,  thanks for your input. I agree the collar tie and block and tackle are good added safety considerations during the rising. I do plan to use a ratchet strap to square and plumb the rafters. Good ideas!
Woodworker, Woodmizer LT15, Stihl 026, MS261CM and 460 chainsaws, John Deere 5410 Tractor 540 Loader,Forks & Grapple, Econoline 6 ton tilt bed trailer

Jim_Rogers

Yes, as I mentioned we put a temporary collar beam board or 2by across the pair after they are assembled laying down before we try and rotate them up. All good advice about straps, and block and tackle. You have to be prepared and test your methods so that when you're up in the air you're going to be safe.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

razor

Nice project. Something to stand on like Jim says will make it all a lot easier for sure.
Maybe you have this issue covered already but those rafters with no ridge and the low pitch will be pushing REALLY hard on your plates. I see the thru tenons on the ties but the plate could roll on you. Just sayin.

Jim_Rogers

Plates don't normally roll. The experts, older and more experienced then me have written about it in Timber Framing. Timber framing is the journal (magazine) of the Timber Framers Guild.

If you don't get it you should join the guild and learn about timber framing.

The frame in that picture has been updated since that picture. Although there are still through tenons on the tie beams.

Thanks for your comments.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

piller

I also saw the frame drawing and with the 5/12 roof pitch and the 24 foot span the first thing that came to my mind was that I would be concerned about the horizontal thrust.  Glad to hear that this is not an issue.  How was the frame updated?

Jim_Rogers

The plates were raised a bit more above the ties.
Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

In a letter to the editor of "Timber framing" in June of 1996 Jack Sobon wrote about a concern of another well known timber frame designer's article about rafter to plate connections.

And he writes: "I was troubled, however, by his last comment concerning overturning plate moment. Since I have know[him] he has had this rolling plate phobia. Perhaps he has nightmares where plate are rolling off his buildings!
I have personally know a lot of old and new plates with common rafters step-lapped into them and haven't and haven't noticed such a problem. What I occasionally see is a plate with spital grain that twisted as it seasoned and caused one end to rotate. (Twisting at the opposite end is prevented by the roof thrust.)
Because the post mortise is typically neared to the outside face of the plate if the timber is boxed-heart, tangential shrinkage can cause the inner portion to lift off the post. I have noticed this new plates even before the rafters are applied. It is minor and not a structural concern.
I have seen plates in old structures that were grooved on their underside for vertical boards, and these plates overhung their posts by 2 inches or more without signs of rotation. So why does the engineering say there is a problem even though reality doesn't? First, the rafter is not an axial load meeting the plate at a point: it has a large contact surface. To rotate the plate, the roof would have to lift up. As the roof pitch lessens and thrust increases so does the contact area in the rafter seat. Engineers always consider an axial force acting along the centerline of a member and in theory such a force would cause rotation. But the geometry of this joint causes the full thrust to occur at the very bottom of the rafter. The bottom inch of [his] rafter could handle the whole load and in face would as soon as the plate rotated a fraction of a degree."

He goes on to say: "In the simplest of terms, the weight of the roof prevents rotation."

Later in his letter to the editor, he states that he is more concerned about the post splitting from rafter thrust.
And he show a drawing to state his reason why.
This drawing:



 

This also shows that the pegs need to staggered so that the both don't line up in one grain line. And to further prevent the risk of the post splitting he writes: "This is especially to be found in frames where the distance between the plate and the tie beam is less than a foot."

This is why I raised the plate. And I have always staggered my peg hole placements.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

RavensWood

I am just a beginner at timber framing and am trying to understand the arguments put forth. In looking at the design it appears to me that all of the thrust from the roof is being handled by two pegs on each side of the tie beam. With a relatively low slope roof and a relatively large span, is this not a little risky? (not sure what the design snow load is in Michigan but up here in Canada it's 55 lb psf). Am I missing something?

razor

Thanks for the tips Jim but I know a little bit about timber framing I think. Not much I know but I try hard.
Try to pick up as much knowledge as I can from the masters like Jack. Especially at the many conferences I've been to, you know, the ones where I buy your stuff.
I know about the magazine also, thanks. Been getting it for almost 12 years, well ever since I joined the Guild actually.
Don't get all caught up on my choice of the word "roll", call it what you like, the plate is going to want to go umm..."sideways". Like Jack's picture. Lowering the tie helps yes but the picture I saw didn't show that.
Relax dude we're having fun here.

Jim_Rogers

Quote from: razor on April 04, 2014, 08:51:45 AM
Relax dude we're having fun here.

I'm relaxed. I was just trying to educate not confront, you, and others who will read this in the future.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

Quote from: RavensWood on April 04, 2014, 08:49:05 AM
I am just a beginner at timber framing and am trying to understand the arguments put forth. In looking at the design it appears to me that all of the thrust from the roof is being handled by two pegs on each side of the tie beam. With a relatively low slope roof and a relatively large span, is this not a little risky? (not sure what the design snow load is in Michigan but up here in Canada it's 55 lb psf). Am I missing something?

I know this frame well as I drew the design.
I was cautioned about the low plate or short distance between the tie beam and plate after I drew it. I notified Stan and I re-drew the design to make it better.

Here is the frame without any gable end boarding or roof decking:



 

The end two rafters are smaller and are just butt joined at the ridge. The inner rafters have a tongue and fork joint at the ridge.

Here is the frame with the gable end boarding and roof decking:



 

Here is a shot of the ridge details, showing the tongue and fork joint:



 

Ok, so now we can all see the frame details.


The rafter seat, where it sits on the plate is called a crows foot. Most people call it a birds mouth but that is not exactly correct.
The crows foot seat cut will be secured to the plate with two (or more) long timberlok screws. These should hold the rafter to the plate.
Now for the plate to rotate as mentioned before, the roof has to raise up. That means all of these screws have to fail in order for the rafter to push the plate so that it can rotate. If not fail at least allow the seat cut to open up along the outside edge of the plate.
If there is such a heavy load of snow to push the roof down to cause the plate to rotate then the thrust would have to overcome the load to raise the rafter feet up. To do that the joint at the ridge has to fail. In order for that to fail the peg has to sheer off. And the tongue tenon or the mortise fork have to crush so that the ridge can drop. Or all three, that is the peg has to fail, the tongue has to crush and the mortise has to crush. Not only on one rafter pair but on all of them, at the same time.
Is this possible? Well yes, it is possible. Is it likely? No, most likely it is not.

If Stan is worried about this he could add a timberlok screw to the ridge connection to secure the tongue to the fork.
Timberlok screws or lag bolts have a very high sheer value. And if he added them then they all would have to fail, again at the same time, in order for the plate to rotate.

Also, in order for the plate to rotate the joint between the post and the plate has to fail. The peg there has to sheer off on two planes one on either side of the tenon. The mortise walls have to crush, the tenon has to crush for the plate to rotate. Again, not only on one post but on all of them. All at the same time.

If Stans joints are cut correctly with fairly tight fitting tenons into the mortises this most likely will not happen. Even with a heavy snow load.

Although there is only boarding on the gable ends, all of these screws or nails will have to fail that holds this boarding on, at the same time everything else fails for the rafters to be pushed down enough to rotate the plate.

If Stan is worried about the plate rotating he could put a steel plate across the inside of the post up the side of the plate and screw it on. This would/could/should totally prevent the rafter thrust from rotating the plate.

I believe the snow load for his location was/is 25 lbs per sqft. And I believe he intends to put boards on the roof with shingles. Boards over the rafters tie all the rafters together combining the strength to support the loads.

I hope this has helped you to understand the design.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

razor

I didn't feel confronted. Your reply was just rude. But I'm over it.
About the roof. Sorry but I don't buy it. A 5:12 roof with 4x8 ash rafters, no supported ridge and spanning 24' is going to have significant forces pushing the building apart. Relying on the peak joinery to prevent horizontal thrust is poor practice. Do a simple test, put the rafters together on the ground and get 2 kids to pull the feet apart, see how much they move.
Over time, not right away, I predict the joint where the tie meets the post will open up a tad and yes, the plate will lift off the post on the inside.
I also don't buy the notion that the roof has to lift up for the plate to push out. I can see what the roof/plate will do with a simple popsicle stick model. A steeper roof would go a long way in helping the situation.
Maybe I'm totally out to lunch here, been out to lunch before, if somebody with more smarts than me or Jim wants to set me straight I'm all for it.

Jim_Rogers

I'm sorry your felt my reply was rude. It was not intended to be rude.

Here is another drawing by Jack:



 

Here you can see what will happen when there is an extreme roof load. For the rafter to come down at the ridge it has to lift up at the plate. In order to do that the screws/nails/fasteners have to fail. Not just at one pair all of them.

I don't want to continue with this. We can agree to just disagree.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

razor


stanwelch

Jim, thanks for all your help with this project. I especially like the discussion and Jack Sobon,s illustrations of the post failure. You gave me a drawing that showed adding a steel strap on top of the cross tie beam and up the outside of the post when we had the shorter post.  I planned to add this strap to the longer post because I already have the straps. Would I be better to run the strap vertical to connect the post to the ridge beam?  It seems to me, based on the Sobon drawings the strap through the tie beam mortise and up the outside of the post would help prevent a post split.
I changed my mind about the shingles and now plan to put on a metal roof.  I think snow would slide off easier.  I'm sure I'll more questions as I get further along.  Thanks to everyone for there input.
Woodworker, Woodmizer LT15, Stihl 026, MS261CM and 460 chainsaws, John Deere 5410 Tractor 540 Loader,Forks & Grapple, Econoline 6 ton tilt bed trailer

Jim_Rogers

Quote from: stanwelch on April 04, 2014, 04:39:44 PMWould I be better to run the strap vertical to connect the post to the ridge beam? 

Stan if you want to run a strap up the side of the post and over the top of the "plate" yes, that would make it stronger and would prevent any possible roll.
It would not prevent the post splitting as I have shown. You'd need to put one through the post over the top of the through tenon/mortise and up the outside of the plate as we discussed to prevent any post split.
However by moving the plate up we should have solved that problem.
It's one of those belt and suspenders type of thing. You could do both.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Aikenback

One quick comment from a career carpenter. The frame would have a "look" that was more comforting with a ridge beam, but it still has to be supported somehow on each end and at each bent. the ridge would help ensure the common rafters tops remain supported between bents, but ultimately would have no benefit keeping the bents' geometry true. A king post at this location might offer someone some visual comfort, but these posts are tension posts holding the tie beam up. If the tie is strong enough to hold it's own weight comfortably, then the strength is in the tension capacity of the tie holding the structure together. Counter intuitive to some , but true.
( Did I say "quick"? )
no whining.

witterbound

Why wouldn't one use a ridge beam in this kind od design?

Remle

Gentlemen
Pardon my intrusion again but a quick question if I may in the interest of education. Is the pictorial frame represented in the post accurate as to the 24X32 frame as proposed by stanwelch ? Or in other words, the picture shows the bent as having only 2 post, with a 24' span would their not be 3 post?

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