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Rafter and purlin sizes story.

Started by Jim_Rogers, May 16, 2011, 11:54:03 AM

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Jim_Rogers

I don't know if I ever posted this story about this barn frame that a local carpenter made for his client.
I was made aware of this as I did some work for this client and I think this was in the late 80's or early 90's before I had a sawmill. Back then I was harvesting timber and selling it to the local "big" sawmill. I had a logging truck on the road and used it to move logs for this client to the big mill to have them cut up for barn siding.

This local carpenter designed the barn and I'm not sure, but I believe he didn't have it professionally engineered.
He designed it to have three bays and four bents. The roof system was a principal rafter system with purlins between the principal rafters. He made up a jig for using a router to cut out the dovetail joints so he could drop in the purlins from above by crane. He figured that the rafter on the gable end bents didn't have to be the same size as the rafter in the middle bents as the gable end rafters where only holding up purlins on one side. I think the middle bent rafters were 8x8 or deeper, and the gable end rafters were 6x6 or deeper.
But anyway the point is that the purlins in the middle bay were a different shorter length then the purlins needed for the outer bays. That is, in the pile of purlins there were two different lengths.

Now comes raising day. Bents when up, plates went on, there were no problems. Three man crew: carpenter, his helper, the client, and of course the crane operator.
The client was the ground crewman. The carpenter and his helper where up on the ladders and the frame putting the pieces in and releasing the straps for the crane operator.

A pair of rafters when up, temporarily braced off with boards. Another set went up. Again braces off with boards. Instructions were given to "fly some purlins up" to hold these two sets of rafters together. Done. Next bent rafters up, more purlins. Why are these purlins too long? 

Let's step back and look......

Why are the gable end rafters leaning in like that? Shouldn't they be plumb?

The client was hooking the purlins up to the crane with the straps and didn't understand that some were shorter then others. And he sent the short ones up where the long ones should go. And sent the long ones up where the short ones should go.

These purlins were dovetailed cut and some has twisted. The carpenter knew that they had twisted but he made up a "purlin fork" so that he could induce some twist into the purlin, and straighten them just enough so that the helper could pound them in from the top of the rafter. Stress fit for sure.

Now they have to be removed and replaced to their correct location. Wow, what a mess. And an additional cost, as they had to cut some of the purlins in half to get them out. Re-cut new purlins and hire the crane to come back another day to install them.

What I learned from this, and it was this carpenter's advice to me, is: "make all the rafters the same size." Even if the gable end ones don't have to be as wide as the middle bent rafters. This makes all the timbers or distances the same, if your bents are spaced evenly.

Label everything correctly. Make sure you've got the right piece for the right spot, the first time. Label the tenon end and the mortise pocket. When raising the end label should match the mortise label. If it doesn't then you've got the wrong piece. We always label the timbers when we do a "full frame fit up". That is check each tenon to each mortise. Each time we do this we label the tenon and mortise with a unique label to show it has been checked. Once it is labeled, it means it has been checked. Keep track of your joints on a joint process sheet. When you call out, during the raising for a piece, call out the label. "I need brace with 3 stars on a tenon." It is easy to see the markings on the tenons and they are hidden when assembled. There are lots of labeling systems. Use one or create your own.

Having different size rafters and different length purlins make more work, more record keeping to keep everything straight.

Sometimes they just don't look right when they are different sizes. What is the costs savings if you make a rafter narrower then the next bent? Not much at all. In the overall price of the entire job, peanuts.

Keep it simple; make it easy, the cost savings can be huge. The time savings can be huge.

By Jim Rogers-5/16/11
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

laffs

im with you on all that. but sometimes its the help you have too. sometimes they dont understand the importance of why something gets done in what order.most of of us have made mistakes and the mistakes of others we blame ourselves for also for not checking their work. so if your running the show you either got to have complete confidence in your partners or double check their work too
timber harvester,tinberjack230,34hp kubota,job ace excavator carpenter tools up the yingyang,

routestep

Jim, your post here on labelling tenons and mortises is very apropos.

My brother and I just raise up the second bent of a four bent frame. He had to leave and I started on three floor joists that will be locked in to their mortises and then pegged. They make up the second floor. The other seven joists I'll slip in later. So I got some five by eights, fourteen feet long and started cutting the tenons.

Due to some shrinkage (and my cutting) each mortise in the cross ties are a little difference in size, so I cut the tenons to fit. Well I looked at my little pile and though I'll get these joists bass ackward if I don't mark them.

So right on the tenon each end got its bent number and position number in the floor. I got two up in place before I had to leave, but I'll raise it up on Friday.

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