Monday, January 30, 2006

Hanging out waiting for Bent D

We used scaffolding to suspend 3 of the 6 girts that will connect Bent C to Bent D. We've decided to raise Bent D in two sections, and at this point we only need 3 of the girts in place in order to raise the first section of Bent D. We've found that the 3/4" steel nail pins that I bought for the concrete form work are very handy for temporarily pegging braces. After assembly we remove these pegs and drive octagonal 1" locust or white oak pegs.

...and what are the people in the picture doing? Well, we've been using ratchet straps and come-alongs to hold the frame together until we're certain we're ready to peg things. We just discovered that we had run out of straps, and we need some for Bent D! Time to drive some pegs and free up some more straps.

Hip Rafters Worked!

top left: frame at dusk

top right: king post with 3 principal and two hip rafters intersecting

bottom left: hip joinery viewed from NE

bottom right: (birdsmouth) feet of two jack rafters and a hip rafter resting on top plate.

Hip Rafters Worked!!

It took two days to assemble the hip portion of the house. This assembly (8 jack rafters and 2 hip rafters tying into the hip king post, three top plates, and two hip queen posts) was never trial fitted as one assembly on the ground, but it ultimately worked up in the air where it counts. Here's how we assembled each corner of the hip roof....

1. fly the hip rafter in to place
2. fly the associated 4 jack rafters, upper ends resting on hip rafter
3. fly the queen post in to place (after cutting to length)
4. re-rig the hip rafter with crane straps
5. nail boards on top plates to keep jack rafter feet from sliding off top plates
6. lift uppper end of hip rafter about 8 inches
7. get upper ends of four jack rafters to fall into place
8. lower hip rafter back into king post
9. guide queen post in as hip rafter lowers back into king post.

The North-East hip rafter fit like a glove, but the South-East hip rafter appeared to be 1" short when we placed it. We thought "oh crap!" because it wasn't 7/8ths short or 1 1/8" short - it was exactly 1" short, which pointed to a cutting error or layout error (which would have meant the SE hip rafter was destined to be firewood). But after tweaking the two principal rafters and king post (they were out of plumb), tweaking the NE hip rafter some, and shimming the foot of the SE hip rafter 1/4", everything came together fine, as you can see in the picture looking at the king post. Of the 1 principal rafter, 2 hip rafters, and 8 jack rafters that form the hip portion of this house, only the upper ends of the 2 hip rafters have been pegged in this picture. Everything else is pretty much being held in by gravity.

[you should skip the next paragraph unless interested in compound joinery!]

This was a very rewarding piece of the frame to put together. Several months earlier, it had taken me 2 weeks just to calculate and draw the hip rafters, jack rafters, and king post. Even though this seemed like a very straightforward problem before getting into it, calculating the angles and translating them into 1/16ths of an inch dimensions that could be laid out timbers was... maddening. There was precious little in the timberframing books and literature that showed how to do this. The one gem that I found was a spreadsheet on the tfg guild web site, but even it was hard to decipher. (you input the various angles of your roof and it spits out two dozen angles - it's up to you to figure out which 3 of these two dozen angles is important to you!) If I ever build a timberframe hip roof again, I'll use the exact same dimensions and angles as I did for this house! I think I would also use a king post again at the top to tie everything together, even though I have only found one other example in the literature of executing a hip roof this way (which I found only after I had designed it this way!) When I found the king post example (in the "Taunton press compilation of fine homebuilding articles on timberframing"), I was somewhat comforted that it had been done this way at least once before (although their example did not use a mortise and tenon joint between the hip rafters and king post - their hip rafters merely rested against, or leaned into, the joint formed between the principal rafters.) Also, a lightbulb went on when I saw how they handled the foot of the hip rafter - they used a "dragon beam" (a beam resting on both top plates) to support the foot of the hip rafter. That makes more sense than the way that I did it - I have the foot of my hip rafter recessed into (and resting on) the two top plates join at the corner. My version makes for some unecessarily complicated joinery there, and even then I was uncomftable enough to use the only metal fastener in the house so far - the stainless all-thread which I thought might be needed to keep the foot of the hip rafter from spreading the two top plates apart at their joint.

Wednesday, January 25, 2006

Dovetail Details

When people think of the joinery that holds a timberframe together, they probably most often think of the "pegged mortise and tenon joint." However the "wedged dovetail joint," is just as important in traditional timber frame houses. The dovetail joint is used on the ends of floor joists (and roof purlins), to connect them to girts and summer beams (and pricipal rafters). If executed properly, these dovetails fit tightly and will actually help pull together the girts (and rafters). Unfortunately, due to their placement, these beautiful dovetail joints are never seen once the house is finished!
We cut our male dovetails a bit undersized ("take the line") and our female dovetail pockets a bit oversized. Then we used a homemade jig to test each piece before calling it done and stacking it in the barn. Assuming one gets the length right (shoulder to shoulder), on raising day, the joist or purlin should theoretically drop right in. OK, maybe 70% of ours have dropped in like they should have... while on average the other 30% put up a fight! After we had the bents square, and the posts plumb, we went around driving dried hardwood wedges into the dovetail joints. As you can see in the lower right picture, this really pulls the joint tight. In the bottom of the uppermost leftmost picture, you can see an 'ornery oak 8x8 joist that has bowed, causing a gap where the oak 6x8 joist is joined to it. When we drove the wedges in, you can bet that 8x8 had no choice but to bow back into shape.
In the old barns here in KY, the old timers used simple lap joints to join the hand hewed sill plates. If you're judicious about it, you can substitute this simple lap joint for a dovetail joint in order to save time. I did this on the 1st floor occasionally, but rarely on the upper floors (as shown in the upper right hand picture).
Of course, cutting the dovetail notch into the supporting beam weakens it, so you don't want the notch to be too big. One book recommended 2.5" long dovetails and one book recommended 3" long dovetails. Not having any real basis for an opinion of my own, I started using... you guessed it 2 3/4" long dovetails. But one day we were invited to a TimberPeg frame raising in Cinci, OH. and there I got to see what kind of joinery their German CNC machine cuts. It was an eye opener, since they departed from traditional joinery here and there to make joints easier (or possible at all) for the CNC machine to cut. I noticed that their dovetails were a scant 1" to 1.5" long! (see the pictures below and compare the joints to the pegs which are presumably 1" in diameter) After this revelation, I decided to begin using 2" long dovetails in my own frame, where I was concerned that 2 3/4" dovetails might unneccesarily weaken the beams. (in leftmost, bottommost picture in the picture group above I have used both 2" and 2 3/4" joints on the ends of 8x10 joists. hmmmm... hard to see a difference... maybe I'm sweating something that doesn't really matter!)

<-- TimberPeg joinery, presumably cut with a Hundeggar CNC machine.

Monday, January 23, 2006

Progress as of January 2006

I'll have a lot more to write about how we got from the ice storm (winter of 2003) to this point (winter of 2006!), but until I can go back and fill in the gaps, I'll be posting our actual progress as it happens. Here's where we are as of January 2006... Bents A,B, and C are up, with rafters on top. When it's done, the house will have 9 bents, one tower, and two shed extensions, so I'd say we're about 25% done with the raising of the frame. Most of the timbers have been notched (there are about 400 timbers in total), so it's mostly a matter of finding them in the barns, sanding/oiling them, assembling them into bents/trusses on the ground, and then raising them.