Timber Frame fit up workshop results

[Jim_Rogers]

Saturday (8/5/17), I helped a client with a "fit up" workshop.
A fit up workshop is where we test fit every joint together so that when we put the frame up everything goes together smoothly.

To begin this, we set up 8 saw horses on the deck of the 18' x 25' cabin.
These saw horses are all the same height so that we don't have to bend over and try to work on everything at floor level. It is much easier, for everyone, to work on timbers at this height.

We started by putting four posts, one from each bent, on the saw horses, aligned with their locations over the deck.
Then we found the braces that will go from the post to the connecting girt of that wall.
It was the south wall.
Here is a picture of the south wall assembly all together:



 


This frame's timbers were all planed on all sides and very nice quality for the most part. And the layout method used was called "mill rule" this is a little different then "square rule".
The distance from post center to post center was 8' 4". From outside post to inside post was also 8' 4" but because it this the girts were different lengths as well as some of the braces were different lengths.
Each time we put a brace, post, and girt together (after trimming if needed, as some were trimmed) we'd check the corner using the 3-4-5 method. All corners checked were right on.

After all the post, braces and girts were labeled this wall was disassembled.

Next we set up the second wall, which was the north wall.

During this text fit we had a rain shower came over us.
We stopped for a short while and had an early lunch while the shower passed.

Using two truck ratchet straps we pulled the wall assembly up tight and checked the end post distances to verify that the frame was 25' long. And it was.

Next we set up one tie beam and a "summer beam" and dropped in some floor joists going between the two and checked that all the joists fit and all were labeled before disassembly. Labeling now makes is easier on raising day to find the pieces and place them where they go.

Using the 8 saw horses we next set up the south roof plane. This roof system was a principal rafter system with purlins going between the rafters. There is an short plate going from rafter to rafter at the top of the post, which is called an interrupted plate.



 

The piece with the bevel cut in the foreground is the interrupted plate.

Each bay had its own short plate. All the floor joists, girts and purlins will be held in place when raised by timberlok screws. Only braces will be pegged.

During the assembly of the roof plane we again had to find the correct length purlins for each bay. Bay "A" and bay "C" had the same length purlins; but bay "B" were longer. We couldn't find enough long purlins to complete the roof plane. Checking the records of the frame cut sheets showed that last May the owner was suppose to cut four purlins of this length and he had only cut one. That's why he had, what he though was extra, 6x6 stock in the timber pile. Careful record keeping is a must.

The reason why we put the entire roof frame together like this is to verify that all the purlins and rafters joints are cut correctly. And that all the distances are correct, before we fly them up on raising day.

When I measured the distance from rafter on bent one to the rafter on bent two, I noticed that the reading was 3/16" too long.
Now, lots of you will say, so what? Well, what happens when the lengths of the other purlins are also too long? When we get down to rafter on bent four the frame is going to be overhanging the deck or posts.

I then measured the purlin overall length and it was 90 inches as per plan. And the housings into the rafters were 1 ¼" so that the end of the purlin wouldn't bottom out in the housing. But the shoulder to shoulder distance which was suppose to be 88" was 1/16 too long. A 1/16 here and a 1/16 there add up and can push a frame out and/or not have the rafters plumb when installed. And if you install the rafters in the correct location and plumb and then try and "drop in" the purling (which are too long), you're going to have a fight.

Bottom line is everything should be cut correctly and to the right dimension. And fit properly when doing a fit up.

The client and his son, as well as the helpers all learned some valuable lessons at this fit up workshop.

Jim Rogers

Rafters set up waiting for missing purlin:
 


 

[Jim_Rogers]

Mapping a timber frame.

Mapping is the method of correctly noting or cutting of joints based on the actual dimension of the timber.

This client had ordered 8x8 timbers planned on all for sides. When I measured rafter for bent one south roof it was 8 1/16". And rafter two was 8 1/8" wide. That's where the 3/16" came from when the dimension didn't match the plans.

So, mapping tells us to measure the width of the timber, as delivered. Then adjust your shoulder to shoulder dimension to account for the difference in the "as requested" size and the "delivered" size.

In this case the purlins in bay "A" will have to be cut back from 88" to 87 13/16" on both roof planes so that all the rafters line up to their posts.

Mapping and recording the "corrected" distance is a special way to make your frame pieces fit and hopefully go together smoothly on raising day.

Once a timber is "mapped" or length adjusted due to the mapping process, it can only be used in that location and it is no longer "interchangeable" with other timbers of the same type.

What I'm saying is you can't take a purlin from bay "A" and now use it in bay "C". Even though they are suppose to be exactly the same.

Personally, this is why I don't like "mill rule" layout.

Jim Rogers