Hi folks....

As I get closer to the day I will need a roof on my hewn log cabin I would like to be able to engineer the roof rafters on my own if possible. The cabin will be 20' deep and I would like roof rafters with no ridge beam, just a collar tie. Rafters spaced every two feet. Any ideas on what an engineer will charge for this if I cannot do it on my own? Keep in mind there will be a knee wall ~ 4' high so I will need something to keep the rafters from spreading under load. Hence the collar tie.

Also was considering getting a copy of the "timberframe Joinery and Design Workbook hoping them may have an example I could work from and just tweak. Any comments from those who have the workbook?

Bruce,

Hi Bruce.

First of all, a picture is worth a thousand words when describing a frame. Second, if you really meant "collar tie", I'm afraid it won't help you with rafter thrust. It will help you with rafter sag, which is quite different. The joinery workbook will help you immensely. CB.

Thx...I will get the book! Here is a pic of what I am trying to do. Actually my desire to timberframe came fromthe fine Norm Hart who came over one day to view my progress on a hewn log cabin. It needs a few more courses of logs but may need a roof this winter!

First you'll need to know your snow load for your area.
Also, the roof pitch.

It's a nice picture but Ed Levin has done structural analysis testing of collar ties and has found that they are not in tension (reducing thrust) until they are within a foot of the top of the wall. In all other places they are in compression (reducing the sag as mentioned).

The name "collar tie" is misleading.

And it has been suggested that the name be changed to just "collar" or "collar beam". Most names indicate location of the timber in the frame. Somewhere sometime ago, someone had three different names for this piece so that each name would indicate the location and function of the piece. A search of the forum or articles in Timber Framing may produce that list.

The joinery book will give you formulas to figure your loads and size your beams.

Good luck with your project.

Jim Rogers

In a bent system, if the post is continuous from ground to eave, past the second floor deck, then the 2nd floor beam acts as a tie in the building. If a collar is used (as Ed Levin's article discusses), it must be placed very low in order for it to take up the tension, rather than the floor beam. In this particular case, the knee wall is a log wall, not a continuous post, so the collar will may be in tension ( a tie ) regardless of where it is placed, depending on how the log wall is constructed.

The design workbook should be able to provide some guidance to help you do some of the figuring yourself. You may just want someone familiar with designing and engineering to review your work, rather than have it engineered from scratch.