Designing Your Own Timber Frame: The Effects of Rafter Thrust

I recently put up a video on designing your own timber frame and what you need to know before you begin.  This will be a multiple part article that I hope you will find useful in trying to design your own frame.  I would also like to add that this process requires a lot of planning as well as a lot of attention to detail, if you are not comfortable with this process you need to consult with a timber framing engineer.

The first step in this process is figuring out the intended use for this structure as well as a rough idea of what you want for square footage and so on.  The beauty of a timber frame is the idea that you are able to achieve large open spaces which makes these buildings ideal for shops, agricultural functions or an open floor plan for a house.

Once you have figured out what type of a building you want and what its intended use will be you will then have to figure out what you plan to use as a building material.  There are several options for suitable material and it seems that most people use what is readily available to them.  On that note there are some species to avoid for a frame due to stability factors as well as rot and insect infestation issues, we will save species selection for another article.

Now that you have chosen what species you will be using the real work for design and planning will begin.  We will assume that by this time you will have a floor plan in your mind and a general idea of what you want to see in your frame.

When designing a frame you need to start at the roof of the structure and work your way down to the foundation.  The reason you need to do this is to make sure that each framing member is able to support the weight of the members and systems above it.  You are also going to try to carry the loads uniformly to the foundation.  In this article we will deal with rafter thrust and its effects on the frame.

Rafter thrust is a major load that actually pushes out against your top plates which in turn push out against your tie beam joinery.  The steeper the roof, the less rafter thrust that you will have to deal with.  The lower the pitch, the more pressure there will be pushing out against you top plates.

So how do you design your frame to fight the load of rafter thrust?  The first and probably most common method would be to design your frame with purlin plates and queen posts.  The purlin plates will support your rafters mid-span and take a lot of load off of your top plates where rafter thrust is concerned, the queen posts will support the purlin plates.  Another method is collar ties.  Collar ties work well if you are not looking for usable space above the top plates.  Collar ties resist rafter thrust when they are placed within two feet of the top plates and connect both rafters and form the shape of a triangle.  These are two basic ways to resist rafter thrust and are probably the ones most commonly used.

Think of rafter thrust as a force that is trying to push your walls apart from the middle out.  The joinery that this force will have the most effect on will be the mortice and tenon joint where the tie beams meet the wall posts.  This joint will have to be big enough to withstand the pressure on it from rafter thrust.  I prefer a through tenon at this joint so that there is a large amount of relish on the tie beam tenon to avoid the tenon splitting out at the joint, relish being the amount of meat left on a tenon past the peg holes.  If there is not enough relish on this joint the tenons will tear out.  The mortice and tenon joint at the wall post and tie beam intersection is a joint that will be under tension, meaning the joint will always be trying to pull apart from the force of the rafters pushing out against it.  Keep in mind that if a scarf joint is needed in your tie beams it needs to be a joint that resists tension.

This article is intended to give you, the reader, a basic understanding of the loads on a timber frame and how each piece needs to be designed to support the forces against it.  This article does not replace the advice of a qualified engineer that is trained in timber framing but it will give you a better understanding of the basic frame design.

Jim, TheTradesman