In the last article we covered the issue of rafter thrust and how to cope with it when designing you timber frame. In this article we will be covering timber species well suited for timber framing.
When you are planning your frame and you are trying to figure out your loads you cannot properly figure out the loads without first knowing what species of wood you will be using for the frame. This is important because part of load calculations will be based on material weight (dead load). So let’s go through some of the pros and cons of certain species choices.
My personal favorite: eastern white pine. We have two species of wood in North America that are very stable and very well suited to use for timber framing. White pine happens to be the second most stable choice and if you live in the east it can easily be had. White pine is fairly light when it is green compared to some choices (average 3 pounds per board foot). This means that your dead loads will not be quite as heavy as many other species. The other attractive feature of white pine is that it is very stable and does not have a large amount of shrinkage or twisting. This can be a good thing if like me it takes you awhile to put your frame up.
The draw backs to white pine are few but really need to be considered. White pine will rot much faster than say oak or hemlock if left exposed for too long. The sapwood in white pine is also favored by insects, and bugs can wreak havoc if not dealt with quickly.
The next species on the list is Douglas Fir. Doug fir is rated the most stable species of wood in North America by many and is probably one of the best choices out there for timber framing. Doug Fir comes in at 3.2 pounds per board foot. Doug fir is a very strong species, seems to do well with insects and is very easy to work with. Most barns and frames out west were made of Douglas Fir and they have stood the test of time. There is also a large market for repurposed Doug Fir out there. If I had access to this species I would be using it instead of white pine for my frame.
White oak…what can we say about white oak? White oak is a very widespread species of many variations throughout the world and there have probably been more frames cut out of white oak than any other in the western world (this is an educated guess). White oak weighs in at 5.3 pounds per board foot and has a high moisture content. This high moisture content lends to a lot of movement in the timbers as well as a large amount of shrinkage. White oak also checks very deeply. Even with those issues white oak is a very strong wood, it also fairs well in the rot and insect department. The pros of using white oak is that your timbers do not have to be as large as say white pine to support the same loads. It is a straight grained wood and is fairly decent to work when it is green, cut your joinery quickly though because when it dries out it is a real bear to tool.
Hemlock…I do not like hemlock for timber framing or much of anything for that matter. That is just my opinion and it would not be fair of me to not write what it is good at. Hemlock weighs in at 4.2 pounds per board foot and is a very strong species even though it is a softwood. Hemlock does well with rot resistance but the carpenter ants love hemlock. The thing that makes hemlock very undesirable for me is that it is a crap shoot to get quality timbers when you mill it up. You could get excellent timbers out of one log and have nothing but problems with another log. Hemlock has a tendency for grain shake which is a condition that occurs when the growth ring separates within the tree. Hemlock is also subject to large amounts of movement which can be very frustrating if you go to put your frame together if the timbers have had time to dry out. Hemlock also checks very deeply but that does not seem to affect its strength.
These are just a few species to choose from but I believe they are the most common that can be obtained fairly easily in many places. Your timber selection will count quite a bit on what you have available to you in your area. Each species has its merits and its downfalls. It will be up to you to decide which is most suitable for you to use.
Jim, The Tradesman