Half Timbering in Modern Construction
About 2 weeks ago, I had someone post a question about how to construct a half timbering detail for modern framing construction techniques. Half timbering was the primary method of construction during much of the medieval era, and many of the architectural styles that emulate these medieval construction will use a false half timbering to express the aesthetics of that era. Of course, the wood timbers in olden days was actually structural, and was infilled with either brick, or mud and stucco.
Today, we relay on much more efficient stick framing (2 x stud construction) for the structural components of our houses. The pine for this construction system can be mass-produced and regrown fairly quickly, and it's modular sizing makes it easy to design and build with. Of course, there are other techniques and methods available, but the platform variation of stick framing is the industry standard for now.
Brick homes in this construction technique are made by applying a single veneer of brick to the exterior of the house. While the brick stacks on top of itself and thus they weight of the brick is self supporting, the lateral resistance (what keep it from toppling over) is provided by the stick framing behind it, and the masonry veneer is fastened to the wall framing behind it with brick ties to make the entire wall assembly. An air space is left between the stick framing and the masonry veneer to allow for waterproofing and insulation purposes.
Some drawback to this construction technique are that brick (and stone) are porous materials- they will allow a small amount of water to leech through them (as will the mortar that hold the individual masonry units together). However, the proper use of flashing, a good vapor barrier and weep holes at the bottom of the masonry to allow any water that gets behind the brick to leave the the wall cavity and keeps the framing and insulation of the structure dry and prevents rot, which obviously wood is susceptible to.
So, in modern stick framing techniques, the half timbering of the exterior is also applied as a veneer and laid side by side with the masonry. This mixing of materials does leave for some structural and sustainability issues that need to be addressed. Masonry will allow for water to seep through, but does not rot as wood does when it gets wet. The heavy timber size will not rot as quickly as a smaller 2 x4 would, and there are certain species of wood that are less susceptible to rot than others are. Still, over the years, a half timber veneer wall will probably need replacing sooner than just a masonry veneer will. Exactly how long is not something I would care to guess on though, as so many factors will come into play; such as how well the timbers are flashed, the species of wood in question, and how well the maintenance is kept up with- (the wood will need waterproofing or painting every so often, as any exposed wood finish would). Still there are many medieval heavy timber structures that have been standing for literally hundreds of years.
I've sketched out some detailing below from the book "Architectural Graphic Standards" by Ramsey and Sleeper (tenth edition). This is a fantastic resource for many construction techniques.
The 1st sketch below is a typical heavy timbered gable for a Tudor house. (French medieval architecture used heavy timber construction as well, but rarely used front facing gables.) Here you can see that the heavy timbers are spaced fairly close together (giving the illusion of them being structural). Also, the brickwork infill was done in some sort of pattern (this illustration shows a mixture of several patterns, but usually just one would be repeated over throughout the whole section). The pattern was not only decorative, but reinforced the fact that the brick was not structural.
I've got a section drawn through the elevation at point "A" below. This is probably one of the more challenging parts of doing a well designed half timbering design- the 2nd floor typically projects out slightly from the floor below it (historically referred to as "jettying") This means that the masonry does not stack from the 1st floor to the 2nd floor, and the load of all the 2nd story brick and timber is carried by the 2nd floor joists, which cantilever out past the stud wall of the 1st floor below.
This can set up a detail that can cause obvious structural issues. Too much brick on the exterior could cause the joists to fail. There are obviously several calculations that come into play, such as how far out the jetty is (less that 1' is probably sufficient for getting the desired effect, and keeps the cantilever to a minimum). Also, if you're using an open web wood truss, as most modern homes are, you'll probably want to reinforce the cantilevered part with solid wood blocking to keep the joist from getting crushed.
Also, if you are using a LOT of brick, you can reduce the weight by having the masons cut each brick in half. Labor intensive, but the weight savings are significant. Just keep in mind that when you do that, you then apply the brick straight onto the exterior sheathing (with water barrier, of course!) probably over a metal lathe, as if it were a tile or "cultured stone". You'll then need to cut the timber to an appropriate thickness as well.
Some other things to note about this detail:
The book is showing a steel shelf angle behind the heavy timber itself- so the weight is transferred to the joists by the steel angel and NOT being supported by the bolts or nails holding up the timber.
Also, the book shows a piece of metal flashing at the top of the timber. You may be tempted to eliminate this as you would have a piece of exposed metal between the wood and the brick, but you will significantly reduce the life span of the wall if water is allowed to come into contact on the top of that timber. Don't forget to provide weep holes for the masonry above any horizontal timber also.
The last detail below is of a typical window head and sill condition. The book is showing a fairly custom wood window with shaped wood mullions, and that would be the more historically accurate way to do the detail, but you can use more generic windows in this technique and still get good results aesthetically. Again, note that the timber is supported by a steel shelf angle, and the use of flashing at the top of the timber.
And, although I didn't mention it earlier, these are large beams of wood- not a 1 x trim applied to the surface. These beams are at least 6" wide, and depending on the placing on the exterior, will probably vary between 8" and 12" tall (taller at the horizontal members, smaller at the angled "bracing" members). Ideally, you'll want the surface of the wood and the brick to be fairly flush with each other.
Unless it's what you're going for, I think what you ultimately would want to avoid (aesthetically) is the sort of "mock tudor" done in the 1970's and 80's- which you can identify by it's white stucco infill and applied 1 x trim slats stained dark brown. They sizing and spacing of the slats (which are supposed to be timbers) are so small and far apart that the are obviously purely decorative. Here are some images of what you are trying to avoid:
And here are some images of half timbering done well:
Anyway, I hope that helps and gives some good information about the subject. I should also add a small disclaimer that I am not a structural engineer, and while I do put a fair amount of trust in the Architectural Graphic Standards book, I would recommend having a registered structural engineer go over any jetty style design that you are doing. It can be done and done well (as the above pictures will attest to).
And, of course, I welcome any comments or questions.
No comments:
Post a Comment