A new regular feature: Preservation 101!
What will you learn in Preservation 101? All those things you’re too scared to ask in Home Depot, the things preservationists give you snotty looks about, and the things that make architects invite you to their cocktail parties…as the entertainment.
Today’s topic, brought to you by the Letter F, is FRAMING.
What is framing?
[Thanks to PoPville for the photo, taken today in Mount Vernon Square- looks like moving these historic buildings to save them from demolition wasn’t exactly successful. One hopes the Association of Medical Colleges building is more sturdily constructed.]
This photo shows an interesting juxtaposition: aluminum framing and brick walls. Historically, framing and walls went together and “framing” meant the timbers that supported the wall materials. Some examples are shown below:
Box framing: The oldest and most basic type of framing. Literally, timbers are joined at angles to create a structure, then infilled with a non-load-bearing medium. Typical box framing creates squares and rectangles out of “posts” and “lintels”. Think Stonehenge, then make it out of wood.
Cruck framing: here at ca. 1700 Wig-wen-fach in Ceredigion, Wales, bent wooden timbers are set into stone walls and infilled with smaller branches and thatch to make the roof. This is also one of the oldest types of framing, in which roof and walls are integrated to create a pitched angle (makes rain slide off instead of pooling and causing leaks). It’s really a very very basic vault based on the same loading principles of the arch.
Things start getting fancy when buildings get bigger than the length of a single tree.
For something bigger, you need to start figuring out how walls create tension and how you can use the load-bearing properties of wood to support the wall and roof materials.
Why can’t you just build massive buildings out of stone or brick rather than wood and save yourself the trouble? Well, you can.
[Barcelona Cathedral, courtesy Dan Hogman]
But it’s kind of expensive, takes forever, and sometimes doesn’t hold up as well.
Timber framing is much cheaper and faster, especially in places like Northern Europe where large timber isn’t hard to find. Also, brick walls, like the brick wall shown above at Mount Vernon Square, are much more vulnerable to stresses and have a tendency to collapse unless they’re well-built and thickly built. Why? Brick is hard, mortar is hard, and the harder materials are, the more likely they are to crack or break rather than bend. Wood is a much more forgiving material because it tends to be (relatively) more flexible. Also, the more pieces that go into a frame, the more likely the frame is to distribute stress rather than fail.
So obviously once you get bigger than the size of a single tree, cruck-framing isn’t really an option (except in lovely, very tricky things like hammerbeam ceilings). Box framing lets you use tree trunks as a uniform material that can be built into all different sizes of truss.
A truss is described in engineering terms as “five or more triangular units constructed with straight members connected by joints”. Why triangles? Because they’re the strongest shape and the easiest strong shape to build with straight (tree-shaped) pieces. In the photo below, vertical and horizontal pieces of timber are braced by diagonal pieces to create triangles:
But again, the diagonal pieces can’t be longer than the length of a tree. So to make buildings taller and bigger, you have to distribute the downward pressure of the building’s weight by adding pillars or posts and the outward pressure of the walls by adding beams, joists, and sills- these are big pieces connecting all the little truss pieces into framing systems.
[Merchant Adventurers Hall in Fossgate, York, courtesy of English Heritage]
Again, triangles are the strongest shapes. But look again at the photo of the brick Mount Vernon square house again- there are clearly no triangles in that sadly bent steel framing. Why not?
Again, money and time. Post and beam construction takes a lot of skill, big pieces of wood, and a lot of time to do correctly. Master carpenters have to carefully cut the wood to size, with lots of tricky angles, and then stick the pieces together using sophisticated joinery techniques such as mortise-and-tenons or dovetails. The wood has to be well-seasoned (this can take years) to prevent contraction and cracking, and no piece of wood handles exactly the same way.
Stud framing developed as a handy way to hang plaster on stone, brick, or heavy-timbered walls. Vertical, non-load-bearing wooden pieces helped infill walls and acted as temper material to improve wall flexibility and resist temperature changes. Later, studs supported small flat pieces of wood called lath that you could smear plaster on to create a pretty, smooth interior surface that conveniently reflected heat.
Only in the late 19th century did someone figure out that whole walls could be made out of vertical studs and they probably wouldn’t fall down. Instead of massive timbers, buildings could be made out of forests of small, light pieces of softwood pine that were cut to a standard size (i.e. 2×4). Instead of bracing the studs into triangles, you can really get away with just block bracing the studs at right angles (making square and rectangles) because that’s so much easier to cut and works sort of as well. Plus, unskilled laborers can put these buildings together, requiring neither the training nor the higher wages of skilled tradesmen.
Even better, plaster could be made in sheets compressed between thick paper and hung on the studs- no lath or plaster needed, hence “dry”-wall. Even better, you could just hang weatherboard on the studs’ exterior and stuff insulation in the cavity, yay cheap, fast, crap construction!
Why did anyone even consider this idea? a) a post-Civil War population and housing boom, b) the modern insurance industry. Build crap houses and by the time they fall apart, you (the builder) will be long gone and it will be easier for people to file a claim with their homeowner’s insurance rather than chase you through the courts. Homeowner’s policies cost so much for the simple reason that most modern homes have significant failures within twenty-five to thirty years unless they are scrupulously maintained. Softwood, vinyl, fiberglass, and plastic- all materials never designed to last.
Now, to be perfectly fair, the steel framing in the first photo wasn’t supporting the wall (I hope), just the interior drywall. But here, a steel frame roof looks like a used napkin after a measly thunderstorm in South Carolina:
And that is why disaster scenes inevitably look like someone spilled a box of matches:
So, next time you’re in negotiations with your contractor:
1) studs are those vertical things behind your drywall
2) framing means (to you) the arrangement of vertical studs and horizontal blocks between horizontal plates that form rectangles
3) triangles are stronger, salvaged old-growth wood is harder, and modern materials are crap