How to form board form concrete.
Average Rating: ( 0 votes)
Without sounding like a broken record–or iPod–we have a lot of board formed concrete in our project. If you’re new to reading this blog and want a description of what I’m talking about, you might check out this post. I haven’t asked quite so bluntly, but reading body language I think the concrete team has [...]
Without sounding like a broken record–or iPod–we have a lot of board formed concrete in our project. If you’re new to reading this blog and want a description of what I’m talking about, you might check out this post.
I haven’t asked quite so bluntly, but reading body language I think the concrete team has more than a little bit of project fatigue, and are ready for the next big pour. To be clear, building board form walls is seriously labor intensive. And we have a lot of walls. Hence a lot of labor.
From early in the design phase knew we wanted board formed walls in our project. Finding out more about the process, and identifying a good contractor though, was more difficult than I expected. Googling “board formed concrete” didn’t help too much. Mostly I found a lot of examples of very expensive residential or commercial projects, with little information about how to do it or what it costs.
Locally, I talked to a few contractors who all said some version of, “man, I don’t know. You’ll probably need to find some old guy to do that for you.” (Board forming is a pretty dated technique.)
The helpful peeps at Olson Collins made a suggestion of someone to speak with. Olson has incorporated board form work into some of their projects, and in fact one of their contractors has built a pretty slick system for building formwork. Check out the pic below. The problem is, we couldn’t get any specifics from him beyond, “that looks really expensive, like maybe a hundred grand or something. I’m not sure though.”
Along the helpfulness spectrum, that suggestion was pretty far to the left.
So, this is all a long build up to walk through how Mikel and his team created our walls.
Our contractor, Carrie, has some experience with board form work, and what we’re doing is similar to how she created walls in her own house.
We start with forming walls like you would for normal, smooth-walled concrete using plywood and snap-ties.
Once the first side of the wall is up though, Mikel nails rough-sawn cedar to the inside. A lot of people use fir 2 x 4s, or 2 x 6s–wood that’s typically used for framing. This gets expensive though, and the fir tends to flake off and get stuck in the concrete. It also doesn’t leave as much of an imprint in the concrete as cedar–a quality we want.
Then Mikel needs to figure out how to work around the snap-ties. These are used to keep the plywood forms stable during the pour. If you’re not familiar with concrete, as it’s poured and set, it exerts a tremendous amount of pressure on the form work. The ties hold things together.
However, the snap ties can get in the way of the cedar boards. The way we worked around this is to use a variety of board heights. You’ll see this in our wall. We may have an 8″ piece of cedar, then a 4″, then a 6″, 2″, etc. I love the variety. Very cool.
The other thing we did was use a variety of thickness of cedar. Again, this will get picked up when the concrete is poured, and I think what we’ll end up with is a wall that looks a bit like an old ruin.
I found the cedar for this project on craigslist for a great price–59 cents per lineal foot. This is the upside of the end of the building season and a down economy, I guess. The material was all 1″ x 8″, and 7/8″ thick. So we sent it to a big shop and had it ripped in half. This effectively doubled the amount of wood we bought and now have some material that’s 7/8″, some that’s 1/4″ and some that’s about 1/2″. Mikel is mixing these in randomly.
According to Mikel, the most challenging part of our walls (beyond the sheer size) is the corners. When he gets to these, things get tight and it’s hard to maneuver the cedar, the rebar and the snap-ties. Mikel is also working hard to try to get the courses of wood to align–a nice touch for sure. He and Carrie also decided to add triangular shaped trim called chamfer strips that ease the corners. I think he was worried about our daughter getting hurt on a sharp concrete corner. Thoughtful, eh? (I also think it looks quite beautiful.)
The big pour happens early next week. The boys have about 30 feet more of wall to construct, then want to take a day or so to get the bracing right (remember the big pressure from the concrete?) and to get the walls perfectly straight.
Unfortunately I’ll be out of town when the forms get pulled off the new concrete, but I’m sure I can convince someone to take a few pictures. As Jennifer Love Hewitt might say, “can’t hardly wait.”