Thursday, July 19, 2012

How to Build a Radius Concrete Curb Form

A spiral staircase is the central feature of Cypress House; the core, if you will. Circlular walls make carpenters like me both nervous and excited: nervous because building anything with a radius is tricky, but excited because we aren't just building straight walls for a change! Circular or straight, a load-bearing wall begins at the foundation. This is how to build a radius concrete curb form on concrete footings.

I started by fastening a full sheet of 3/4" plywood to the cut station. I then added a half sheet to start cutting. Here you can see the marking stick, with four 2" common nails, each of which will serve as a different radius: the inside and outside of each curb. As you can see, one arc is already scribed, the largest of the four. These were determined by the architect and are noted on the blueprints. 
At the far end of the marking stick, I marked centre of the fixed sheet of plywood with a line and made a mark every 3 1/2". After scribing an arc, I pulled the nail, moved the marking stick back to the next line and tacked the nail into the centreline. This step helps to create consistent pieces. 
The second scribe in progress. It is important to note that this method (marking one line and moving the nail back) will create cutout pieces that are narrower at the ends than the centre. This is fine for curb forms, but not for a whaler or strongback in wall formwork. 
Here is a shot of the four different radii.
I kept the full sheet fixed to the cut station, and cut up scraps of 3/4" plywood, leftover from the wall and curb formwork taking place at the same time. Here you can see some cutouts as well as the differing radii. I used a jigsaw with a speed blade. 

With enough cutouts produced I started with the bottom plates of the form, laying them out overlapping each other (expanded in this picture for clarity)
In this photo, notice the small square of concrete near the circular saw with the orange dot on it. We poured that little cube with the rest of the footings, on top of what we thought was close to the centre of radius. When we had the surveyors on site to lay-out the building corners, they also found the centre of radius of the stairs, marking it with a concrete nail and flagging tape. I subsequently pulled a tape measure off the centre to mark on the footings each of the four radii. With the bottom plates nailed together I nailed them to the concrete.  
With the bottom plates fastened, I began fastening 3/8" plywood to them with 2" common nails. An old hammer with the claw cut off is your best friend for a tight nailing job like this.  
Here, with the top plates fastened in an arc, I nailed the 3/8" plywood to them. At this point, the plywood, which was previously somewhat floppy and misshapen, assumes a nice curve. 
Here it is all finished up. Of note: cleats were added to hold the tops at a consistent 8" width, there are bulkheads where we determined the curb needed to end, it ties into another straight curb (top of photo) and on the large radius, both ends turn into short straight curbs. 
Here, Joel is wet-setting anchor bolts soon after the concrete was placed (this method was approved by our engineer)
And finally, here they are, stripped of formwork!

Author: Sam


  1. Excellent. I really appreciate that you documented this curved footing process.

    1. Glad you found this post useful. Thanks for the feedback!

    2. Can you tell me how to figure concrete when pouring steps at a radius?

    3. This is a 2 step process and all the measurements are taken from the center of the stair treads in meters. Measure the width and length of the center of the flight of stairs and multiply by the throat thickness. measure the rise and run of the treads and multiply by length of tread, then multiply by number of treads and divide by 2. Add the two numbers together and add 10% extra. hope that helps

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  3. lets take it one step further, now how do I cut the plywood face if it was on a 45* batter?