Part One: Framing
I can still remember the first time I worked on a job with a curved stairway. By the time I got there it was sheet rocked with temporary treads. Up to that point I had built several straight stairs and even one or two that hit a landing and changed directions, but this curved thing was a complete mystery. The guys who had worked there the whole time shared just enough to really get me interested but they weren’t giving up many of their secrets. I made up my mind that if they could do it so could I.
It took several years, and a lot of time asking questions and a few false steps, but I did get the chance to build a curved stairway for a friend. I didn’t make a lot of money, but the education I got was priceless. The number one thing that I learned is there isn’t that much difference between a curved stairway and a straight one. It is still all about rise and run and getting the layout right. I have built several freestanding curved stairs and a few that only have a curved wall at the outside radius, but the one we built for this article is supported on both sides. I’ll have to save building the unsupported stairs for another article.
A curved stairway actually has two rise and run layouts—one at the inside or tight stringer, and a different layout at the outside stringer. The good news is for my simple method you only have to worry about the inside layout. The first thing I have to do to layout a stairway is figure out how many rises it will take to get from one floor to the next. It is always a good idea to talk to the local building inspector, but where I live 7 3/4 in. is the maximum rise allowed per step. I start by measuring the distance from one floor to the next, then divide by 7 3/4 (make sure you make allowances for the finished floor on both floors) to get the number of rises and then adjust the rise per step to be equal. Since the staircase starts with a rise up to the first tread and there is one more rise from the last tread up to the next floor, I always have one less tread than the number of rises. Everyone has different ways of working.
For me doing a vertical layout on a story pole helps me double check my math.
I actually layout the subfloor and finished floors along with the top and bottom of each tread to make sure I have the rise for each tread correct. On my jobs those are always one-inch thick wood custom made to match wood flooring in the house.
As soon as I am set on the number of treads my mind has to change gears to thinking horizontally. I need to know the size and location of each tread so I do a full-size layout on the floor exactly where the stairway will be built. My curved stairs are usually a little more the one quarter of a circle and it takes a little bit of trial and error to figure them out. I’m sure that there is a way to do that with a CAD program, but that’s just not the way my mind works. I have built stairs where the framing carpenters have already built the outside curved wall based on the plan before I got there, but that really shuts down my options. If the depth of floor truss grew (that increases the rise) or someone moved a door or made it bigger (that decreases the potential run), the wall on the plan may not work. I really recommend building your own walls or at least getting to the job early and doing your own layout.
To find my pivot point, I start at the point directly below what will be the far back edge (at the outside curved wall) of the top step and measure back fourteen feet. That is just working from experience so you can see where the trial and error part comes in.
I use trammel points that clamp onto a 2X4 at 10 ft. 6 in. and I draw well over a quarter of a circle using the 14-ft. pivot point. That is the radius for the inside wall. I extend the points to 14 ft. and draw the outside radius. Now I am ready to do a trial layout for my tread size.
There are two places you have to meet a minimum size with a curved stairway. The minimum size where I work is 6 in. at the inside radius and 10 in. at the walk line, which is 12 in. in from the inside radius. These measurements don’t include any tread overhang, but I always think the best thing to do is call your local inspector and make sure you are on the same page with him.
|This is the really simple part of my system. I start at the chalk line and make a mark at 9 and 18 in. (just based on experience) on the inside radius. Then I go back and drive a screw at my pivot point leaving just enough to hook my chalk line.|
|I pull a line through the 9-in. mark all the way to the outside radius, then I pop a line and move to the 18-in. mark to pop another line. That gives me a tread layout.|
As long as it meets the width requirement at 12 in. in, I am ready to lay out the rest of my treads. If it doesn’t, I just have to make my marks along the inside wall line longer and pop another set of lines with a different color of chalk. We double checked to make sure 9 in, worked then made a mark every 9 in. along the inside radius, one for every tread. If space is really tight you can draw a radius 12 in, in from the inside (that is at the walk line), and then make marks every 10 in. to pop a chalk line through. That will give you the absolute minimum size tread that will meet code. After I made the last mark I popped another chalk line through the last mark to locate the start on my first step. This house had a double front door and the stairway came about halfway in the front of one door. But I had made sure it wasn’t the active door and there was room to swing it open with a couple of feet to spare. That much room usually feels pretty good to me. If you don’t like where the first stair starts, then the only options are to increase the rise per step to lose a tread or shrink the tread size. If you are already at the maximum rise and the minimum tread size, then the only other option is to change the wall radius.
When I am happy with the location of the first step I go back and pop a line through all the other marks all the way to the outside wall line. These lines actually represent the front of each riser. I like to number each tread at my inside wall line and I number the treads on my story pole too. It really helps me keep track of where I am when I start to combine the horizontal layout on the floor with my vertical layout on the story pole to build a curved wall. I learned a long time ago to build a curved inside wall with steps in it. That is a lot easier than laminating a stringer, building a wall under it, and then cutting steps in the stringer. The curved wall that I build actually becomes the form for my inside stringer. The steps in my curved wall give me the location to cut the stinger for each step.
The bottom plate of my curved wall is actually a series of wedge shaped pieces of 2×4 that fit between the studs at the front edge of each step. For this stairway they were 7 1/2 in. long and cut on a 2-degree angle. It usually only takes a couple of cuts to find the angles and length I need, then I can cut enough wedge shaped pieces for the whole stairway. A miter saw on a good stand with a way to set a stop for repeat cuts really speeds up the whole process. It takes two studs at each tread, and it’s pretty easy to make a stud cut list from the story pole. The front stud (I am calling these the “A” studs) at each stair is the length to the bottom of that stair tread minus 1 1/2 in. The back stud is 3 in. shorter than the bottom of the tread (I will call these the “B” studs). I am always careful to save my straightest material to make the tall studs toward the top of the stairs.
The chalk lines on the floor mark the front edge of each riser. Since the riser has to run past the front stud at each stair I move the stud location one inch behind the chalk line to allow for a 3/4 plywood riser and a little room to shim. The first wedge piece gets nailed on the inner wall line 2 1/2 behind the chalk line (one inch for the riser and 1 1/2 for the stud), then the first stud (“A1”) nails into it. The lead stud for the second step (that would be “A2”) nails into the back of the block, then the 3-in. shorter back stud (“B1”) nails to the face of the “A2” stud.
About every 3-4 steps I add a temporary diagonal brace to keep the wall plumb. Later on I will add blocking for drywall on the bottom side of the stairs, which connects the inside and outside wall so I can get rid of the braces.
On some stairways the entire outside wall is complete and I just have to laminate a curved skirt board or stringer for the treads to die into.
|If the wall is already sheet rocked, I glue and nail the layers straight to the drywall.|
|I plow pockets into the skirt board with a router to receive the stair treads…|
…and I glue on a cleat to support the tread:
Other times I get there first and just keep building my curved wall with the steps in it inside the 2-story curved wall.
|A third option is to laminate an extra thick (to allow for the sheetrock) stinger right to the 2-story curved wall studs and let the sheet rocker work to me.|
The first four steps on this stairway were open on both sides so I started building a curved wall at the outside radius. All the stud lengths were the same at each step (same amount of rise at this layout) but the wedge shaped pieces were longer (the run increases at the outside radius).
|I actually built the wall for the first four steps to the outside of the outer radius. That makes the first four steps about 4 1/2 in. wider.|
That extra width let me move the centerline of the handrail over just enough to die into the wall.
Since I beat the drywall guys on this job we just moved to the inside of the radius line and kept building our wall with steps in it after the first four steps.
We added short joists between the walls for a drywall ceiling under the stairs after both curved walls were complete.
The last step in the framing part of our curved stair job was to nail down temporary treads to walk on and put up a temporary rail for safety.
I couldn’t wait for the sheet rock work to be done because I really enjoy the second part of a stair project like this one. Part two of this article covers laminating skirt boards, building custom pie shaped stair treads, bending a curved stair rail, cutting, fitting and installing handrail/balusters, and final trim details. That might sound like a lot to take in but it is just a step-by-step process. I am sure that, just like me, once you see it you will realize there is no mystery to it—just some hard work that really pays off.
Check out Jed Dixon’s article for more information on story poles for stair layout.