Circular stairs are not as difficult as they seem the first time you think about doing them. They do, however, take some planning and careful work. There is no one way that curved stairs need to be built, as long as they are strong enough to bear the traffic. The method that follows is commonly used.
First of all, unlike straight stairs, we will not use stringers. Instead, each tread will be supported independently, by either a wall or a header. The header method allows for space to be usable under the stairs. The system outlined here uses a header to create what are called tread walls.
• It is not uncommon for a set of plans to be drawn up with the stair headroom less than the 6′-8" that the code requires. To check the headroom before you frame the stairs, you need to find the point that is plumb, down from the lowest point above the stairs, and then measure to the line in a plane with the nosing of your stair treads.
• Since the stairs are not built yet, the hardest part is finding that nosing plane. You can either work off the plans, if framing has not started, or work with the framing if the frame is ready for the stairs. To find this plumb point on the nosing plane, start from the first nosing, count the number of risers, and multiply that number by the riser height; then add the partial riser.
• To get the partial riser height, you just multiply the partial tread length by the riser percent, which is the riser height, divided by the tread length. Once you have found this length, you can measure either up or down, depending on which direction you used, to see if you have enough headroom.
8 Steps for Building Circular Stairs
To get started, you first need to find your riser height. Quite often it is given on the plans, in which case you want to check it to make sure it works with the actual floor heights. If the height is not given on the plans, consider the following points when figuring the riser height.
• As with straight stairs, you want the steps to feel comfortable, so remember:
— Make sure all risers are equal, so the stairs
will not cause people to trip and fall.
Drawing the circumference lines
— The lower the riser, the longer the tread needs to be to feel comfortable.
• Common dimensions for riser and tread on straight stairs are 7" for the riser, and 10%" for the tread.
• For circular stairs, the tread width varies, so it is more difficult to figure the riser and tread dimensions.
• The comfortable range for circular stairs is harder to determine than the range for straight stairs because of the varying tread width.
The code for residential buildings requires a minimum of 6" at the narrow end and 11" at a point 12" out from the narrow end. For other buildings, the code requires a minimum of 10" at the narrow end and 11" at a point 12" out from the narrow end. Because most of the length of the tread is greater than 11", the rise will typically feel more comfortable if it is less than it would be for a comparable straight stair.
2) Mark the Circumference Lines
With the rise figured out and the number of treads known, you can start marking your circumference lines. The best way to start is by making the stair footprint on the floor in the position where the stairs are going to be built. If the plans show a radius dimension and location, then you can use the plans to locate the radius center point. To make your circumference lines (which represent your stairs and the walls on the sides of the stairs), set a nail partway at the located radius center point. Then hook your tape to the nail and mark your circumference lines by swinging your tape around the nail and holding your pencil on the required dimension.
(See photo.) Most tape measures have a slot in the hooking end for a nail head. (See illustration later in this section.)
If the radius or the radius center point is not given, you will need to find it. You can vary the radius length, but make sure you can maintain the following four requirements:
• 6" minimum tread width at the narrow end of the tread (10" in non-residential).
• 11" tread width at a point 12" in from the narrow end.
• A minimum stair width of 36" in the clear to finish.
• In non-residential buildings, the smaller radius should not be less than twice the width of the stairway.
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End of tape hooked to a partially set nail
The first thing you need to do to find your radius is to establish two points on the circumference opposite each other. They can be any two points. Look on the plans for points that are already established. If there are no established points, then select points that fit with the location of the stairs. Once you have established two points, it is merely a matter of bisecting the line between these points, finding the radius origin, and drawing your circumference lines from the radius origin. (See the “Bisecting a Line to Establish the Radius Origin" illustration.)
3) Marking the Tread on the Footprint
Now that you have your circumference lines, you need your tread lines. Since you have figured your riser height, you know the number of treads that you will have. Knowing the number of treads, you can find the exact tread point along your stair circumference. To do this, divide the stair circumference in half, and then divide those halves in half again and again until you are down to single treads. (See “Divide Circumference for Treads" illustration.)
If your stair has an uneven number of treads, then you have to subtract one tread before you begin dividing into halves. To subtract one tread, you first have to know the width. The width will be equal to the total stair circumference length divided by the number of treads.
It is difficult to measure the stair circumference, and so your one tread will probably not be exact. Therefore, when you are done marking all the other treads, re-mark the tread you measured first.
Once you have all your division points for the treads, then chalk lines from the radius center point through the division points to the longest circumference line, and those lines will make your tread footprint. (See “Tread Footprint" illustration.)
The bottom plate of the tread walls will not be parallel to the top plate, as it would be in a straight stair. The bottom plate will follow the circumference and serve as the bottom plate for all the tread walls. A good way to make the bottom plate is to use two pieces of %” plywood. If the radius is not too small, you can cut the plywood with a circular saw. To mark on the plywood, set a nail anywhere and mark the plywood with a pencil and a tape measure. Use the dimensions from the stair footprint to get the radius length.
To build the stairs, start by nailing the bottom plates in place. (See “Bottom Plate Nailed in Place" illustration.)
The walls supporting the treads will be built as header walls. Built this way, they will provide the riser and allow space for storage below the stairs.
The wall will consist of a 2 x 12 single header that will serve as the riser, a top plate, a double plate, trimmers for under the 2 x 12 header, and king studs next to the trimmer. A ledger to support the tread below will be nailed onto the header. (See “Section of Tread Wall from End" illustration.)
Each tread wall should be higher than the one below it by the riser height. The height of the first step will have to be figured separately to equal one riser height, adjusted for any difference in floor covering. The top step might also have to be adjusted for a difference in floor height.
Nail the tread walls in place using the footprint lines. The bottom of the studs will be toenailed into the bottom plate already in place. (See “Tread Walls Nailed in Place" illustration.)
The treads should all be the same. They will be nailed onto the top of the tread walls and the ledgers. An equal nosing should be maintained the full length of the tread. Make sure the walls stay plumb both ways. Glue each tread to prevent squeaks. (See “Treads Halfway Up Stairs" illustration.)
