DEAR TIM: It appears I must use a spiral staircase to solve a tight access problem between two floor levels in a room addition I am building. This circular staircase was a shock and I need to get up to speed quickly with respect to sizing, appearance and structural issues. What do I need to know about spiral and circular staircases so I do not regret owning one? Sharon R., Manchester, NH

DEAR SHARON: The first thing you need to do is ensure you are using the correct terms. I have found that some people say one thing and mean another when talking about curved, circular or spiral staircases.

A curved staircase is one that has a gentle bend between floor levels. The total curvature or change of direction from one floor level to the next may only be 90 degrees or less. These sweeping, gentle curves make for dramatic staircases and have had many cameo appearances in movies and television shows. A curved staircase takes up a generous amount of space on the floor plan of a home.

This five-foot wide set of spiral stairs has been heavily used for nearly 20 years. The oak treads are screwed to the metal treads making for a classy, yet contemporary look. PHOTO CREDIT: Tim Carter

Spiral and circular staircases are two different terms that generally describe the same product. These staircases resemble a corkscrew and may twist 270 degrees or even up to 360 degrees as the stairs turn a fairly tight radius. Spiral staircases that connect floor levels of 12 feet or more can sometimes have several complete 360 degree rotations in the staircase. These staircases are often used when space is at a premium and a normal straight, L-shaped or curved staircase will not work.

The least expensive spiral staircases usually have a center support column that each tread connects to. You can purchase helical spiral staircases that have no center support column. The support for the spiral comes from two stringer boards that connect to each flat tread. These stringers are twisted like taffy and each one has a different radius. These spiral staircases are extremely dramatic and seem to float in the air, defying gravity.

I have installed several different spiral staircases. One of the things I've learned is that bigger is better. The overall diameter of the staircase is critical, especially if you think there might be people going up and down the staircase at the same time. If you feel this will happen, then you better consider a spiral staircase that has an overall diameter of six feet or greater.

It is vitally important to talk with building code officials and develop an accurate plan of how the staircase will be built. The railings, tread area, baluster spacing, riser height and overhead clearance are all important considerations with respect to the building code. Do not order a spiral staircase until you have a written approval from your building department. Then be sure the staircase is built exactly as it is drawn.

Spiral staircases can be very heavy. If you plan to have one that will have a center support column, this post needs to bear on a solid floor. If you have a wood subfloor system, you will undoubtedly need to have the center support column directly over a floor joist below. If the staircase is extremely heavy, a structural engineer may have to design a special solution so the staircase does not deflect the subflooring or cause the wood floor to crack.

This spiral staircase was made in one piece by some talented welders. It took four people to carry it and lift it into place. PHOTO CREDIT: Tim Carter

A spiral staircase that rests on a concrete slab is usually not an issue so long as the slab is on compacted fill. But the building department may require special concrete footers beneath the spiral staircase to help spread the weight over a wider area. Do not underestimate the weight of a spiral staircase, especially when four people may be going up or down at the same time.

The variety of spiral staircase styles seems endless to me. They can be ultra-modern to classic Victorian. Any number of manufacturers exist who specialize in these unique, but useful, staircases. It would be very wise for you to consult with several before making a final purchasing decision.

If you decide to have a spiral staircase built in one piece, do not forget to plan how it will be installed. Although the staircase may have a finished diameter of five or six feet, you might be able to make them fit through a smaller three-foot wide doorway. A set of spiral stairs can be brought through a doorway on its side by actually screwing the staircase into the house as you would install a screw into wood. I have seen them dropped through a hole in a roof by a crane. The point is, plan ahead for how the staircase will make it into the house.

The distance between finished floors is very critical. Be sure the distance is calculated from the upper landing to where the lowest tread ends. It is vital the floors are level and measurements are double checked. If possible, schedule a time for the manufacturer to do a field visit to ensure the spiral staircase they are building will work at your jobsite.

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Wood Staircase Length | This is the stairs going up to the second floor in my daughter's new home. You need to do a little math to calculate how long of a space you need for stairs to fit.

Staircase Length - It's All About Distance Between Floors

DEAR TIM: I am making rough plans for a new home. What is the footprint for a normal set of stairs from the first floor to the second floor of a house? William Worthington, Pearl City, HI

DEAR WILLIAM: Unfortunately there is no exact answer. You can blame the mathematicians who claim there are an infinite amount of points on a line. But all joking aside, I can get you pretty close.

What is a Normal Staircase?

You didn't give me enough information to give you a highly accurate answer. First I am going to assume that normal to you means a straight staircase. The first thing a carpenter needs to know is the exact distance between the two finished floor levels. There can be many other variables including, but not limited to, the wall height for the first floor walls and the thickness of the actual second-floor structure.

Can You Adjust Riser Height and Stair Tread Depth?

It actually is more complicated as you can adjust the riser height of the stairs as well as the tread length. These have to fall within certain limits for the building code, but for my example below I used the most comfortable tread-riser combination known to man: 7.5-inch risers and 10-inch treads.

What Controls the Length of Stairs?

The length of stairs is controlled by the distance between the two floors the stairs connect.

The first-floor wall height can be any dimension, but common ones seem to be a standard eight-foot-high wall or perhaps a nine-foot wall. The floor joists for the second floor could be 2x8's, 2x10's, 2x12's, and even deeper floor trusses or wood I-beams.

For the sake of this column, I did two quick calculations for you. I assumed you would have a standard eight-foot-high wall and I did a calculation for a nine-foot wall system. In both examples, I calculated 2x10 floor-joist material.

What is the Total Stair Height for an 8-Foot-Wall House?

In the eight-foot wall system, you have a total rise of 108 and 5/8 inches. Here's how I arrived at that number:

• 9 and 1/4 inch for the 2x10 floor joist
• 3/4 inch for the second-floor subfloor thickness
• 92 and 5/8 inches for the stud height for an interior bearing height wall
• 4 and 1/2 inches for the top and bottom plates for the bearing wall

This number is not evenly divisible by 7.5 inches, but it is close. It will yield a staircase with 14 risers.

Why Do Stairs have One More Riser Than the Number of Treads?

All staircases have one more riser than they do treads because you have to step up to get to the first tread.

In my example above for the 8-foot-high wall home, this means you will have 13 treads for a total length of 130 inches or 10 feet 10 inches. If the stairs end at a wall, you must also figure in a minimum three-foot landing or clear floor area at the bottom of the steps. For an eight-foot wall height, figure on a 14-foot-long footprint for a straight staircase.

What is the Height for a 9-Foot Interior Wall Home?

If you have nine-foot-high walls on the first floor, your total height is 120 and 5/8 inches. Once again this is not equally divisible by 7.5, but it yields 16 risers. Multiply the 15 needed treads by 10 inches and you get 150 inches or 12 feet 6 inches. Add to this the three-foot clear area and you end up with 15 feet 6 inches for a straight staircase in a house with nine-foot-tall interior walls.

How Can you Shrink the Footprint of a Staircase?

You can minimize the footprint by installing an L-shaped stairway if needed. The absolute minimum footprint for the stairway would be a circular staircase.

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