ArcaMax

You may be like most folks. I’m guessing you give very little thought to the stairs you go up and down on a regular basis. They might be constructed from wood, steel, or concrete. The vast majority of stairs in residential homes are made from wood.

Wood is very easy to work with, and it can create very strong stairs. I stood just 10 feet away from a marvelous circular staircase made from wood. It was in the Loretto Chapel in Santa Fe, New Mexico. This staircase has no center support column. Many feel that’s a miracle, but the truth is no central support column is required. The two twisted helical stair stringers that support the treads are the structural beams that support the stairs and anyone going up and down them.

I feel it’s important for you to understand the structure of a set of stairs before you get out your saw and framing square. This basic knowledge will allow you to construct a strong set of stairs. These stairs will support you, your buddy, and that 300-pound refrigerator. Think about the concentrated load of two corn-fed boys/men plus a heavy appliance! Weak stairs have collapsed under such circumstances.

Simple stairs have just two components. The flat board that your foot steps on is called a tread. The treads connect to side stringers and sometimes to one that is in the middle of the stairs. The stringers are no different from wood floor joists, steel I-beams or steel-reinforced concrete beams that support many tons of weight.

I’m sure you can relate to wood floor joists. The building code permits the use of 2x8s in houses. They must meet certain specifications. These 2x8 floor joists can create a floor that resembles a trampoline, and still be code-compliant. It’s unnerving to walk across a floor that bounces up and down, in my opinion.

You can choose to use 2x12s or even giant floor trusses to create a wood floor system that has no springiness. The floor resembles walking across bedrock with absolutely no give whatsoever.

Now think about a set of stairs. A narrow set usually has only two stringers, one at each end of the treads. These stringers are not level. They are angled up or down in an opening so they connect one floor to another.

Imagine if you lifted the lower ends of the stringers and made them parallel with the upper floor joists. Then imagine the treads were rotated to sit on top of the stringers. Your stairs are nothing more than a narrow, tilted floor, much like a child’s slide at a playground.

I see stairs all the time that have stringers that have the strength of weak 2x6s! Perhaps you have a set of these leading from your deck down to the ground. I’m talking about stairs where a solid 2x12 was used for a stringer. However, you or the carpenter proceeded to notch the wood, creating the flat spots to attach the treads. These notches resemble the teeth on a saw blade. They transform a strong 2x12 to a much weaker 2x?, depending on the width of the lumber leftover from the notching process.

I avoid this issue, and you can too, by creating shallow 1/2-inch-deep mortises in the 2x12s. The treads fit into the mortises and are attached to the stringers by driving nails or screws through the outer face of the stringers into the ends of the treads.

The mortise method preserves all the strength of the 2x12. It’s a bit more work to create the mortises, but it’s worth it in the long run. You need a framing square, a circular saw, a router, and a wood chisel to make perfect mortises.

For the sake of discussion, let’s say you’re building a simple set of stairs for a deck, basement access, or up to an attic. You’ll just be using 2x12s for both the stringers and the treads. This lumber will make a very strong set of stairs.

The framing square is used to lay out on the face of the stringers where the mortises will be. The front nose of each tread must be the exact same distance from the edge of the stringer. I prefer to create stairs that have a 7.5-inch riser and a 10-inch tread. Using a 2x12 as the tread provides the code-compliant 1-inch overhang for foot safety.

A normal 2x12 is 1 1/2 inches thick. I create a mortise that’s just under 1 5/8 inches wide. I do this because the 2x12s are often not flat. They may have a hump or dip in the center. The larger mortise allows you to insert the tread into the stringer with minimal effort.

I set my circular saw blade depth to 1/2 inch. I then start to make parallel saw cuts within the 1 5/8-inch lines. Each cut leaves a thin wafer of wood that’s about 1/8 inch, or less, wide. These wafers snap off when tapped with a hammer.

The router is used to create a smooth face inside the mortise where the wafers used to be. The wood chisel is used to square up the two inside corners of the mortise. I have to tell you, creating these mortises is therapeutic for me. The process creates an enormous feeling of satisfaction when you see the treads fit into the mortises like a hand fits into a glove.

I wrote a short ebook filled with photos showing the step-by-step process of marking out and creating the mortises for a set of simple stairs. You can get a copy for just $7 by visiting this link:

http://go.askthebuilder.com/StairsEbook

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