Oct 10

# Stair Stringer Layout / The Calculation Procedure

Stair stringer layout is not difficult, quite the contrary. Once you do the calculations (basic mathematics), it’s easy to layout stair stringers using a framing square and a pair of square gauges.

Allow me to take you through the simple process of how to layout stair stringers, because frankly, I’ve seen a lot of “do it yourself” people waste their money on pre-made stair stringers, which won’t work 9 out of 10 times anyhow (in a moment you’ll see why). Also, because there is no reason for anybody to be intimidated with the thought of building stair stringers.

Basic Rules of Thumb for Residential Stair Stringer Layout

• Stairs consisting of more than 15 risers should include a landing, preferably somewhere in the middle of the flight.
• Each stair riser in the same flight must be equal.
• Each tread in the same flight must be equal.
• There is always 1 less tread than the total number of risers.
• 1 tread plus 1 riser should equal between 17″ to 18″. This will ensure a comfortable fall line of roughly 35 degrees.

For example, if the riser is 7.5 inches, than the tread should be 10 inches (7.5 plus 10 equals 17.5).

There are more rules of thumb to consider when building stair stringers such as headroom, handrail and nosing stipulations (see your local building codes) but for the purpose of this tutorial, the above rules and following tutorial will be sufficient for building deck stairs.

Why Don’t Pre-made Stair Stringers Usually Work?

Store bought stair stringers are cut to a predetermined unit rise. Unless the total rise of a deck (measurement from concrete landing to decking) is a multiple of the predetermined unit rise of the pre-cut stair stringer, it won’t work. To make matters worse, these stringers are sometimes pre-cut to accommodate a predetermined tread thickness.

However, for those who are bent on using store bought stringers, there are two possibilities of making them work. On second thought, there is only one logical possibility of making them work;

• Buy the stringers first, pour the landing slab and then mathematically determine the finish height of the deck.
• Take a chance that pre-cut stringers will work. What do you think of the chances, 1 in 3,215?

Why Treads and Risers Should be Equal

There is a reason as to why treads and risers must be equal in the same flight. If they are not equal within reason (building codes usually allow a 3/16″ variation), your steps become a trip hazard, because as we walk up steps we subconsciously measure the rise by lifting our feet the required distance to move up one step.

If one step is suddenly taller, there is a good possibility that you’ll stub your toe and trip. I’ve seen more people trip going up stairs than going down. When there is a short tread thrown somewhere in the middle of a flight of stairs is when you’ll see folks fall as they walk down, because they missed a tread with their heal. That is why winders are so dangerous (pie shaped treads).

Calculations for the Stair Stringer Layout

First, we must determine the number of risers in our stair stringer layout. Determine the number of risers by dividing 7.5 into the total rise (in inches) from finish floor to finish floor. Seven and one half is used, because it is the most desirable riser height.

Example, total rise equals 96″ divided by 7.5 equals 12.8. Therefore, you can have 12 risers, each being 8″ or 13 risers, each being 7.38″ (multiply .38 x 16 = 6.08 = 6/16 = 3/8, so the rise is 7-3/8″).

It is better to go with 13 risers at 7-3/8″ each. Since our riser is 7-3/8″, the tread (run) should equal 10″ according to our rules of thumb, right?

Now that the rise and run are established, it’s time to use the framing square with your square gauges. Square gauges are used to hold the square in the correct position for laying out the treads and risers. It makes no difference as to weather you start your stair stringer layout at the top or bottom of the stair stringer.

Personally, I start my layout at the top and slide the square down the stringer, marking-out each riser and tread until I get to the bottom or last riser.

Layout Stair Stringers

Set a square  gauge on the tongue of the square so that the  7-3/8″ mark is on the top edge of the stringer, set another square gauge on the blade or body of the square at 10″. Using a sharp pencil, draw lines along the outside edge of the square (see drawing, sorry but you’ll have to right click on drawing to view image full size).

Next, slide the square along the top edge of the stringer, moving the 7-3/8″ mark in line with the previously drawn 10″ mark. Number each tread 1,2,3 and so on until you reach the required number of treads (figure C).

In this example there are 13 risers and 12 treads. One riser is actually the floor framing at the top of the stairs (figure B).

Now your stair stinger is almost laid out. But the top and bottom of the stringer must be configured to accommodate the tread material so that the risers end-up equal after installing the treads (figure B). Thickness of the tread material will determine this configuration.

Measure down from the finish floor the riser distance (7-3/8″) plus the tread thickness. This is the elevation at which you will set the top tread cutout on the 2 x 12 stringer. In all likelihood, you’ll have to cut a notch at the top of the stringer so as to make it fit snug at the upper floor framing (figure B).

The bottom riser will be cut so as to make it equal with the other risers after fastening the tread. Simplified, cut off the bottom a distance equal to the tread thickness, minus the depth of the finish floor.

For instance, your tread thickness is 1-1/2″, your stringer is going to rest on the sub-floor and the finish flooring is 3/4″ hardwood, which will stop at the base of the stairs. So, 1-1/2″ minus 3/4″ equals 3/4″… Cut 3/4″ off the bottom of the stringer.

The riser height (stringer cut) from the sub-floor should be 6-5/8″. After a 1-1/2″ tread is added and the 3/4″ finish is installed, you’ll have 7-3/8″ risers along the whole flight of stairs.

Congratulations, your stringer is laid out! Now you just have to cut and install it.

Cutting Stair Stringers

Before cutting all the tread and risers into the stair stringer, cut the top notch and make the level cut on the bottom of the stringer. This is done so as to test fit the stringer layout and see how level the treads are.

You or your boss has just bought four 2 x 12′s 14 to 16 foot long at around \$30 each, so let’s not mess this up! That is why it is wise to make this test fit. When testing the accuracy of the stringer layout:

• Locate the elevation mark previously made and line this mark up with the top tread layout line (elevation at which top tread will be set).
• Check the fit at the bottom of the stringer where it sets on the floor.
• With a torpedo level, check to see how level the “tread” layout lines are, they should be within reason (very close to level).

Now that you’re satisfied with the test fit, you’ll want to cut the rest of the treads and risers. This stringer will be the pattern when laying out the other stringers in this flight of stairs, so the accuracy of this stringer will determine the accuracy of the stairs.

I start by cutting all the tread layout lines and then the riser layout lines but you can do the opposite, because it doesn’t matter. What does matter is that the saw kerfs stop no more that about 1/4″ past the intersection of the tread and riser layout lines. Finish this inside corner cut with a handsaw or a reciprocating saw (if you have one).

Cutting too far beyond this inside corner may compromise the structural strength of the stringer.

Okay, the Pattern is Cut, Now What?

Now you just have to transfer the pattern onto the other 2 x 12′s to finish the stair stringer layout, cut and install them. Start by nailing a temporary 2″ x 4″ block (top and bottom) on the bottom edge of the stringer pattern. This will aid in keeping the bottom edges in line while you trace the pattern onto the other stringers.

Building stair stringers, trick of the trade… When tracing the pattern onto another stringer, make sure that the layout face of the pattern (side in which the layout lines where drawn) is face down on the other stringers when tracing the layout, because this ensures accuracy.

When cutting the two outside stringers, leave the layout line. When cutting the inside stringers, take away the layout line. After the stringers are cut and installed, take a straight edge and check to see how well the risers and treads line up with each other. The inside stringers should not protrude beyond the line of the two outside stringers at any point.

It would be ideal to everything line up like ducks in a row but that doesn’t happen too often in the real world. That is why it is better to have all risers on the inside stringers shy of the risers on the outside stringers (shim riser boards straight). The same applies for the treads.

Fasten the Stair Stringers

Fasten the stringers at the top with toe-nails to the upper floor framing, reinforce with banding iron or metal strapping nailed to the bottom edge of the stringers and the backside of stairwell header.

A 2″ x 4″ is fastened to the concrete slab or floor joists at the bottom of the stair flight. The stair stringers are notched around and nailed to this 2 x 4, this will lock the stringers in place.

Put Your Stairs Together

The 4065 Construction Master Pro

Finish by installing the risers first. Then install the treads. Start at the bottom of the flight when installing treads, so the treads can be back-nailed through the risers as you go up the flight of stairs. You’re done, except for the handrail.

See, stair stringer layout is not all that difficult. Using simple math, a few rules of thumb and the framing square is all it takes. No need to buy pre-cut stair stringers.

Note: There is a great tool for doing all your stair calculations, it’s called the construction master pro. Included are a number of built-in time-saving functions, including calculations for risers, treads, stringer length and incline angle.

Carpenter’s usually have their own way of building stairs. There is more than one way to skin a cat, so to speak and I’ve never had a problem with that, as long as we have the same end result. The video below shows another way to layout stair stringers.

For comprehensive instruction on building a set of stairs, you should read “Smart Guide: Stairs & Railings“. This “step-by-step” guide is excellent. The text is concise, organized and key information is well highlighted.