Carpentry Technique:
Designing Stairs And Laying Out Stair Stringers
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In This Article:
An illustration of the process used to design and lay out stairs and stair stringers. |
Related Articles:
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Skill Level:
3-5 (Intermediate to
High)
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Time Taken:
A Few Hours
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Author:
Bruce W.
Maki, Editor
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Project Date:
June 2003
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Stair building is one of the most complex aspects of carpentry. My experience is that a good and accurate design is the only way to approach any complex geometry problem. I made these drawings on an inexpensive computer-aided drafting program called QuickCAD, which I bought for $60 several years ago. At first I hated QuickCAD because it was bass-ackwards from all the CAD programs I had used years before. I ignored the program for a couple of years, and eventually I took the time to struggle with it and (most importantly) read the manual for solutions to problems that kept arising. Eventually I figured out the darned thing, and now I can't live without QuickCAD.
Without CAD software, stairs can be laid out on paper, using ordinary drafting tools. When complete, you can use a ruler to take measurements from the drawing, measurements that would be difficult and time-consuming to calculate. But... care needs to be taken to create precise linework. I would recommend using as large a sheet of paper as possible, even using poster board, which can be bought at office supply stores or Wal-Mart. The larger the scale used (say... 3 inches on paper equals one foot in real life), the more accurate your results will be.
Another alternative is a special spreadsheet for stair stringers from www.Shalla.net. This spreadsheet calculates all the necessary dimensions for stair stringers, and lets you print out a diagram and list of points that can be transferred to the stringer stock with a tape measure and marker. The spreadsheet does the hard work for you.
Identifying The Stair Parts:
Stair stringers (red arrow) are the notched boards that
are placed at an angle.
Stringers are the structural support for the stairs. You can think of stair stringers as sloping floor joists.
On this project the riser boards are painted white and the treads are painted green.
Riser boards are not necessarily required. Without riser boards, you have open riser stairs.
Note that there are 3 stair treads but 4 risers. (Three of the risers are painted white, and the top riser is bare wood.)
Riser vs. Riser Board: What's The Difference?
Technically, the riser is just the vertical distance from one stair tread to the next.
Riser boards cover the riser area, thus preventing you from seeing under the stairs.
This set of tall stairs have open risers, which simply
means that there are no riser boards fastened across the front of the
stringers
Note About Tall Stairs: Your local building code may have rules limiting the height of any single "run" of stairs. In my area, the maximum height of a single section of stairs is 10 feet. (The stairs shown here were just under that.) If the vertical drop was taller than 10 feet, I would've had to build a landing part way down.
Stair tread nosing is just the small overhang (red arrow) at the front of each step.
Stair nosing may or may not be required by your local building code, but nosing is a good idea because it makes walking up the steps feel more natural.
AND... Without nosing, the stairs will actually take up more space, because the total run will be longer. Since nosing allows a slight overlap you can fit the entire series of stairs into a shorter horizontal distance.
Nose depth of 3/4 inch to 1-1/4 inch is usually acceptable. I typically use a nose depth of one inch when designing stairs.
If the top-front corners of all the stair treads were connected with an imaginary line, this would be called the Nose Line.
All of the tread "noses" should touch this line, but small deviations are not a problem.
When designing indoor stairs (such as basement stairs), the nose line is very important where the stairs pass under the end of the stairwell opening (the place you'd bang your head if the stairs weren't built right).
Building codes normally require 6'-8" of clearance above the entire nose line. This dimension is the same height as a standard door opening. The reasoning is: if a person doesn't hit their head on a standard doorway, they won't hit their head when going down the stairs.
Creating The Stair Layout:
Step 1
Determine heights of
finished floor, deck, or sidewalk surfaces. This determines the rise
of each step.
The distance from the deck surface to the floor, sidewalk or deck surface below is called the total rise.
Then some math must be done. If a certain riser height is preferred (say 6 inches), then divide the total run by the preferred riser height. Or... use the riser height of the pre-cut stair treads, otherwise use a good starting number like 7 inches.
This gives the number of risers (steps) needed. Let's use an example of 27 inch total rise. 27" divided by 6" gives us 4½ steps. Oops... you can't have half a step. We can have either 4 or 5 risers.
27" divided by 4 gives a riser height of 6.75 inches.
27" divided by 5 gives a riser height of 5.4 inches, which is kinda short. So 4 risers will be our choice.
Note that the number of risers is not necessarily the same as the number of steps. You always get one free riser. Suppose a house has one room that is 7 inches lower than the next room, you don't need any stairs... you just have a step down to the next platform. A deck could be built this way, with several different levels, each 6 to 8 inches lower than the previous. That's a lot of trouble to avoid building stairs, though.
Step 2
Determine the thickness
of stair tread material.
On most decks this is simply one inch, the thickness of standard 5/4 x 6 deck boards.
But... there are extra-thick deck boards available (I keep running into these darned things), and there are synthetic decking materials that can be any thickness the manufacturer wants.
Step 3
Lay out the heights of
the UPPER SURFACES of the stair treads.
These are the surfaces you walk on, and the surfaces that the building inspector measures from.
First... We subtract the riser height from the finished deck surface, which gives the location of the top of the upper tread.
Then... Then we subtract the riser height again from that first line (the top surface line) to get the top surface of the second tread. And so on...
Laying this out is much easier on paper (or a CAD system) than trying to create layout lines on the actual project. I suppose you could draw layout lines on the deck support posts, or you could drive some stakes into the ground and mark them.
Step 4
Subtract the thickness
of the stair treads to determine the horizontal
cut lines that will be marked on the stair stringers.
If using pre-cut stringers, hold the stringers in a position so the highest horizontal cut meets this line. Of course, the stringer must be held with the horizontal cuts level, or else your stairs will suck.
Step 5
Determine a starting
point for the outer (front) surface of the risers.
On many decks and porches, the top riser will determine the starting point, because the top riser is often the outer joist of the deck structure.
Step 6
Determine the final
position of the top tread:
The important geometry is the location of the front edge of the first tread.
Common tread materials for exterior decks are:
- Two 5/4 x 6 deck boards, which gives a tread width of about 11 inches.
- Two 2x6's which gives a tread width of about 11 inches.
- 2x12, which gives a tread width of about 11¼ inches.
- 2x10, which gives a tread width of about 9¼ inches. This certainly works for indoor treads but may not be acceptable for deck stairs.
In this example we'll use a pair of 5/4 x 6 deck boards, which creates treads one inch thick and 11 inches wide.
Step 7
Subtract the nose
overhang distance to get the location of the front of the next
riser.
The nose is usually one inch. Nose distances of 3/4" to 1¼" are usually acceptable.
Step 8
Subtract the riser
material thickness to get the vertical cut
line for the stair stringers.
Riser materials are usually:
- 1x6 or double 1x4 treated wood (3/4" thick)
- 5/4x6 deck boards (1" thick or more)
- Sometimes 2x lumber is used (1½" thick)
See the next drawing for this...
Step 9
Repeat steps 6, 7 and 8
for all the other treads.
In other words, now that the first riser outer surface has been determined, the tread width can be laid out in front:
- This gives us the location of the front edge of the second tread.
- Back off the nose overhang to get the outer surface of the next riser
- Back off the riser thickness to get the vertical cut line on the stringer.
Step 10
Convert stair layout
to stringer cutting layout:
Draw a line to connect all the points on the stair stringers. Note that this is parallel to the "nose line" of the finished stairs, but is not the same as the nose line.
Make another line parallel to the first line, 11.25 inches apart. This denotes the width of a 2x12, the standard material used for stair stringers.
In the drawing below, note some interesting patterns:
- The 7.75" top vertical dimension is not part of the stringer... this is a dimension that locates the position of the top of the stringer relative to the deck surface.
- The middle vertical dimensions (6.75") are the riser heights.
- The lower vertical dimension (5.75") is just the riser height minus the tread thickness.
- The 10.00" horizontal dimensions are the "effective tread width". This is the actual tread width minus the nose overhang distance. If you took a "bird's eye view" of the steps from above, you would see only 10 inches of the 11 inch treads.
- The 9.25" horizontal dimension is the effective tread width minus the 0.75" thickness of the riser. When the risers are nailed onto the front edges, the effective tread width will return to 10 inches. And all the remaining tread supporting areas will simply "shift forward" by 0.75", if that makes any sense.
One benefit of a simple 2-dimensional CAD program is that you get easy (and very accurate) measurements of the overall length, and the angle between the stringer bottom and horizontal. Note that some of the tread cut-out angles are the same 34 degree angle, and the other lines are the complement of that angle, 56 degrees. You remember the Complementary Angle Theorem from high school math, don't you?
It's kinda intuitive... if a line is 34 degrees above horizontal, then the angle between that line and vertical is just 90-34, or 56 degrees.
A really powerful benefit of CAD software is the ability to quickly get distances from the end of the board to the stair "points" on the stringer. From these points the angled cuts can be laid out, and inaccuracies are held to a minimum.
This is NOT how most carpenters lay out stair stringers. The
traditional practice has been to use a rafter framing square with
little hexagonal brass buttons (sold separately) that are clamped
onto the framing square to establish fixed dimensions. I'm not sure
I can explain this process... you can read about it in the book
Basic Stairbuilding
Before you hurt yourself, read our disclaimer.
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Step 11
Build Stairs
Note that many decks are not built with the decking overhanging the edge by one inch, so stairs built against such a deck may have a missing nose at the top. This is not normally a problem.
The procedure for building stairs is typically:
- Install the stringers. Stringers would be fastened at the top, to the deck. It may also be desirable (or required by code) to fasten the lower end of the stringers to posts in the ground.
- Install the riser boards.
- Install the treads.
For more information on building stairs see these articles:
- Building enclosed stairs.
- Building basic stairs with composite treads and risers
- Building a deck, part 2
- Building porch stairs
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