How to Measure for and Cut Custom Exterior Stair Stringers

Home Improvement: With a logical approach, and a little know-how, creating custom stair stringers can be an efficient and enjoyable project.

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How to Measure for and Cut Custom Exterior Stair Stringers



Illustrations by Len Churchill (Lead Photo by Dreamstime)

Just because pre-cut stair stringers are sold in many building supply stores doesn't mean they will be best for your outdoor project. Stairs are a connection between two different elevations and the best results are reached by creating a custom stringer built for the specific site conditions you will encounter. The result of using off-the-shelf stringers to construct a set of deck stairs will almost always lead to an inequality in riser height at the top or bottom of the stairs, which can be unsafe for climbing. In the built environment your body makes assumptions that stairs will be consistent in height, and when they're not people can trip and fall.
 
I think people often avoid cutting their own stringers because they are intimidated to do so, and yet it's an easily attainable and rewarding skill to possess. I'll walk you through how to measure for, mark and cut a pair of stair stringers for a deck that will fit consistently every time so you can approach your next stair-building project with confidence.
 

Rise and run

Stair basics include understanding that each set of stairs has a total rise and total run, and an equal incremental rise and run for every step of that stair. The individual run of a stair step is called its tread, typically a 2x piece of lumber that your feet will step on, and a 1x riser forms a barrier to stop your toes from going through, and gives strength to the tread's leading edge. First you need to figure out the total rise and run of the set of stairs, and to do so you'll need to determine what you'll be using for stair treads, how steep you want the stairs to be, and locate exactly where they will land.
 
Using two 2x6 stair treads separated by a 1/4" gap for water drainage is standard, but you could go with one 2x6 beside a 2x8 if you want a deeper tread and more gentle stair climbing experience. You'll next need to figure out if the risers will be open, or if you will be adding a riser board. When you add up the width of the tread boards you'll be using, plus the spacing gap between them, subtract the width of the riser board and subtract the overhang you want and you will be left with your stringer tread cut length. For example, with a double 2x6 tread with riser it would be 5-1/2" board + 1/4"gap + 5-1/2" board - 3/4" riser - 3/4" overhang, equaling 9-3/4". So you know that for every step you will come out horizontally 9-3/4" from the edge of the deck's skirt board.
 

Measuring rise and run

If you're building a set of stairs from one level deck or platform to another the next step is relatively easy, but if you're trying to figure out where the stairs will land on sloping or uneven ground you'll need to hone in somewhat through a process of estimating, and then refine your measurements when you figure out how many risers and treads there will be. The bottom of your stringers will ideally land on a flat surface that will not shift seasonally — such as a concrete pad with good drainage beneath it, or asphalt, limestone screenings, etc.

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To start to measure for total rise place a level (or a level on a straight piece of lumber on edge) on the top of the deck and cantilever it overtop of the lower area where you want the stairs to land. You may need to raise or lower it to get it level, then measure from the underside of your straight-edge to the ground. Let's say your measurement is 47". Take that measurement and divide it by a 7" average riser height. You get 6.7 risers, so you'll need to round up to seven risers for your stairs. Dividing 47" by seven risers you'll end up with seven risers at 6-11/16" each. You'll need one less tread than you will number of risers if you step down from your deck to your first stair tread. So, knowing that your tread depth is 9-3/4" (or whatever you have decided from your earlier calculations) you will multiply that by your six treads and come up with 58-1/2." Ensure your level or straight-edge projects out at that distance from the leading edge of the deck and check your total rise measurement. If the ground has sloped you will need to recalculate a more accurate riser height. Let's say you re-measure 47-3/4." That will be your total rise, which divided by seven risers will give you a stringer riser cut height of 6-13/16".
 
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Checking Rise – Using either a long level, or a level attached to a long piece of material, rest one end on the upper deck surface, and adjust for level. Measure from the underside of the level or straight-edge to the point where the stairs will sit to determine your overall rise.

Marking

The next step is to mark your individual rise and run triangles onto a piece of lumber. What material should you use for your stringer? Pressure-treated or cedar 2x10 or 2x12 can be used depending on the depth of the cut-outs you'll be using. By code you will need a minimum of 3-1/2" of material left on the stringer after the cut-outs and that area should also be free of large knots which would weaken the stringer. Make sure you crown the lumber and have the bow of the board facing up so it will better resist sagging over time.
 
Place your stair stringer board on top of a pair of sawhorses lying flat. You can now use one of two methods to mark out your stringer. One simple way to do this is to take a piece of scrap plywood which is square on one corner and measure your riser height on one side and your tread side on the other. Draw a line between the two marks and cut the line cleanly and accurately (inaccuracy at this stage will compound later on). The hypotenuse, or the long side of the resulting right-angled triangle, will travel up the leading edge of the board always staying flush with it. You can keep the long edge of the triangle aligned with the edge of the stringer by screwing a wood cleat to the edge of the triangle.

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Triangular Plywood Guide – One option to mark the cuts is to cut a piece of plywood into a triangle, with one edge the rise and another the run. Attach a cleat to the hypotenuse and use it to mark the stringer.
 
Alternately you can use a framing square and a set of stair gauges, small screw-on stops that will allow you to use your framing square more consistently to slide up and down the board referencing its edge. Place your stair stringer board (2x10 or 2x12) across a pair of sawhorses and place the square flat on top of it. Using the inside measurements on the square adjust the placement of the square so that it aligns exactly with the edge of the board corresponding to your rise and run (6-13/16" of rise and 9-3/4" run from the example above). Set your stair gauges so that they will allow you to repeatedly mark that consistent triangle on the stringer board.

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Carpenters’ Square Guide – A carpenters’ square, equipped with stair gauges, will allow you to mark the cuts needed.
 
When laying out the stringer you will need to understand that the first riser will not be on your stringer — it is the distance between the top of the deck and the top tread of the stairs. Starting at the top of the board you will replicate a series of triangles, which will become your step cut-outs. For the example stairs above you will draw on seven triangles with the tips of each triangle meeting at the edge of the board exactly, sliding your triangle guide or square along the board edge.
 
Next you will make two marks, which will allow your stringer to meet the ground and the deck. The bottom riser you will have to cut shorter than all the others by one thickness of your decking, then draw a line parallel with the bottom tread cut-out mark above it. That will drop the entire stringer, but you will add that height back to each step as you place the actual treads on and a consistent riser height will be maintained.


For example, if you will be using 2x6 decking (that's 1-1/2" thick) you will be cutting the bottom riser short by 1-1/2". At the top of the stairs you will have to mark a line down through the stringer perpendicular to the back of the top tread. This is where the stringer will butt into the rim joist or skirt board of the deck to which it is fastened. At this point double check that you have the right number of risers and treads before proceeding to cutting.
 

Cutting the stringer

With the stringer across two sawhorses take a circular saw and make cuts into the right-angled intersection point where the riser and tread meet, raising the guard on the saw for starting the cuts where it will otherwise bind. Take a hand saw and finish both cuts making sure not to over-cut and weaken the stringer. You will be left with a saw-toothed stringer and a series of triangular cut-out scraps. At this point take a minute to clean up any of the cuts which might require it because you're going to use this stringer as a template to save marking out other stringers from scratch.

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Make the Cuts – A circular saw is a common way to make most of the cuts. The remaining material on the underside of the stringer will have to be cut with a hand saw.
 
Test the stringer out where the stairs will go to make sure it will fit. Remember that the top riser will be the correct height only when the treads go on. If it's correct you can then replicate the stringer by laying it on top of the next pieces of lumber and tracing it. When you cut out a series of stringers you can line them up and ensure that they are all accurate before installing.

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Identical Stringers – Rather than risk making stringers that aren't identical, after cutting the first one, and ensuring it's correct, lay the cut stringer on top of the other, clamp it in place, and mark all of the cuts.
 

Code compliance

Before you plan your project take a look at your local building codes and make sure you comply or exceed minimum expectations. Here are some general guidelines. Generally wood stringers can be spaced apart by code a maximum of 2'-11" in single dwelling units, unless a riser supports the front of the treads in which case it's 3'-11." Generally 2x lumber treads with no riser (actual thickness of 1-1/2") will allow for the spacing mentioned above, however if a 5/4" tread (actual thickness of 1") is used and the distance between stringers is greater than 2' 5-1/2" then a riser must be used to increase the strength at the leading edge of the tread. Tread depth limits are between 9-1/4" and 14" and riser limits are between 4-7/8" and 7-7/8." The maximum depth of overhang on the leading edge of the stair tread is 1". Code limits allow you to customize stairs for different situations that will ensure they remain safe and functional.
 

Step on up

While it may seem like a bit of a process with a number of decisions to make along the way, once you do this a couple of times it's actually quite simple and very rewarding. To walk smoothly and safely from one level to another on a set of stairs you have created will feel great and will enhance your connection to your entry or backyard immeasurably.
 
 

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