Plywood Stairs DIY

Building a set of stairs is the actual test of a master carpenter. That may be true for a complex staircase, but not all staircases are that complicated. You can build a relatively simple set of stairs, mainly if you use plywood.

It usually complicates a set of stairs because they are typically built using a 2”x 12” stringer. If the angles aren’t exactly right, all you end up with is scrap wood, not something usable to climb up to the second floor of a house or down into the basement. People always mess up the angle because they are trying to work with those stringers.

This problem can be solved by making stairs out of plywood, especially using the plywood to make the stringers. Because the plywood provides a right angle and an edge that can be placed against the floor, there’s minimal angle to deal with when calculating the angle. However, there is a tradeoff for this, as plywood stairs aren’t ordinarily open like other staircases are. They can be made to be open. They just usually aren’t.

Nevertheless, plywood stairs are rarely used for a home’s main staircase, although they can be used for basement stairs. They are also usually used only when a short staircase is needed, such as for a mobile home. But that’s an aesthetic decision more than anything else.

Calculating the Stair Rise and Run

The first part of making any stair is calculating the rise and run of the stairs themselves. The rise is the height of each step above the next lower one, or, to put it another way, the amount you have to lift your leg to get to the next step. The run is how much depth your foot must climb before running into the next riser.

That brings up some other important terminology: risers and treads. The risers are the vertical pieces in a stair that go from a lower step to the next higher step. Some staircases are built with an “open” design, which leaves the riser out, but that is not recommended for plywood stairs and is limited to staircases with a maximum total rise of 30” by building code. The tread is the part you walk or tread upon. We need to calculate the rise and run to know how big to cut the risers and treads.

The reason why these dimensions have to be calculated is that they are dependent upon the space available. While it may seem like all home steps are pretty much the same size, they’re not, although they are within a relatively small range of dimensions. Nevertheless, it is impossible to ensure that the rise and run of a particular stair are consistent without calculating it.

Consistency is essential for safety. If one stair has a smaller rise than all the others, it becomes a trip hazard for everyone going up and down the stairs. Therefore, the building code requires that all stairs in a staircase have the same rise, with a maximum of 3/8” variance. The building code also specifies a maximum seven ¾” rise on residential staircases.

To determine the rise of a stair, measure the distance from the lower floor’s surface to the upper floor’s surface. Then, take that dimension and divide it by 7.75. This will probably give you a number between 14 and 16, but that also includes various digits to the right of the decimal point. Round that number to get the total number of steps (including the top floor) and divide that number into the exact measurements from floor to floor. This will give you the actual stair rise needed for that staircase.

The run is calculated in much the same way: dividing the available space by the number of steps determined while calculating the run minus the one that will be the top floor. Once again, the building code steps in here, as the minimum run for a step tread is 10” if the stair treads will have a bullnose and 11” if they do not. The stairs must also have at least 36” of clear space at the bottom.

While the length of the staircase’s run is expected to be calculated, it is not necessary. If the required space is available, all that must be done is multiply the 10” or 11” of the run by the number of steps in the staircase to determine the overall run.

Laying Out and Cutting the Stringers

With the math over, it’s time to start making some sawdust. As I mentioned, laying a stair stringer on plywood is much easier than on 2”x 12” dimensional lumber. A simple framing square can be used, but it’s best to use a drywall square.

The building code does not list a minimum thickness requirement for plywood stringers. However, the fact that they are replacing 2”x 12” boards indicates they need enough thickness to provide strength. I would recommend a minimum of ¾” of plywood for stringers with a total rise of not more than 48” and 1” to 1 ½” of plywood for longer stringers.

Start by marking the bottom for reference. Then, measure up the edge of the plywood and mark out each step’s rise. With the square, draw lines at those marks to indicate where the tops of the increase will be. This will be the bottom of the stair’s treads.

Do the same, working across the plywood base to mark the run for each step. The jagged line of the stringer’s edge, formed by the intersection of those lines, is then visible. Cut along that line to make the stringer. Sand the cut lines to make the lines straight, if necessary. If properly laid out and cut, the scrap piece should be able to be used as the stringer for the other side of the stairs.

To make thicker stringers out of thinner plywood, it is a good idea to glue the plywood together before cutting. This will help ensure that both pieces have the same cut line to support the step treads equally.

Using a third-stringer under the middle of the steps is not uncommon with wide stairs, but this is not normal for most homes’ 36” wide staircases.

Cutting the Risers & Treads

The building code specifies that home stairs must be at least 36” wide and have a minimum of 31 ½” free space (from the handrail to the opposite wall), so they’re typically found in homes. Once the stringers are put in place, the width of the staircase is provided.

The rise calculated earlier will provide the other dimension for the stair risers. Risers do not have to be made of thick material, as they provide only minimal structural support. Therefore, they can be made of ½” thick plywood or OSB. Those are usually installed first, with the treads following. On a tall staircase, one can work one’s way up, installing a riser, tread, and tread to the top.

While ¾” thick plywood or OSB is considered sufficiently thick for subfloors, using a thicker material for the stair treads is not uncommon. One inch of total plywood thickness is ideal. It can be made from two layers of ½” plywood, bonded together. The bullnose can either be routed into the wood or bullnose made of aluminum can be nailed on. Either a 1” bead router bit or a ½” roundover bit can be used to cut the bullnose in the plywood.

If a bullnose is used on the step tread, it should be a minimum of 11” deep. However, it should also be between ¾” and one ¼” more profound than the space cut into the stringer for the step tread. Be sure to check, before cutting, to ensure that the step tread is deep enough to reach the full depth of the space provided for it, along with the necessary projection for the bullnose.

Recovering Worn Steps with Plywood

Some people, especially in older homes, have recovered their existing stairs with plywood. This is no problem as far as the risers are concerned. Even a thin layer (1/4”) of plywood over the existing wood will clean it up and provide a good surface for painting. However, this can be problematic with the step treads.

The problem is that adding a ¾” plywood step tread, let alone a 1” or more thick step tread, on top of the existing one will make it so that the rise for the bottom step is too high. The surge for the top step is too low unless installed in a way that makes allowance for that extra material thickness. In most cases, a better solution is to remove the existing step treads and replace them with new ones, making sure to make your new ones the same thickness as the old ones.

Finishing Plywood Stairs

Plywood can be finished as any other wood product. However, it is common for stairs to paint the risers and stringers with semi-gloss white paint to match the home’s trim. The step treads can be stained and then varnished using a heavy-duty floor varnish.

Avoid painting the risers, and step treads white or varnishing them the same color, as this can create a visual reference problem for some people, making it difficult to distinguish the edges of the steps or differentiate between the tread and riser. Use a two-color system to provide the best visual reference and ensure excellent safety.