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How to build exterior stairs that last

These are custom-cut stair stringers. A special joist protection tape with butyl adhesive will help resist wood rot at the notches. (Tim Carter)
These are custom-cut stair stringers. A special joist protection tape with butyl adhesive will help resist wood rot at the notches. (Tim Carter)

DEAR TIM: I need to build some simple outdoor steps to replace ones that fell apart and rotted (even though they were made with treated lumber). I’m fairly handy and have a decent grasp of the math but would love some tips to get professional results. I’d also like to prevent the new steps from falling apart, so what can be done to ensure that doesn’t happen. – Blondie C., Raleigh, North Carolina

DEAR BLONDIE: Most weekend warriors shudder at the thought of having to build steps because it appears to be fairly complex. In reality you just need some simple grade school math skills to master the process. As for the rot issue, there’s good news about how to prevent or eliminate it.

Over the years, different companies have developed all sorts of products that allow you to take regular lumber and add metal brackets and connectors to simplify the construction of stairs. These are not as easy to install as they seem, but some of them are perfect for the average person. Just realize there are many options when it comes to DIY deck stairs.

You can even buy precut stringers from some lumberyards and home centers. Some old-fashioned lumberyards may even make custom-cut stair stringers for you if you provide the relevant measurements. Then you assemble the parts at your home.

If you are able to modify the landing area where the bottom of the stairs will rest, then you can take advantage of my most valuable tip. I’ve discovered over the years – with input from master stair builders and architects with decades of experience – that the most comfortable stairs to go up and down are ones that have 7 1/2-inch risers and 10-inch treads. These are also some of the safest steps to travel.

Decades ago the building code had a very commonsense approach to the formula for building stairs. Because there’s an infinite number of possible riser/tread combinations, the code said something like, “The combination of any two risers and one tread must not be less than 24 inches and not exceed 26 inches.” You’ll note that the 7 1/2-inch riser and 10-inch tread produces the sweet spot between the two extremes: 25 inches.

Keep in mind that the wood stringers used to support the flat treads and vertical risers are the beams that support the weight of the steps and anything that travels them. Think about hauling a heavy refrigerator or piano up a set of steps. To ensure the stairs don’t collapse, it’s wise to have the stringers spaced around 12 inches on center.

This is vital if you take giant 2-by-12s and notch them to create the flat tread area and vertical riser. When you cut away lumber to create these 90-degree cuts or notches, you transform the 2-by-12 into a 2-by-6 or less! Long sets of steps may need support mid span so they don’t feel spongy as you go up and down.

The rot issue has perplexed many a homeowner; you’re not the first person to ask about this. When you take regular dimensional lumber like 2-by-12s and cut notches into them for the treads and risers, you expose, at an angle, the tiny vertical tubes that make up the average tree.

If you were to use a microscope and look at the end of a log, you’d see thousands of tiny tubes. Imagine taking a handful of drinking straws and holding them in your hand. That’s what a tree looks like on end.

If you glued the straws together and then put them in a miter saw, set at a 37-degree angle and cut them, you’d end up with enlarged openings for water to enter. This is exactly what you have on each flat space you cut for a stair tread after you cut the lumber with your saw. Water that gets under the stair tread will easily flow down the open tubes in the lumber.

Even though the lumber was treated, the treatment process may not have reached deep into all the tubes and now you’re allowing water to seep into the wood. This water promotes the growth of fungi that eat the wood fibers.

One way to stop the water from entering the wood is to cover the flat and vertical cuts with a continuous piece of joist protection tape that stops water from getting into the wood. This tape is readily available at most traditional lumber yards and online; I prefer the ones that use butyl adhesive rather than asphalt.

If you can’t locate this tape, then you may want to apply two coats of paint to the notched cuts of your stair stringers. This paint will plug up the tiny tubes and prevent much of the water from entering the wood. Three coats of paint are better than two.

Finally, be sure to use screws instead of nails when fastening all the lumber. Nails tend to lose their holding power over time because most exterior lumber expands and contracts from the endless cycles of getting wet from rain and then drying out. This causes cracks to develop in the lumber and the cracks get wider and wider with each successive wetting.

Need an answer? All of Tim’s past columns are archived for free at www.AsktheBuilder.com. You can also watch hundreds of videos, download Quick Start Guides and more, all for free.


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