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I'm a custom stair builder. When possible, I prefer to build a staircase in my shop, then take it to the site in one piece and install it right over the site-built rough stringers that are already in place.

The rough stringers are cut by the builder to dimensions that I provide. But because I need a perfect fit when I drop the finish stair into place, I usually have to tune up the stringers first.

This would be a tall order for a 4-foot level and a measuring tape. Instead, I use a Laserjamb fitted with a Gizmo level. Following the calibrated marks on the Laserjamb, I can cast a level laser line at precise increments from a couple of inches off the floor to over 12 feet in height.

I build the finished stairs against a set of "air studs" -- tall, rectangular aluminum tubes with a pneumatic ram affixed at the top -- that I designed to quickly replicate on-site framing right in the shop. I first tack up the uncut skirt boards at their precise location. Then I use the laser to accurately mark the tread lines on both skirt boards at the same time. After cutting and routing the stringers, I set them back up and assemble the stairs, a tread and riser at a time. As the stair goes together and gets heavier, I add temporary supports to keep the treads true to the laser line, to ensure that cumulative errors don't occur.

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The author sets up "air studs" -- aluminum tubes with pneumatic rams at the top -- to support the finish stringers while he builds the stairs. A pole-mounted laser makes it possible to mark the tread cuts on both skirts at the same time.

Once on site, I again set up the Laserjamb, this time in front of the rough stringers. Using the same riser settings, I cast a level line at each tread height. All it takes to true up the rough cutouts is to grind or plane down the irregularities until the red line just disappears.

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On site, the laser provides a perfect reference for tuning up rough stringers. The finished stair then slips into place.

After that, the finished assembly clicks into place as if it grew there.

Richard Harkis a stair carpenter in Harwich, Mass.