Foolproof Layout For Winding Stairs - Continued
My next step is to mark tread divisions on the face-skirt line by dividing its total length into equal segments. Depending on the total run and configuration of the stair, there may be a number of "common" treads — full-width treads that occur in a straight section of the U. You can determine this by trial and error, trying to keep the narrow ends of the wedge-shaped treads as close to the minimum 6 inches as possible. The stair shown here had two commons.
With the tread spacing marked equally along the face-skirt line, I connect those marks to the divisions along the walk line and extend each line to the wall. This gives me the complete finish riser line for each step; from there, it's easy to mark the back of the 3/4-inch finish riser, then the back of the plywood subrisers, which tells me where the rough stringer plumb cuts go. (I also add other details to the drawings as needed — nosing, wall skirts, newels, baluster, and railing.)
I use LVL for stringers; it's 1 3/4 inches thick and very stable, and comes in a variety of widths. Because the winders cross the stringers at different angles, the stringer shapes become complex and require wider-than-normal stock. For this stair, I used 16-inch-wide LVL.
All the information I need to lay out each stringer is right on my full-size plan. I draw 3/4-inch offset lines behind the finished risers and inside the skirtboard line to represent their respective thicknesses. Then I draw secondary offsets 3/4 inch behind those lines to represent the thickness of the plywood subrisers. I draw the stringers according to their actual spacing on the plan. This stair was only the minimum allowable 36 inches wide, so I used three stringers. On any stair wider than 36 inches, I use four stringers. I draw the stringers with a red pencil to distinguish them from all the other pencil lines.
This is where the story pole becomes indispensible. By holding it at right angles to the stringer lines, I can accurately mark the risers where they occur on each stringer — no two stringers will be alike.
By laying the story pole perpendicular to the plan and snapping out riser lines.
The author can project full-scale templates for the rough stringers.
I make all of my stair carriage components in the shop — this one took a couple of days — and then haul them to the site for assembly as a rough stair. Installation usually takes two guys about a day to complete. We lag the stringers together at the turns, and to the walls, with 5/16-inch-diameter GRK star-head screws (GRK Fasteners, 807/474-4300, www.grkfasteners.com), driven with an impact driver fitted with a Torx bit. The screws are rated for shear and provide a rugged connection.
Starting at the bottom and working up, we glue and screw the plywood treads and risers, using construction adhesive to eliminate squeaks. We continually check the stringer heights with the story pole and check the subtreads for level, front-to-back, and side-to-side accuracy. We try to keep the face side of the stair about 1/8 inch higher than the wall side to counteract deflection. Once all the treads and risers are attached and the glue dries, the entire carriage becomes one strong, monolithic structure. Before leaving the site, we install all the blocking needed to secure finish components — for instance, where railing meets wall — and install a temporary 2x4 handrail that's screwed, not nailed, together for easy removal.
Shear-rated lag screws connect intersecting stringers and fasten the inside members to the walls. Plywood subrisers and treads, installed with screws and construction adhesive, convert the individual stringers into a strong monolithic structure.
Temporary, 2x4 guardrails, screwed together for easy removal, provide fall protection for workers using the rough stair.
Once the flat plastering is done, we return to fur out the underside, or soffit, of the stair so that it can be plastered to a pleasing, sculptural curve. To do this, we first fasten flexible wood battens or contoured plywood strips to the wall, following the underside of the wall stringers. We adjust the battens by eye to form graceful, flowing lines that connect the upper and lower ceilings. The face stringer line requires no additional adjustment, as it follows an equal offset from the tread nosings. However, keep in mind that a slender face skirt looks better than a heavy one; I control this by leaving about 4 inches of solid lumber under the tread-riser cutouts.
Carpenter Mike Kennedy adds a plywood batten beneath the wall stringer to create a visually fair line for the plaster furring to follow.
We bridge between the face and the wall with metal "high-hat" furring, named for its hat-shaped cross section. High-hat is easily cut using sheet-metal snips, and is fastened with drywall screws on 12-inch-average centers. It's important to install each piece of furring level, to keep the soffit parallel between the top and bottom of the stair. The pitch of the stringers is not the same at the wall side and at the face side, but the high-hat can be readily twisted to accommodate the difference.
Pieces of metal high-hat furring channel get individually leveled across the underside of the stair to ensure a level start and end to the transition between upper and lower ceilings.
Leveling automatically fans the furring across the plane of undercarriage. The eye is the final judge of appearance: To ensure a pleasing line, individual furring pieces may need slight adjustment.
Once all the pieces are installed, I adjust pieces up or down to fair the overall plane. Then the plasterers screw expanded wire lathe to the furring and apply a plaster scratch coat, the first in a three-coat job.
Newel posts. Because they would be partly embedded in the plaster, we installed the newel posts at the rough stage of this project, using GRK screws to fasten them. The assembled rough stair carriage was strong, but the furring and continuous diaphragm of plaster made it even stiffer. Before the finishing coats could be applied, we had to install the face skirt, because the plaster would finish directly against it.
The author lays out the newel notch directly on the winders.
Then transfers the information to the post. After notching, the post is positioned.
Then shimmed plumb and bolted from the back. Plugged bolts are added later through the face for reinforcement.
In any job, the skirt, like the stringer, has to be cut from wider-than-common lumber — in this case, glued-up walnut boards. We fit the face skirt between newel posts, scribed the soffit profile on the back side, and cut it to shape (Figure 9). Then we held the skirt in place to scribe the tread and riser cutouts. Although the winders didn't form 90-degree corners at the skirt, I made all the skirt cuts at 45 degrees and adjusted each riser angle to fit.
A plaster scratch coat establishes a rough plane against which the face skirtboards can be scribed.
The author cleans up the skirt's curving profile before gluing on a decorative edge bead.
After the face skirt is installed, the final plaster coat will fill any gaps and provide a smooth surface that flows between ceiling levels.
The author uses a 45-degree default angle for the vertical skirt cuts and adjusts the finished riser angles individually.
The shim space behind the skirt allows the miter joints to be adjusted plumb and tight.
We custom-make balustrade components in the shop. This is not only the best way to match existing architectural details, but, in some cases, it's the only way to get the unique transitional pieces needed to assemble a complex, flowing handrail. The balustrade on this job was a relatively simple post-and-rail configuration, in which railing segments butt into the newel posts at all points of transition. Nonetheless, the handrails had to be custom-shaped to follow the stair's irregular fall line.
We handle this kind of situation by producing and delivering the balusters first, so that the painter can prefinish them before installation. This approach saves time and gives the finished railing a crisp, professional appearance. Ideally, we also install all of the landing nosings before the finish flooring goes in, so that it scribes to the stair rather than the other way around.
The woodwork — treads, risers, skirtboards, moldings, balusters, and railing — is installed using conventional, high-end finish carpentry skills and a bag of special stair-builder tricks, but that's another topic for a future article.
Jed Dixonowns North Road Stairbuilders in Foster, R.I.