We'll describe the construction method for a common scenario here, but most of the following information will apply to any home bouldering wall configuration.
You should frame each of your walls separately. Build one framework section and attach it to the support structure before building the next piece of framework. Begin with the largest or most important wall and progress to the smallest or least important. This will allow you to make any necessary adjustments that the space may require with minimal trauma. Once the framework is in place for the primary walls, you will connect them with additional framing members to create the secondary walls.
If you're not familiar with framing terminology, don't be confused by the terms "stud" and "joist" For our purposes, they are essentially the same. Technically, studs run vertically in a house's walls while joists run horizontally in the floor or ceiling. Few bouldering walls are purely vertical or horizontal, so we vary the term depending upon the situation. You may also hear these framework members referred to as "stringers"
Walls that aren't too steep (say, up to 20° overhanging) can be framed flat on the ground then hoisted into place as a unit, much like the walls of a house. Lay the top and bottom plates and the two outer studs on the floor. Measure diagonally from opposite corners, and compare the length of both diagonals to ensure that the frame is square. Adjust the frame until the diagonals are equal, and secure the four corners with screws.
Measure along the top and bottom plates and mark the locations for the inner studs every 16 inches. Make your measurements from the same side of the frame at the top and the bottom. Measure from the outside of the outer stud to the center of the first inner stud, then center-to-center for the rest of the studs. If your wall is wider than a sheet of plywood, make sure that the sheet will end in the middle of a stud, so the next sheet will have room to attach to the same stud. If you use 16" or 24" spacing and measure correctly, this will happen automatically.
If you're framing a vertical wall or horizontal roof, the ends of the studs or joists will be cut straight across or perpendicular to their length. However, if you are framing an overhanging wall, you'll need to cut the studs at an angle, so the top and bottom plates will sit flat on the ground (or flat to the wall) and flat to the ceiling. Use a T-bevel to mark the correct angle and cut one stud. Check it for fit, then use it as a template to cut all the other studs identically. Because you've cut the end of the stud off at an angle, it will now be wider than the plate where it attaches. If the wall will sit flat on the ground, align the front (or climbing side) of the stud with the front of the plate, then make a second cut on the backside of the stud, so it is flush with the plate.
Lumber is rarely straight, so sight down each stud to see which way it curves. Assemble the framework with the high side of the curve (the crown) facing toward the back side of your wall. Set the studs in place using a framing square to make sure each stud is square to the top and bottom plates. Recheck that the outer framework is square, and screw everything together.
If you've framed the wall on the ground as described above, simply tilt it up into position, recheck it for square, and screw through the top and bottom plates directly into every ceiling and floor joist along their spans, using two #14 screws per joist. Exposed framing will make this task easier. However, fire codes require wallboard on all interior walls and ceilings, so you will probably need to use a stud finder to locate the framing members. Your house's wall studs are usually spaced on 16" centers (i.e., the distance from one stud's center point to the next), and floor or ceiling joists are usually on 24" centers, so once you find the first stud or joist it should be easy to find the rest. Since you'll be screwing through the drywall, you won't be able to visually verify the quality of the attachment. Make sure that the screws go in solidly and there is resistance all the way in. Plates can be attached to concrete walls or floors with concrete expansion anchors.
If your wall runs parallel, or at any angle other than perpendicular, to the ceiling joists, run sleepers perpendicular to the joists, spaced on 24" centers, picking up at least three joists with each sleeper. Then attach your wall's top plate (or header joist) to the sleepers.
Overhanging walls transfer their loads to the support structure in a different manner than near-vertical walls. They are more akin to residential floors than walls. Frame steep walls by first attaching header joists (the equivalent of top and bottom plates) to your house's ceiling joists and wall studs. Screw each header joist into every stud or joist along its span using at least two #14 screws per stud or joist, just as described earlier in the top and bottom plate method.
Make sure your header joists are level and square to each other, so your wall framework and plywood sheeting will fit correctly. It's rare for residential walls, floors or ceilings to be perfectly square and level, so don't base your positioning on the existing structure. Verify the relative position of the header joists with a level and tape measure.
Then run joists (the equivalent of studs) between the headers every 16 inches, again, facing the crown of each joist up (or back). Attach the joists with joist hangers. Cut a short flat section at the end of each joist to bear against the hanger.
Run a string where the first joist will be positioned. Then, use a T-bevel to determine the correct angle for the cuts at the ends of the joists. Cut one joist and check its fit, then use it as a template to cut the remaining joists.
Attach 2"x 4" blocking between the studs every 48 inches (where the plywood panels will eventually meet). This will strengthen the framework and give you something to screw the panels to.
Finish your framework by adding framing members for kicker panels and secondary walls.
Before the plywood is attached to the framework, you need to install the T-nuts. Stack 3 to 6 panels on saw horses and clamp them together. The ACX plywood sheets have an "A" side and a "C" side, meaning one side is a higher grade of laminate and has fewer voids or blemishes. Face the "A" sides up. These will face out to make the climbing surface when the panels are in place.
Mark the locations where the joists will be when the panel is in place (each end and every 16 inches). Usually it's easier to position the panels horizontally on the framework, but if they will fit your wall configuration better in the vertical orientation, take note of it now and mark the joist locations accordingly. You can layout a grid pattern or drill holes randomly. Either way, don't put any holes where the joists will be. You'll want 100 to 250 T-nuts per sheet, the more the better. Sheets that will go at the bottom of the wall will take mostly footholds so you can install fewer T-nuts and use screen holds liberally.
Use a 1/2" Forstner or spade-style drill bit to drill the T-nut holes. It is critical that you drill the holes straight. Inexpensive drill press attachments are available for your hand drill that will insure straight holes.
If your design calls for anything other than full 4'x 8' sheets, cut the necessary panels to size now. Make sure you cut the panels and check their fit before installing the T-nuts.
Turn the panels over and knock off any splinters from the drilling. Install the T-nuts on the "C" side (the opposite side that you drilled from). Tap the T-nuts into the holes with a hammer. Make sure you set the T-nuts straight. A little extra time and care now will save an enormous amount of headache later.
Now that the panels are cut to size and the T-nuts are installed, it's time to put the panels in place on the framework. Screw a couple of jug holds on panels to help maneuver them into place. Attach the panels with #8 x 2" self-drive screws spaced about 6 inches apart all the way around the perimeter and along each joist or stud. Make sure the screws are going into the center of the stud (i.e. 3/4" from the edge of the panel except where panels meet, in which case, 3/8" from the edge.) The 2"x 4" blocking between the joists should be right where the panels meet so you will have a solid attachment and a tight joint between panels.
Bare plywood is fine for indoor walls, but a coat of paint is crucial for protection if the wall is outdoors or in a humid environment. Any exterior-grade paint will work. Metolius, however, offers textured paint in a variety of colors to add a touch of realism to your wall. Plug your T-nuts with golf tees during the process to keep paint out of the threads.
The area underneath and around your wall must be made safe for falling. Ensure that this area is free of any objects that you could hit or land on in a fall. Never boulder over unpadded concrete, asphalt, wood, or any other hard surface. Falls directly onto an unprotected surface can result in serious head injury or even death. Even grass and turf will lose its ability to absorb the shock of a fall through wear and weather. Your fall zone must extend far enough to protect you from swinging falls or dynos. If your wall is steep, this could mean that you will need to extend your fall zone several yards past the edges of the wall.
There are several options for padding your fall zone including commercially available bouldering pads, old mattresses, or several layers of carpet padding. However you choose to prepare your fall zone, you must take the responsibility of making it safe very seriously. Artificial climbing holds regularly spin or break, resulting in violent, out-of-control falls. You must never put yourself or others in a position that could result in injury in the case of an unexpected fall.
The most common maintenance procedure you will need to perform on your bouldering wall is tightening and inspecting the holds. Remove any damaged or cracked holds immediately, even if the crack is very small. Holds will loosen with surprising speed and regularity, especially when your wall is new, after temperature changes, or in a humid environment. Spinning or breaking holds can result in very dangerous falls, so it is important to inspect and tighten them frequently.
T-nuts will sometimes spin in the plywood or become cross-threaded or stripped. If a T-nut has spun or is severely cross-threaded and you cannot tighten or loosen the bolt, fit a pry bar under the edge of the hold and pry outwards. Then try to loosen the bolt under tension. If this doesn't work, you'll have to slip a hacksaw blade between the hold and the wall and cut the bolt. Tap a damaged T-nut out from the front of the wall using a bolt, then install a new one by threading it onto a bolt that is through a hold (use a strong, thick one) and drawing it into place by tightening the bolt. If the plywood is severely damaged, do not reuse the hole. If there is only minor damage, you can epoxy the new T-nut in place or anchor it from behind with short wood-screws to keep it from spinning.
Periodically check bolts, screws, joints, and any other hardware on your wall for looseness, wear, or damage. Look for signs of stress like expanding joints or seams, that may indicate that your wall requires reinforcement or repair.