Building Stone Arches, continued

Forming an Arch

Although a stone arch is ultimately self supporting and load bearing, it doesn't start that way. The shape of the arch is first created by laying the stone over a precise wood form provided, in our case, by the GC. I've probably laid a hundred arches over the years without ever repeating the same radius. But you never know; I find the forms too nice to toss and have a sizable collection in my storage yard. The typical form is made from a pair of plywood or solid lumber face panels that describe the arch. The face panels are spread and reinforced with framing, built to the approximate finished wall thickness. The top of the form is covered with narrow planks or bendable plywood to support the stone as it's laid.

The form is set to span between bearing points and is elevated slightly on wood spacers and beveled shims. Pulling the shims and spacers makes the form easy to remove later (Figure 3).

Figure 3.Wood shims under the arch form ensure easy removal later. The author relies on the builder to construct the form.

The arch ring is built of individual ring stones, typically wedge shaped, that follow a radial pattern from the center of the arch opening. I usually attach a length of mason's line to pivot from the center of the bottom edge of the form and pull it across the ring stone blank like a chalk line to mark the radial lines. The arch ring is laid out one stone at a time, working from one side, then the other, toward the center. I do the spacing and proportions pretty much by eye unless a strict, formal geometry is specified.

The top of the arch is closed with a keystone that locks the construction together. With the keystone in place, the form can be removed and the arch will stand independently. When working with mortar, we leave the form in place for an initial curing period of at least four days. Mortar not only binds the stones but also fills gaps and distributes stress, adding even more strength to the arch.

On this job, the architect stipulated that the ring stones should not be different in shape or pattern from the "spandrel" — the general vertical facing above and around the arch. That made me a little uneasy, so I still worked a wedge in among the ring stones wherever I could. Most stone arches are deeper than the thickness of the ring stones. But filling the underside, or soffit, of the arch follows similar principles, relying on compression between the stones and mortar for structural integrity.

I back-cut the soffit stones on all edges to minimize the face joint and laid them face down on the form. Back-cutting also created wedge-shaped spaces between the stones, which I filled with mortar. After the mortar hardens, the soffit's compressive strength and structural performance are like that of the ring stones. Before setting each soffit stone, I spread sand on the form to protect the face from mortar staining. And because the joints can't be raked out while the mortar's still green, I filled them with a 1/2 inch of dry sand to displace the mortar. When the form was pulled, the sand spilled out, leaving a recessed joint (Figure 4).

Figure 4.The underside, or soffit, is laid between the ring stones. Mortar and masonry rubble fill the void behind the face stones. As he places each stone, the author spreads sand on the form and packs a layer into the joints before the mortar (top and bottom left). The sand helps keep the finished face clean and falls out when the form is pulled, leaving recessed joints. Only minor pointing is needed to finish (bottom right).

On its own, Portland cement­based mortar would do an adequate job of holding the stones together. But in pursuit of the mason's tradition of building for eternity, I leave no stone unturned. Every bonding face gets a liberal brushing of Silpro Weld-O-Bond Plus, a water-based latex bonding agent formulated to bond Portland cement to a variety of surfaces (Figure 5).

Figure 5.Although Portland cement forms a naturally strong bond with stone, the author brushes all mating surfaces with an acrylic binder to improve adhesion.

Until the cement begins to set up and stiffen — from 1 to 2 hours or longer — some of the face stones are wobbly and easy to dislodge. When working a short run, I could be forced to stop laying up until the cement hardens. Instead, temporary shims and wire twists allow me to continue stacking stone without the downtime (Figure 6).

Figure 6.Mortar is slippery when wet. Shims and wire twists stabilize individual stones until the cement hardens.

To control the lay-up, I stretched a reference string line across the spandrel, offset an inch from the face to avoid irregularities in the stone surface.

Cutting Stone

A 7 1/4-inch wormdrive saw equipped with a diamond blade is my primary stone cutting tool. I've modified ours with a length of plastic tubing attached to a mini sump pump that douses the blade with a continuous trickle of lubricating water (Figure 7). Even cutting with water, we burn through at least a couple of blades a week. If bought individually, diamond blades go for about $90 each. I buy them by the case, which cuts the price to about $60.

Figure 7.The author's saws are modified with a pump-fed water tube to lubricate the stone during cutting. Dry-cutting is also possible but drastically shortens blade life.

After back-cutting the stone, I "rock," or roughly bevel, the edges to soften the saw-cut appearance, using a maul and cold chisel or a masonry hammer (Figure 8). Rocking can be further refined to a more natural-looking surface by "thermaling," or finely flaking the rocked edge with an acetylene torch. That would have been a good way to add a few more months to this job.

Figure 8."Rocking" the cut edges with a chisel or masonry hammer gives the stone a more natural look.

Mortar

I work with modern cement mortar, using ratios of Portland cement, lime, and sand that produce a mix with extremely high compressive strength. I add plasticizers to reduce the need for water, which can weaken the bonding strength of the mortar. Over time, every stone mason develops a personal mortar recipe. Mine calls for 15 shovels of sand, half of a 94-pound bag of Portland, two shovels of masonry cement, and one shovel of lime. I add about 24 ounces (or two paper coffee cups) of Silpro C-21 (Silpro, 800/343-1501, www.silpro.com), an acrylic plasticizer, to a five-gallon bucket of clean, potable water, which brings the mortar to a workable consistency.

Weather protection. Cement mortar is sensitive to freezing. If ice crystals form in the mix before the cement can fully hydrate and set, the mortar may be weakened. Ice expands 9% beyond its liquid state and can forcibly separate the binder (cement) and aggregate (sand), making the mortar crumbly. Calcium chloride can be added to the mix as a setting accelerator; I use Simpson's Winter Ad-Mix (William S. Simpson Co., South Easton, Mass.; 508/230-0900), a liquefied form of calcium chloride. The Ad-Mix has no adverse effect on the mortar itself. But calcium chloride is suspected of causing corrosion in nonstainless metal wall ties and reinforcing steel, so if there's a concern, don't use it.

It's best to maintain the mortar at above-freezing temperatures of 40°F minimum for the period of setting, at least 48 hours. One simple way to make sure there's no frost or frozen aggregate in the mortar is to use hot water in the mix, but heating water isn't practical on site. In really cold spells, we'll sometimes heat the sand before mixing the mortar, by tenting it under a tarp and blasting it with a propane-fired salamander. Completed work has to be protected from freezing overnight and during the initial 48 hours of curing. Insulating "thermal tarps" are made for this purpose, the heat supplied by a portable electric heater. This is an effective method for protecting limited areas of newly completed work. But it's a waste of time having to thaw a wet and frozen pile of stone and sand in the morning before you can get to work. In order to keep the work moving ahead in all weather, we set up a temporary enclosure over the general work area (Figure 9).

Figure 9.The crew constructs a temporary shelter of pipe scaffold, lumber, and poly sheeting over the primary work area. Propane-fired heaters keep the materials and masons above the freezing mark in cold weather.

Pipe scaffolding and furring lumber make a handy framework for the reinforced poly membrane canopy that we use (A.H. Harris, Newington, Conn.; 860/665-9494, www.ahharris.com). I used to use cheaper, woven poly tarps for the canopy, but the translucent membrane is much more durable and doesn't block the daylight. The poly comes in 100-foot rolls from 10 to 40 feet wide and costs $45 per 1,000 square feet. With a couple of salamanders blasting away, we're able to stay relatively warm, dry, and busy.