The Art of Slipforming

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

The gable ends are easy to fill in with stone, once the rafters or trusses are in place. just make sure you line up the rafters so that one set will get concreted into each gable, preferably along the inside edges of the walls (L). Then continue with the slipforming, using the rafters as a guide inside the forms. You will need to stop at the rafter bottoms, then pull the forms off, and construct the floating rafters (M) with supports resting on the stonework Now finish the remaining stonework between the supports (N). This will be freehand work, without the use of forms.

Please note, when you are building an insulated double wall, the inside wall should stop short enough to allow a continuous line of insulation from the wall into the roof.

Grouting

The stonework is not quite finished when you pull the forms off the wall. You will see gaps of all different sizes between the stones, and globs of concrete loosely stuck to the face of the wall. The concrete is naturally lumpy anyway, so you need to trowel a finer mortar over it between the stones. This is called grouting or pointing the wall.

Grouting the wall fills all the little spaces; it makes the rock work stronger and protects the wall from the weather. First, however, you need to use a hammer, a chisel, or a rock pick to chip away the concrete on the wall face. Also chip the concrete back an inch or so in between the rocks; this will make enough space for the grouting mortar. Obviously it is best to chip away the concrete within a day or two of pouring it, before it becomes hard. Just be gentle around the stones. A bad hit can easily loosen a rock.

Grouting does not need to be done right away, but it is good to do it while you still have momentum on the project. Besides, grouting brings out the beauty in your stonework. It is the masonry equivalent of framing a picture.

We have tried many different mixes for grouting work, and we now prefer a mix of one part lime to two parts cement (Type I & II) and eight parts masonry sand. We measure this with coffee cans and mix it in the wheelbarrow with a hoe. The mortar should be moist enough to work easily into the wall, without being so wet that it sags in the joints or smears all over the rock faces. Hose down the wall before you start grouting, so the old concrete does not suck the moisture out of the fresh grout.

Additional tools you will need for grouting are a mortar board and small trowels. A mortar board can be made by attaching any kind of a handle to the middle of a 12-inch-square piece of 1/2 inch plywood. Hold the board from underneath and pile the mortar on top. Then place the edge of the board against the wall where you are working, and shove the mortar directly off the board into the mortar joints around the stones. There are many sizes of grouting or pointing trowels commercially available, but my favorite tool is still a cheap, flexible putty knife, about 1 1/4 inches wide.

RSS | Comments | E-mail | Print