The Art of Slipforming

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Measure the amount of water used in this first batch, then pour the same amount of water in the mixer to clean it. Save this water and add the gravel, cement, and sand—in that order—to the water to mix subsequent batches. Adding the sand last allows it to work down into the spaces between the gravel, instead of clumping up in the back.

You must wear rubber gloves to protect your hands from the cement. Rubber gloves are also useful for working the mortar between the rocks. We use coffee cans to bucket the concrete from the wheelbarrow into the walls, then plunge our gloved hands down into the wall to vibrate the concrete in between the rocks. Pour concrete to the top of your stonework, then place a new row of rocks on the fresh mortar and continue stacking as before.

It is okay if some of the rocks stick up above the forms, but tilt them back at least a quarter inch, otherwise they tend to bulge out the forms on the next level.

At the end of a day you should cover masonry work with blankets or cement sacks, especially in hot, dry, or freezing weather. And always soak old masonry work with a hose before you start adding to it again. Otherwise the dry mortar absorbs the moisture out of the new material before it properly cures.

Moving Up the Walls

The simplest way to move the forms up the wall is to set new forms on top of the previous layer. However, it is usually more desirable to pull off the old forms and chip away any globs of concrete on the rock faces before they become too hard. Move the forms up and hold them in place with stilts. The stilts may be merely propped underneath, or screwed onto the sides of the forms. The forms are wobbly at first, until you clamp them tight against the walls with the wire ties.

It is easier to move the forms up the wall in places where the door and window frames are already in place. Just hoist the forms up to the right height and screw them into the frames. You do not need wire ties or spacers at these points.

Windows and Doors

There are many ways to save money and recycle old resources while building your home, but you should be very careful about reusing windows and doors. We tend to think, for example, that it is economically and environmentally sensible to salvage windows and reuse them. But the cost and energy expended to manufacture a new window is trifling compared to the amount and cost of the energy that will leak through the glass over its lifetime. Often the most sound course of action is to discard the old windows, especially singlepanes, and replace them with double- or triple-glazed units.

Granted, there are always trade-offs, especially when you are paying cash-as-you-go. Sometimes it is more expedient to install old windows, and to simply accept higher fuel bills in the future. We installed used double-glazed sliding glass door panes in our greenhouse because they were virtually free, and we were building our house almost without money anyway. Our house is reasonably efficient with these windows, but in order to eventually achieve 100 percent passive solar heating we will have to replace these units with the new triple-glazed, low-E, Krypton gas windows coming on the market.

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