The Art of Slipforming

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If you do install used windows and doors then you should at least make sure that the sizes are interchangeable with those on the market today. Otherwise, having the new windows custom-made to fit odd-sized openings will at least double your costs when you do decide to upgrade.

For installing new windows you should pick them out before you begin construction and build the frames according to the "rough size" specified for each unit. You may also want to build the frames extra large to insulate across the thermal gap that occurs through the stonework (I). We add three inches in width and height to the frames to leave room inside for rigid insulation and plasterboard. We nail in 2 x 4s at the outside edge to attach the windows (J). In our next project we will have the window frames custom-cut from a highdensity polystyrene beadboard to eliminate both the thermal leak and the wood framing (K). We will use windows that can be glued in place rather than nailed.

Build the window and door frames on the ground, and put braces across the corners to keep them squared. Drill in screws partway all around the frames to anchor the wood to the concrete. Large frames should be temporarily braced inside so they do not bow inward from the sides or top while you are doing stonework.

The door frames are often installed directly on the footings, and you should leave a gap in the forms through at least one doorway. so you can go in and out with a wheelbarrow. For symmetry we usually align the tops of the windows with the tops of the doors, even if the windows are of multiple sizes. Measure down from the top of a door frame to find the starting point to install the windows. Mark this with a level and a pencil on the inside of the forms. Sometimes the hardest part is remembering to stop putting stones in before you rock in the whole opening!

The stonework should end one or two inches below the windows. Then lay in a thick bed of mortar and drop the window frame into the forms, tapping it into the mortar. Check with a level in all directions, and recheck it as you work, until the end of the day. Keep in mind that both your successes and your mistakes will be permanently "cast in stone."

Window and door frames up to about four feet wide can be spanned with stone by laying out multiple lengths of 1/2-inch rebar in a bed of mortar one-inch deep across the tops of the frames. Cover the rebar with more concrete, then add the stones. Longer spans may be accomplished using 6-inch rebar.

The Roof

The form setting at the top of the wall must be very level. Fill the forms with stonework, but stop one to two inches below the top. Cap the top of the wall with a thick bed of mortar, but be careful to keep the mortar back from the outside edge so it does not run down the stonework. Trowel the mortar smooth with the tops of the walls, then tap anchor bolts down into the fresh masonry, about every four feet along the walls. Attach a "sill plate" to the anchor bolts when the masonry is cured, and attach the roof system to the sill plate.

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