A unique shallow frost-protected foundation (SFPF)

Fastfoot forms at Endeavour Centre

In the pursuit of foundations that use little or no concrete, we have two common strategies that we often use. The first is rubble trench foundations (you can see an example here), and the second is shallow frost protected foundations (SFPF). Our extremely flat and low lying site this year dictated the use of a shallow foundation to avoid potential drainage issues in a deep trench.

The teachers’ union office building has two extensive curved sections in the foundation, and this would have made a conventional footing formed with 2×8 lumber difficult to achieve. A great new product called Fastfoot allowed us to make this footing very quickly and easily, and is a product we’d definitely use again. Made from a woven polypropylene, the Fastfoot uses lightweight stakes in the ground and small dimension lumber fastened to the stakes to support a fabric formwork (in many ways similar to doing earthbag footings) into which concrete is poured.

The Fastfoot system has several advantages over conventional formwork. We were able to use lightweight stakes and 2x4s (or doubled 1x4s to achieve the curves), despite the footing being 8 inches deep, rather than needing 2×8 lumber. As the formwork is draped by the Fastfoot fabric, the footing lumber does not get covered in concrete. This means when the forms are disassembled we can reuse the lumber without having to clean it. The fabric of the Fastfoot system is also a barrier to rising dampness from the soil beneath, adding a layer of protection from underneath. We were able to wrap the “tails” of the fabric up and over the footing and tie it into the foundation wall moisture proofing layers, adding a positive lap to the intersection between footing and stem wall.

The lines stamped onto the Fastfoot fabric make it easy to line up within the forms, and allow for many different widths of footing. There is no waste with the system, as the fabric stays in place. Joints in the fabric are overlapped by about a foot, and a folding pattern allows for corners to be handled easily. Our curves required us to make folds at regular intervals, and this too was easily done.

We had a few small issues when we poured the concrete into the form and had the fabric slide to one side under the force of the concrete. This pushed the fabric out on one side and left a concave section on the other. In these areas we had to shovel the concrete a bit to get the form centred again. We wouldn’t make that mistake again as it’s easily avoided.

Fastfoot is an excellent addition to our sustainable building toolbox.