The Rubble-Trench Foundation

Rubble-trench foundations were used extensively by architect Frank Lloyd Wright in the first half of the 20th century. They have fallen out of favor as concrete has become the norm and many code inspectors are unfamiliar with them. After finally deciding that a concrete slab was unacceptable to us in terms of meeting our project goals (because of extremely high embodied energy, high cost---and hard on feet and legs!), we decided that a rubble trench was time-tested and would provide a fairly simple, low-cost, environmentally friendly alternative. We also decided, after much deliberation, to use wooden grade beams (rot-resistant hemlock) on top of the trench, rather than concrete grade beams. We are betting the long-term soundness of these wooden grade beams on the steps we have taken to ensure that very little water ever touches the beams, that any moisture that comes in contact with them immediately drains away, and that, if for some strange reason water finds its way into the trench, it will not freeze before it drains away. A number of people have tried to persuade us to place a moisture barrier under the hemlock grade beams, but we feel that such a barrier would only serve to trap any moisture that found its way to the beams rather than allowing it to drain away. The following features of the foundation and house should keep the grade beams dry and sound:
> appropriately constructed trench dug all the way to ledge, with raised berm of washed stone and good bottom-of-the-trench drainage
> ground sloped away from the outside of the trench/house
> blueboard insulation angled over outside of trench and backfilled with washed stone
> a 2.5' roof overhang to keep water off the walls and grade beam
> a berm and a swale on the upslope side of the house to direct water away from house
(Excavating and filling the trench, and the graywater septic, is the only part of the construction for which we hired a professional contractor.)


Anonymous said...

Y'all need to put some gators in that moat. :)

-Laura L.