RUBBLE TRENCH FOUNDATION PROS
Lower cost than a concrete footing.
Uses much less concrete (production of concrete requires a great deal of energy and generates greenhouse gases).
Can use recycled crushed concrete fill.
Provides excellent drainage, and thus a "static" foundation system.
RUBBLE TRENCH FOUNDATION CHALLENGES
Soils with low bearing capacity may require an extremely wide trench (or some other footing alternative) to achieve adequate bearing area.
Not specifically addressed in Building Codes; requires additional dialog with permitting officials.
A rubble trench foundation meets the requirements and the intent of U.S. Building Codes. However, since this system is not specifically outlined in current Codes, acceptance is provided on a case-by-case basis. Since this puts permit approval at the discretion of individual building officials, it is recommended to initiate a dialog prior to submitting for a building permit. This provides an opportunity to inform and educate permitting staff and provide adequate information to satisfy their desire to ensure a safe structure. The article written by Elias Velonis for Fine Homebuilding provides excellent technical information to this end. It is recommended that stamped structural drawings be provided so the burden of proof is not purely conceptual.
My experiences with rubble trench foundations have been rather positive. I interact with the permitting office well ahead of time, and have not encountered rejection or delays. In one case the building inspector required that the structural engineer be present to verify tamping. I have needed to increase the trench width to 24 inches when a 16-inch backhoe bucket proved too difficult to find. The only impact was the need for additional gravel mix to fill the trench; even with additional gravel, the cost of the foundation was lower than a standard concrete footer would have been.
by Rachel Preston Prinz and Michael Curry
Floors are part of both a building’s structural and enclosure systems. A floor system may be constructed of joists, beams, and decking, or of a continuous solid material like concrete. The depth of the system required is directly proportional to the distance between supports and the strength of the materials used to make it.
The ground floor of many houses is often a concrete slab-on-grade. The slabs are usually a minimum of 4 inches thick with steel or fiber reinforcement to control cracking. To prevent water vapor and mineral deposits seeping up from below onto your slab-on-grade floors, use a layer of film (at least 6mm thick) under your floors and above the ground to prevent leeching and water vapor transmission. If you use a gravel basecourse (recommended as a capillary break to prevent soil moisture from migrating upward) under concrete or earthen floors, put the film above the gravel.
When a floor is also a ceiling, it often doubles as a chase for mechanical, electrical, and plumbing lines to pass through. A floor’s acoustic characteristics can also have a detrimental effect on our comfort. It can act like a drum, making sound above or below reflect loudly in the space we are in. Designing a floor that ‘feels good’, limits sound transmission, and is not too thick can be a delicate balancing act.
There are lots of ways to build an elevated floor system. The two basic elements of a wood floor structure are the decking or sheathing and the joists. We recommend you consult with your local building department, lumber supplier, or structural engineer to give you a safe and serviceable floor or roof structure. Some of the most common materials, and their relative spans and spacings, are: