How to Choose Gravel or PVC Vault Drainfields
PVC vaults have replaced old-school drainrock (sometimes called drainfield gravel) in most, but not all new drainfields. The vaults have open bottoms and no central pipe, and like drainrock, serve to provide storage space for the sewage effluent giving it time to soak into the soil below the trenches. Sometimes drainrock is preferred when the gravel pit is close by, and cost is a major factor in the job. Vaults are a little more expensive, but in cases like a steep access road, or on an island, vaults can be stacked in a pickup or on a boat for delivery to the site without heavy equipment. In constructing the drainfield, vaults can be handled and placed by one person once the trenches are dug.
The plans included in the Promo system show a gravity system for a 3 bedroom house with plans for both traditional drainrock technology and PVC vaults. All the appropriate notes, specs, and construction details for both drainfield designs are shown.
Modern drainfields use the flat floor of the trench to provide distribution of the effluent across the entire drainfield so each trench in a gravity system whether PVC vaults or drainrock, is excavated dead level end-to-end with zero slope regardless of advice from anyone, especially people with a long history in the building business.