When it comes to pouring and detailing a foundation slab, you have one shot to get it right—at least without the major expense and headache of a retrofit. Correctly detailing the control layers for water, airtightness, vapor, and thermal management is integral to the assembly’s successful performance. This is our approach to constructing a basement that will stay dry and in a healthy condition.
Water management always comes first
Under-slab water management requires what we call an exclusion tactic—meaning preventing water from collecting right from the get-go. It is achieved with a perforated perimeter drainpipe. In this detail, the pipe is installed to the interior of the footing. It is not uncommon to put it to the exterior of the footing or to have one in both locations. What determines this is the level of risk given the local water table. The higher the water table, the higher the risk, and therefore it is advisable to have more than one path directing water away from the foundation.
If using just one drainpipe, whether you put it to the interior or exterior of the footings doesn’t matter, so long as the pipe is installed below the bottom of the slab. Also note that a perforated drain is typically set in a bed of stone to encourage water to move laterally, making its way into the drainpipe that will divert it away from the slab. The stone bed also acts as a capillary break. If the stone and perimeter drain are installed properly, the bottom of the slab should not see water in its lifetime.
Lastly, there are two places water can exit the pipe. If the site is sloped enough, the pipe will drain to daylight down grade. If the site is closer to level, the drain…