House walkout basements opening to a patio (or just a patio located below the grade) sometimes require a dedicated drainage system. A combination of a drain tile and / or sump pump ensures that moisture intrusion will not threaten the living space located at the same level; it will also prevent your patio from turning into a pond … and staying one.
In warm climates, even shallow sump pump pits / drain tile installations perform perfectly all year round. However, in climates where temperatures drop below freezing, such installations require a little more planning.
The most critical situation would be when your patio is lower than the surrounding grounds / surfaces and at the same level as your walkout basement or any living area. When designing a sump pump assisted drainage system for such purposes, you should take into consideration the frost line depth in your area (you can check this frost depth map for approximate values).
Any sump pump and pipes located on the exterior and above the frost line would freeze up. Even if the sump pump is only partially submerged under water, freezing might cause permanent damage to the assembly.
If you have a shallow pit that only collects surface water (a pit with solid walls and solid bottom with no drain tile connected to it), remove the sump pump for a cold season.
You can equip it with a flexible hose and use it periodically, whenever it starts getting warmer and water accumulates in the pit. If the temperature drops below freezing, remove the pump and flexible pipe (you can leave the pipe outside but make sure that it is empty).
Since general purpose sump pumps are not designed to carry anything other than clean water, you should not add salt, antifreeze, etc. to the sump pump pit in order to prevent it from freezing. Chemicals might accelerate corrosion, damage seals, cause electrical failure and create hazardous conditions. It can also contaminate your soil and make it impossible to garden / plant anything.
Exterior sump pump pits that provide below the frost line depth will only work properly during the cold season if:
- Discharge pipes are also installed underground or somehow protected from freezing
- Sloped away from the pump
- Drain pipes are discharging into the reservoir that does not freeze up during the cold season (municipal underground rain water drains / storm drains or sewer systems – make sure that your local authorities permit connection of your sump pump discharge line to them – most will probably not)
Do not connect your sump pump discharge line to the septic tank, it could significantly compromise its performance.
Sump pump pit installed within the area of a concrete patio. If the top edge of the sump pump pit / well is level with a concrete patio surface, there should be an expansion space left between the well wall material and a concrete slab itself.
When you remove the sump pump for the cold season and the pit fills up with water all the way to the top edge, freezing water and moisture-saturated concrete will expand at slightly different levels. This might cause the concrete slab to crack since the sump pump well penetration has already weakened its integrity.
If there is no gap between the pit and the concrete, try to keep the water level inside the well below the bottom of the slab (usually 4”).