Crawl Space Drainage and Sump Pumps
Some crawl spaces collect liquid water: puddles after a week of rain, a stream along one wall, or a shallow pond that never quite dries until July. A vapor barrier alone cannot handle that. Water has to be collected and moved out of the crawl space. We install interior drains and sump pumps that do it automatically.
First, find out where the water comes from
Not all crawl space water needs a pump. Before proposing drainage, we check:
| Source | How to tell | Fix |
|---|---|---|
| Gutters and downspouts | Water appears along the wall below a downspout, worst in storms | Extend or reconnect downspouts, clean gutters — not a crawl space job |
| Grading toward the house | Soil or landscaping slopes toward the foundation | Regrade or add a surface drain outside |
| Plumbing leak | Water in one spot, even in dry weather | Plumber repair |
| Condensation | Damp plastic and framing, no puddles | Ventilation or encapsulation |
| Groundwater | Water rising from the soil across the floor after prolonged rain | Interior drain and sump pump |
Many wet crawl spaces in our area improve dramatically with downspout corrections alone. We will tell you if that is the case. Our guide on why crawl spaces get wet goes further.
What a drainage system includes
- Interior perimeter drain. A trench along the inside of the foundation wall, perforated pipe wrapped in filter fabric, bedded in washed drain rock.
- Sump basin at the low point of the crawl space, sealed with a lid.
- Submersible sump pump with a float switch and a check valve on the discharge.
- Discharge line from the pump through the rim joist or foundation to an approved outdoor location.
- Liner over the system, so the drain and basin sit under the new vapor barrier or encapsulation liner.
- Optional battery backup or high-water alarm, recommended in crawl spaces with ductwork or a furnace.
Electrical and permits
A sump pump needs a dedicated, properly protected circuit. That work requires an electrical permit. In Seattle and some other cities the city issues electrical permits; elsewhere in Washington the Department of Labor & Industries does. A licensed electrician handles that part of the job under our contract.
Where the water can go
Sump discharge cannot be connected to a sanitary sewer line. Depending on the city and the lot, it may go to the storm system, a splash block far enough from the foundation, a dry well or a rain garden. We confirm the allowed discharge point for your address before installing.
Drainage and sump pump prices
| Scope | Typical range |
|---|---|
| Sump basin and pump at the low point, no perimeter drain | $2,500 – $6,000 |
| Partial interior drain along one or two walls + sump | $5,500 – $10,000 |
| Full interior perimeter drain + sump | $9,000 – $18,000 |
| Battery backup or high-water alarm | $500 – $1,500 |
Price depends on the length of trench, the clearance in the crawl space and how far the discharge line has to run.
Drainage questions
Will a sump pump run all the time?
It runs only when water reaches the float. In a typical wet crawl space here, it may cycle often in November through March and not at all in summer.
How long do sump pumps last?
Most quality pumps last seven to ten years. Check it each fall by pouring water into the basin until the pump starts.
Do I need exterior foundation drains instead?
Exterior footing drains are a larger excavation project, and when they fail it is usually a foundation-contractor job. For crawl spaces, an interior system is far less disruptive and handles groundwater well. If we see signs of structural movement, we refer you to a foundation specialist.