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Community and Q&A

French Drain Against or Away From Slab On Grade?

lance_p | Posted in Energy Efficiency and Durability on

My Stepson lives in San Diego with a bungalow and detached garage both slab on grade.  He’s got water coming into the garage in an area where the grade seems to be a bit higher than the slab.  This area is bare dirt with a concrete walkway poured about 24″ from the garage slab.  I’m not local so I can’t take precise measurements.

He’s thinking about pouring concrete right up to the garage so there’s a concrete drainage plane to shed water.  I suggested if he was going to go that route to make sure the area is below the slab when completed, that it slopes away from the slab, that permeable fill was added against the slab, and that it might be a good idea to bury a french drain pipe even if it’s level and doesn’t technically drain anywhere simply because adding a drain later would require pulling all the new concrete.

His contractor is telling him it would be better to have the french drain added beyond the existing concrete walkway as this is where the bulk water would be draining to.

I’m assuming the lot has proper slope and drainage, and that the pipe (if added) should be where the anticipated problem is; against the slab.  His contractor seems to be thinking about what to do with the bulk water after it runs off the concrete walkway.

Either way, a drainage pipe may not be necessary, but I thought I’d ask those here for their input on A) pouring concrete right up to the garage, and B) to install or not to install a french drain pipe, and if yes where to put it.

EDIT: or if another approach makes more sense.

Thanks!

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Replies

  1. BSBot | | #1

    Addressing water ingress in a garage, especially in a situation where the grade is higher than the slab, requires careful consideration of both immediate water diversion and long-term moisture management strategies. Your stepson's situation in San Diego involves several potential solutions, and both your and the contractor's suggestions have merit. Let's explore the options:

    A) Pouring Concrete Right Up to the Garage
    Pouring concrete to create a drainage plane adjacent to the garage can be effective if done correctly. Key considerations include:

    Slope: Ensure the new concrete area slopes away from the garage slab at a minimum of 1/8 inch per foot to promote positive drainage away from the structure.
    Height Relative to Slab: The finished surface of the new concrete should ideally be slightly lower than the garage slab to prevent water from pooling against the slab edge.
    Joint Sealing: If concrete is poured close to the garage slab, ensure there is a sealant in the joint to prevent water from seeping between the new concrete and the slab.
    B) French Drain Installation
    The decision to install a French drain and its placement depends on the specific water flow and landscape grading around the garage. Both suggested locations have their advantages:

    Adjacent to the Slab: Installing a French drain close to the garage can intercept water before it reaches the slab, effectively reducing the amount of water that could potentially enter the garage. This is particularly beneficial if there's a significant amount of water flowing towards the garage from the higher grade.
    Beyond the Existing Concrete Walkway: Placing the French drain further away, as the contractor suggests, aims to manage and redirect bulk water runoff before it even reaches the garage area. This approach can be effective if the primary source of water is coming from beyond the walkway and if the grading directs water towards this area.
    Other Considerations and Approaches
    Grading and Landscaping: Re-evaluating the overall landscape grading around the garage and the walkway could provide a more natural solution to water diversion. Ensuring that the entire lot slopes away from the garage and implementing landscaping solutions like swales or rain gardens could mitigate water issues without extensive hardscaping.
    Permeable Solutions: Instead of solid concrete, consider permeable paving options that allow water to percolate through the surface and into the ground, reducing runoff towards the garage.
    Gutters and Downspouts: If the garage has a roof, ensure that gutters and downspouts are correctly installed and directed away from the foundation to reduce the amount of water saturating the ground near the slab.
    Conclusion
    Both pouring concrete with proper slope and considering a French drain have their places in addressing water ingress issues. The decision should be based on the most significant source of water and the most effective way to divert it from the garage. A combined approach, using thoughtful grading, appropriate landscaping, and strategic hardscaping, might provide the most comprehensive solution. Consulting with a local landscape architect or drainage expert who can assess the specific site conditions could offer tailored advice for your stepson's situation, ensuring the chosen solution is both effective and sustainable in the long term.

    1. lance_p | | #2

      Thanks for your thoughtful and thorough response. You’ve certainly given us some food for thought.

      I just got the notification today so apologies for the delayed reply!

      1. Malcolm_Taylor | | #3

        lance_P,

        BSBot spent a day or so spamming the board with AI generated answers.to a variety of questions. I would be cautious about any advice he gave.

        1. lance_p | | #4

          Thanks Malcolm! That makes total sense when I re-read it.

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