Foundation Waterproofing 101

Foundation Waterproofing Cutaway

Credit: Basement Waterproofing Baltimore (2018, February 20).

Designing buildings with water protection in mind is critical to protecting buildings from future damage, difficult/costly repairs, and potential litigation. Foundations are by necessity in the ground. So is water. Foundation waterproofing is intended to keep them separate, by providing a layer of protection between a below-grade structure and the moisture present in the surrounding soil and fill. Waterproofing is especially important when the foundation lies below the water table or in a flood zone. Read on to learn about different approaches and materials used to waterproof foundation walls and slabs and specific detailing needed to provide a watertight enclosure. And, check out Part 2 of this series for specific guidance and examples to achieve a watertight enclosure.

Why is foundation waterproofing necessary?

Did you know? Water intrusion makes up more than 70% of construction litigation.Water

Foundations are basically holes in the ground that want to fill with water. Poor site drainage, through-wall penetrations, concrete cracking/mortar joints and movement, door/window/vent openings, flooding, high water tables, hydrostatic pressure – all contribute to the propensity for water to fill the subterranean void we have established. Foundation leaks are difficult and costly to rectify, not to mention designer/contractor financial liability. Water in a basement is water in a building. Excess moisture within a building is a recipe for higher RH and increases the potential for condensation, and mold and other allergens.

Luckily, foundation water intrusion is usually preventable. The goal is to identify all the potential water transport mechanisms, and address them, through good design practices, proper detailing, and quality execution.

How local code requirements and flood zone classification could affect you

Before we get to the solutions, you need to understand how water tables, flood zone restrictions, and code requirements could impact you. What is the difference between waterproofing and dampproofingIf local code requires compliance with International Building Code (IBC), waterproofing is required where a hydrostatic pressure condition exists and dampproofing is required where it doesn’t. If the foundation lies below the local water table, a hydrostatic condition is present and waterproofing is required. Appendix G additionally defines requirements for flood resistant construction in regulated flood zones. You can determine if your building is in a regulated flood zone (high risk, AE & VE zones) by looking at the FEMA flood zone maps or Flood Insurance Rate Map (FIRM).

Below slab vapor barriers are not considered waterproofing!Understanding the three foundation waterproofing system types and when they are used

There are three basic types of waterproofing systems: blind-side and positive-side (new construction), and negative-side (new or existing construction). All types rely on the waterproofing membrane to be continuous and water-tight.

Blind-side Waterproofing

  • Waterproofing installed prior to concrete placement, but in continuous contact with concrete. Typically sheet membrane systems are used, such as HDPE and TPO sheets, sometimes bentonite sheets, sometimes PVC+”expanding core”. Sometimes a hybrid system (liquid+sheets) is used. The membrane is typically placed over rigid insulation boards. Sometimes a “rat”/working slab is provided. Often, but not always, the slab is first in installation sequence followed by foundation walls. Support of excavation and adjacent building walls provide the substrate and the PROBLEMS!
  • Key takeaway: It is critical to schedule concrete placement as soon as possible after waterproofing is installed, and to carefully check all details for continuity prior to the pour (no second chances).

Blind-side Waterproofing

Positive-side Waterproofing

  • Waterproofing installed on the foundation exterior after concrete placement, in continuous contact with concrete. Sheet membranes (e.g., self-adhered mod-bit sheet membranes) or liquid membrane systems (e.g., roll-on or spray liquids) are typically used. Usually tie in to blind-side waterproofing below slab/footing.
  • Key takeaway: Substrate prep is CRITICAL and protection is needed as soon as possible after installation! For sheet membranes, primers are often required and wrinkles must be minimized during installation. For spray-applied liquid membranes, use certified experienced spray machine operators.

Negative-side Waterproofing

  • Waterproofing/water control methods installed on the interior, which is typical for existing building remediation/retrofit, since the other two options are not feasible.

What’s Next

Once water enters your foundation, the damage can be very difficult and expensive to repair. The good news is that with proper attention to foundation waterproofing, most damage can be prevented. The key is understanding how and where water enters your foundation, and implementing a design with proper details and construction oversight to avoid the typical failures.

Check out part 2 of Foundation Waterproofing 101 where we discuss proper installation and common mistakes.

Bill Zoeller


By Bill Zoeller, Senior VP

11 replies
  1. Avatar
    Kairi Gainsborough says:

    My husband and I notice that water is seeping into our basement, so we know that we need to do something about it before it damages our foundation. I noticed you mentioned that water in the basement can increase the amount of mold and allergens in the air, which is definitely something I have noticed in this house. I will plan on calling a contractor to come out and completely seal the basement so that no water can get in.

  2. Avatar
    Bob says:

    I didn’t know that waterproofing was put in prior to the foundation installation. My wife and I are thinking about building a new home and we want to build a waterproof basement. We’ll have to consider your tips so that we can get a nice basement in the future.

  3. Avatar
    Md Hasan says:

    after reading this article I got able to noticed you mentioned that water within the basement can increase the quantity of mold and allergens within the air, which is sincerely something I even have noticed on this residence. thank you for sharing something new.

  4. Avatar
    Dean Phillips says:

    I had never considered the fact that foundations are basically holes in the ground that want to be filled with water. My brother lives in an area of the country that is prone to large rainstorms and flooding. It may be beneficial for him to hire a professional company to go to his house and help waterproof his basement.

  5. Avatar
    Penelope Smith says:

    This is some really good information about waterproofing your foundation. It is good to know that it would be smart to think about checking what the building codes in your area when it comes to waterproofing. That wouldn’t have been something I would have thought about checking before starting a waterproofing project.

  6. Avatar
    Eileen Benson says:

    It made sense when you said that waterproofing is essential because leaks can be hard to identify and fix after they occur, in addition to being costly. My husband and I want to have a large custom home built on the land we purchased last month. Thanks for helping me understand why hiring a basement waterproofing contractor is something we should prioritize!

  7. Avatar
    Rebecca Gardner says:

    Thanks for explaining that most local codes require waterproofing if the conditions meet minimum standards for hydrostatic pressure. My husband and I have been wondering if we’ll need to hire a waterproofing service for the custom home we’re planning on have built later this year. Your article should help us ask the right questions to determine if exterior waterproofing would be a good idea!


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