Applying Commercial Waterproofing Membranes Below-Grade: 8 How-Tos

Commercial waterproofing products are heavy-duty, built-to-last systems (unlike dampproofing products). But as your customers know, they’re really only as heavy-duty and built to last as the installer allows them to be.

Think of applying waterproofing membranes underground much like you would air and vapor barriers. They’re close in composition and application, except that below-ground membrane installation doesn’t ask you to deal with as many penetrations (and perhaps detailing work).

That said, your attention to the specifics of applying waterproofing membranes affects the performance and useful life of the product. And the surrounding structure!

Let’s walk through how to install a waterproofing membrane for a basement, foundation, or similar below-ground commercial area.

8 Steps of Applying Waterproofing Membranes

Already inspected the job site and its requirements? There are eight stages to actually applying below-grade commercial waterproofing products:

1. Prep tools & materials
2. Repair the surface
3. Prep the surface
4. Install the membrane
5. Seal
6. Inspect
7. Add protection board and/or insulation
8. Backfill

1. Tool & Material Prep On-Site

Refresh yourself on the job site. The technical report should provide a solid overview of the conditions you’ll be dealing with (including any waterproofing failures). Reinforce your understanding of how the ground, climate, and any nearby contaminants might affect installation.

Tools for Waterproofing Installation

The tools your job requires will depend on whether you’re installing a fluid or sheet membrane. Tools that will directly work with flammable membranes should be sparkproof to prevent safety hazards.

A note on fluid-applied products: Low-speed mixers keep you from bringing unwanted air into the membrane. Use a mixer with flat paddle blades.

Also, when using liquid membranes on large areas, consider an automated applicator. They speed up application and provide more consistent thickness.

Material Storage & Prep

You should also prepare accordingly at the material level. If your construction material storage practices are careless or disorganized, the product may not perform the way you’d hoped.

To keep your materials ready:

• Avoid storing primers and membranes in extreme temperatures or humid conditions. 
• Store liquid-applied products near room temperature at least one night before use to preserve their pot life.
• Unload only what you'll need for the day.
• Cover the materials until it’s time to use them.

When in doubt, follow the manufacturer’s guidelines for storage.

2. Inspect and Repair the Surface

Closely check the surface for pinholes, cracks, and voids. These are sources of leaks. Addressing these imperfections can:

• Seal away water
• Improve adhesion
• Prevent premature membrane aging or failure

You might also find efflorescence, which is a bit ugly, but not necessarily a structural concern itself. However, heavy efflorescence is like a blinking, red billboard telling you, “Moisture lives here.”

To repair a surface needing waterproofing, apply hydraulic cement or appropriate fillers as necessary to prevent further leaking.

3. Prep the Surface

Let the substrate cure for at least 28 days before beginning waterproofing procedures. This guidelines can vary from product to product.

From this stage on, timing is everything.

Remove all debris, rocks, and sharp objects that could damage the membrane. 

Waterproofing membranes and primers generally love three surface conditions:

• Clean
• Smooth
• Dry
• Structurally sound

Take off all dirt, contaminants, and old coatings. Your choice of cleaning and removal methods should depend on the surface material, level of damage, and any contamination present. 

(Related post: Concrete Surface Prep & Repair Guide)

The manufacturer's instructions for waterproofing membranes also usually include applying a primer. If that’s true in your case, check the surface’s porosity to gauge what type and how much primer you’ll need.

Apply the primer evenly (and generously) to enhance the adhesion of the waterproofing membrane. If you’re priming a wall, work from the top down.

Other tips and potential tasks at the surface prep stage include:

• Pretreat surface irregularities and voids more than 1⁄8 in. deep with a concrete mix, grout, or the membrane material itself.
• Remove windrows, form match lines, and high spots more than 1⁄8 in. tall.
• Apply sealer to very porous or rough concrete; this will help the membrane bond.

Again, wait for curing to complete before moving to Step 4.

(Diagram of Blueskin PreSeal sheet membrane courtesy Henry)

4. Install the Membrane

It’s finally time to do what you came to do! For starters, space all penetrations at least 2” apart so you can do detail work around them without trouble.

Installing Liquid Membranes

If the waterproofing product is fluid-applied, you’ll have to prepare it first. Mix it out of direct sunlight so it doesn’t cure prematurely. Faster is better – you’ll want to minimize the time your material has to mingle with the air.

Since liquid is self-leveling, there’s no need to overwork a fluid-applied membrane. You can remove imperfections like pinholes and bubbles with a spiked roller. 

As you spread the next bit of product, overlap the previous application area by at least a few inches, depending on the manufacturer’s recommendations. It’s easy to gouge the previous layer when adding a new one, so work carefully! 

Installing Sheet Membranes

In this case, you’ll be subtracting product rather than adding it together. Roll out and cut the membrane sheets to fit the working area. 

Securely overlap edges as the manufacturer specifies. For overlaps, a double-sealing technique can ensure a watertight bond. This involves applying an initial adhesive, then adding a secondary seal using heat or pressure. 

5. Seal All Joints

Promote a watertight seal by getting meticulous with:

• Overlaps
• Edges
• Joints
• Penetrations

Penetrations include pipes, conduits, and anything else that requires putting a hole in the membrane’s coverage.

To put these risks to rest, apply a compatible sealant or additional membrane pieces. This might require a specialized tape or welding with a heat gun.

At structural penetrations, use custom or standard boot-type seals. Encase all penetrations in concrete and solid-grout them in place. They should integrate with the membrane to create a seamless barrier. 

At corners, use preformed pieces and joint covers where the membrane bends or joins another section to reinforce these vulnerable areas.

6. Inspect Your Work

Now’s the time to check for obvious mess-ups:

• Bare spots
• Gouges
• The need for touch-ups at flashings

If you fix these issues before the product fully cures, you can avoid having to abrade and clean the area again.

For liquid waterproofing products, also verify that you’ve met the minimum thickness requirement for application. 

Many times, on high-profile below-grade projects, a third party will handle inspection on behalf of the owner. The manufacturer may be there as well.

To cover your butt, keep detailed records of all inspection and test results. Note any repairs or adjustments your team had to make. These notes will act as a reference for any future warranty claims or maintenance.

Finish all inspection work and testing of the membrane before you cover it up for good.

7. Install the Protection Board and/or Insulation

Say it with us again: Wait for curing before moving to the next step.

Once that happens, apply a protection board to shield the membrane from damage. This board is typically rigid and provides a durable layer over the membrane. Choose a rigid board that’ll not only protect the membrane from punctures, but also against damage during backfilling or future maintenance.

Integrating Insulation

Integrating insulation will boost the membrane’s thermal performance, with the bonus of added physical protection.

Insulation materials you can use alongside protection boards include:

• Extruded polystyrene (XPS) – excellent moisture resistance and high compressive strength
• Expanded polystyrene (EPS) – budget option for good thermal & moisture resistance, but with less compressive strength

Integrating a Drainage System

For certain applications, you’ll also want to integrate the drainage system, making sure nothing’s blocking water from flowing to the outlet. Install a drainage board, mat, or panels that direct flow away from the foundation, reducing hydrostatic pressure on your commercial waterproofing system. 

Vertical systems generally focus on direct waterproofing measures, such as wick drains, weep holes, and drainage boards. With gravity on their side, vertical installations typically don’t need extensive water management.

8. Backfill the area.

Once the technical inspector has signed off on your work, restore the overburden in a careful, controlled manner. Avoid using big rocks and debris that could put a hole in the membrane. 

Fill the area with materials that drain well. This can reduce hydrostatic pressure on the membrane. Compacting in layers can also help your work last longer, as it prevents settling and shifting that could stress the membrane.

Every Waterproofing Membrane Application Is Unique

Mission accomplished: You’ve nailed the basics of installing a commercial waterproofing membrane.

While these basics will serve you well, there’s no catch-all guide that’ll cover every unique job site and situation. Consult the product’s manual, official training videos (if any), or your distributor for the deeper details.

If you want more hands-on help, check out the range of training options available through our technical product experts: