Below-grade waterproofing is a continuous membrane on the outside of foundations, walls, pits and decks below ground, built to hold back groundwater under pressure. It belongs in new construction and major renovations, installed before backfill or topping slabs bury it. It is the right call wherever water in the soil could reach occupied space, equipment or a process area.
Below-grade waterproofing protects foundations, retaining walls, and plaza decks from hydrostatic pressure and groundwater intrusion — failures here are expensive and disruptive to fix once the structure is backfilled or occupied.
We install sheet-applied and fluid-applied waterproofing systems below grade, detailing every wall-to-footing transition and penetration before backfill or deck installation covers the work.
Below-grade waterproofing gets one chance to be right — once it’s backfilled or covered, a failure means excavation or demolition to fix. We pressure-test and inspect every seam and transition before the work is covered.
That verification step is what protects you from a callback that costs ten times more than getting it right the first time.
Below-grade work fits into a construction schedule controlled by other trades. These are the steps we take to keep the membrane intact from the first detail to the last load of fill.
| Spec | Typical Value |
|---|---|
| System types | Typical: sheet-applied (self-adhered), fluid-applied or bentonite membranes |
| Application side | Positive (outside) face where excavation allows |
| Tight sites | Typical: blindside (pre-applied) membrane against shoring at property lines |
| Protection | Protection board or drainage composite before backfill |
| Drainage | Drainage composite tied into a footing drain, as designed |
| Quality control | Thickness checks, detail inspection and photo records before covering |
| Horizontal decks | Flood or electronic leak testing where specified |
Below-grade waterproofing costs far less to install correctly than to repair after the structure is buried. These factors drive the installed price.
A high water table calls for a heavier system, more reinforcement and better drainage than a site with dry, well-draining soil.
Sheet, fluid-applied, bentonite and blindside membranes differ in material cost, labor and the substrate prep each one needs.
Pits, sumps, penetrations, cold joints and wall-to-footing transitions take far more labor per square foot than a plain wall.
Narrow excavations, shoring, deep pits and work that must be split around other trades' schedules add mobilizations and time.
Flood tests, electronic leak detection and detailed QA records add cost, but they are often required on decks over occupied space.
| Below-Grade Waterproofing | Air & Vapor Barrier Systems | Polyurea Coatings | ||
|---|---|---|---|---|
| Where it goes | Foundations, pits and decks below ground | Above-grade wall assemblies | Exposed concrete, steel and tanks | |
| What it stops | Groundwater under pressure | Air leakage and vapor movement | Water, abrasion and chemicals | |
| Typical materials | Sheet, fluid-applied or bentonite | Fluid-applied or self-adhered | Spray-applied polyurea | |
| Installed | Before backfill or topping slab | Before cladding goes on | During construction or a shutdown | |
| Access after | Buried; hard to reach | Behind cladding | Usually exposed | |
Waterproofing resists water under pressure; dampproofing only slows moisture moving through soil. Building codes generally require true waterproofing where groundwater can push against the wall, and allow dampproofing where it cannot. The geotechnical report and the design team set which one applies, and we install to that requirement.
During construction, after the concrete is ready and before backfill or the topping slab. That is the only time the outside of the wall is fully reachable. Adding it later means excavating around an occupied building, which costs more and disrupts the site. Plan it into the schedule early.
Both work when installed correctly. Sheet membranes give a factory-controlled thickness and suit large, flat walls. Fluid-applied membranes form a seamless coat and handle complex shapes, penetrations and tight corners well. Many projects use both, with fluid-applied material at details and sheets across the field.
Yes, though the approach depends on access. Excavating and waterproofing the exterior face is the most reliable fix because it stops water before it enters the concrete. When the outside cannot be reached, interior treatment or crack injection is a separate approach with its own limits. We assess the site before recommending either.
Most failures start at details, not in the middle of the wall. Common causes are unreinforced footing transitions, poorly sealed penetrations, membrane damaged by backfill or other trades, and drainage that lets water sit against the wall. Inspection before covering and protection board during backfill prevent most of them.
Yes. Below-grade scopes include machine pits and heavy equipment foundations, underground tanks and sumps, wastewater basements, pipe trenches, conduit galleries and tunnels. These areas often combine groundwater with chemicals or vibration, so the membrane, protection and details are selected for those exposures as well as the water.
Yes. Strong has worked on large new-construction semiconductor and technical facilities, where below-grade waterproofing sits alongside roofing, air and vapor barriers and coatings in one tightly coordinated schedule. On these projects, strict security, documentation and sequencing with other trades matter as much as the membrane itself.
