An elevated deck wastes the ground beneath it because nothing manages the water coming off the walking surface above. Rain drips straight through the joists, so the space below stays too wet for storage and too damp underfoot for a patio.
Walk-out basements, sloped lots, and decks built for a view over the yard all tend to push the walking surface well above the ground. Once a deck sits several feet up, the space underneath becomes an awkward in-between zone.
A standard deck surface, whether wood boards or open vinyl planking over joists, lets rain pass straight through to whatever’s below. Every storm turns that space into mud, then into a magnet for weeds and mildew.
A waterproof deck covering system solves that by catching the water before it ever reaches the joists. This article walks through how that system actually works, and what it opens up underneath once the water is gone.
A waterproof deck covering system works by sloping a continuous membrane across the deck’s substructure, collecting every drop of water that lands on the walking surface, then channelling it through a trough and downspout network away from the space below. Nothing drips through the joists.
The slope is the starting point. Installers pitch the substructure, usually a very slight grade across the joists, so water never sits still on the membrane. Standing water, or ponding, is exactly what a properly sloped system is built to prevent, since pooled water eventually finds the smallest gap in a seam or fastener.
Below that sloped membrane sits a trough or gutter running the length of the deck, usually tucked just under the joists so it’s out of sight from below. Water sheets off the membrane into that trough, then travels to a downspout that carries it down a post or along the house wall to grade, well clear of the foundation.
The membrane itself has to stay continuous everywhere water can find an edge. Seams between membrane runs are heat-welded rather than taped or glued, so the seam is as watertight as the sheet itself. Deck posts, ledger boards, and any post that punches through the walking surface get flashed and sealed individually. A single unsealed penetration can undo the sloping and channelling happening everywhere else.
For the material science behind the membrane itself, including seam welding and lifespan, see DeckRite’s guide to vinyl deck membranes. This piece stays focused on where the water goes once it lands.
Once water is fully captured and routed away, the ground-level area under an elevated deck becomes genuinely usable space.
That’s the payoff most homeowners are actually chasing when they ask about deck waterproofing. A second-storey deck with a proper drainage system underneath turns its footprint into real square footage: a shaded patio for a table and chairs, a covered spot for a kids’ play area, or dry storage that does the job of a garden shed without adding a structure to the yard.
In our experience, the most common uses homeowners land on are:
None of that works if the drainage underneath is only partly doing its job. A membrane that sheds most of the water but leaks at one seam still drips onto whatever’s stored below, which is why every seam and flashing point in the system needs to hold. DeckRite’s guide to vinyl sundeck covering walks through the broader installation process if you’re planning the deck itself alongside the space underneath it.
The square footage was always there. Most homeowners just weren’t using it.
A complete waterproof covering system relies on four parts working together: the sloped membrane, sealed seams and flashing at every penetration, a perimeter gutter, and downspouts sized to the area they’re draining. A gap in any one of them reopens the leak path the others were built to close.
The membrane does the bulk of the work, but only where it’s continuous. Flashing is what keeps it continuous at the edges: around deck posts, along the house wall where the deck meets siding, at stair openings, and anywhere a railing post or light fixture punches through the surface. Each of those points gets its own sealed detail.
The gutter collects what the membrane sheds and needs to run the full perimeter without dead spots where water can back up. Downspouts then move that collected water to grade. Undersized downspouts are a common weak point: a gutter can be doing everything right and still overflow if the downspouts can’t keep pace with a heavy rain event.
Membrane quality is part of the system too. PVC waterproofing membranes used in Canadian construction can be evaluated against CAN/CGSB-37.54-95, the Canadian General Standards Board specification for PVC roofing and waterproofing membranes, through the National Research Council’s Canadian Construction Materials Centre evaluation program. It’s a reasonable question to ask any supplier: what standard is the membrane evaluated against, and can they show the documentation?
Treat these four parts as one system rather than a shopping list. A crew that gets the slope and membrane right but skimps on flashing around a single post has built a system that will still leak, just more slowly and less visibly than an unwaterproofed deck. For DeckRite’s own installation specifications, see the product specifications page.
Coastal BC deck systems need to handle rain on a different schedule than Prairie or Ontario builds do, so gutter size and downspout placement should be sized to local rainfall patterns rather than a single national default.
Rain frequency matters as much as rain volume. Abbotsford, BC averages 174 days of measurable precipitation a year, and St. John’s, Newfoundland averages 212, among the highest rates of any Canadian city, according to Current Results‘ compilation of Canadian climate data. A drainage system on a coast that wet is moving water on far more days of the year than a system on the Prairies, even in a year when total volume looks similar.
| Region | Typical rainfall pattern | What it means for drainage design |
|---|---|---|
| BC Coast (Vancouver, Victoria) | High annual precipitation, sustained fall and winter rain | Larger gutter capacity and more downspout outlets to keep pace with volume |
| Prairies (Calgary, Saskatoon, Edmonton) | Lower overall precipitation, but sharp freeze-thaw and chinook swings | Slope and seams need to handle ice-damming risk as much as raw volume |
| Toronto and Ontario | Summer thunderstorm intensity mixed with winter freeze-thaw | Downspouts sized for short, intense summer bursts as well as winter freeze-thaw cycles |
The BC Coast and the Prairies don’t just get different amounts of rain, they get it on different schedules, which is why regional fall maintenance looks different too. DeckRite’s regional guide to fall care for vinyl decks covers that split in more detail, and the guide to decking materials for Toronto’s climate covers the Ontario freeze-thaw pattern specifically.
A dealer sizing a drainage system should ask about your postal code as much as your square footage.
Gutter capacity and downspout count both depend on your local rainfall pattern and your deck’s exact dimensions, which is why that sizing is best confirmed with the dealer working on your home. Speak with a decking dealer in your province to get that right before work starts.
Trapped moisture under a deck is exactly the sustained damp condition mould needs to take hold. Canada Mortgage and Housing Corporation identifies moisture control as the key factor in preventing mould growth in housing, and the same logic applies to the ground under a deck.
An uncovered deck with open joists drips onto the ground below every time it rains, and that ground rarely dries out fully between storms, especially through a wet fall or a rainy coastal spring. The space stays damp for weeks at a stretch, close to ideal conditions for mould and wood rot to set in on anything stored there, and on the framing itself.
A working drainage system removes that moisture source at its origin. Water never reaches the ground under the deck, so there’s nothing sitting there to evaporate slowly and keep humidity elevated in that pocket of the yard.
This protects the deck’s structure as well as the space underneath it. Joists and framing that stay dry last longer than framing that spends every fall sitting in damp shade.
A waterproof covering effectively creates a finished space under your deck, so it’s worth confirming two things before you commit: what your municipality requires for guard rails, stairs, and enclosed space at that height, and whether your deck’s structure was built to carry a fully covered, moisture-managed space underneath it.
Guard-rail and enclosure rules vary by municipality, so the exact requirement for your deck depends on where you live rather than a single national number. Confirm the specific requirement with your local building department before any work starts on the space below.
Structural load is worth confirming too. A deck built to carry furniture and foot traffic wasn’t necessarily engineered with a fully enclosed, weatherproof space underneath in mind, so confirm that load path with whoever’s doing the work before you finish the space below.
DeckRite’s supplier checklist covering building codes and safety compliance is a useful starting point if you’re vetting who does this work. Beyond that, the specifics depend on your deck’s age, span, and municipality, which is exactly what a dealer can walk you through.
If you’re weighing a waterproof covering system for the space under your own deck, speak with a decking dealer near you. They can assess your structure and confirm what’s realistic before any work begins.
