TL;DR
Both TOPSIDE® and UNDERSIDE® drainage systems create dry, usable space beneath your deck — the right choice comes down to your deck, your goals, and how you want to install. TOPSIDE® is DEK Drain’s most popular system because it keeps your framing dry and installs with the grain of the work, not against it. UNDERSIDE® is the go-to for adding drainage to a deck you already have, without tearing up the surface. Either way, you reclaim the space below
It’s time to choose a side…
When it comes to creating dry, functional space beneath an elevated deck, DEK Drain® gives homeowners something most manufacturers don’t: a choice.
Both our patented systems — TOPSIDE® and UNDERSIDE® — divert water away from the area under your deck, transforming a dark, dripping dead zone into space you’ll actually use.
The goal is the same. The path to get there is what’s different.
What Both Systems Have in Common
Before we get to the differences, it’s worth knowing that our two systems share most of their DNA. Both are fabricated from the same 45-mil rubber membrane, both fit 12”, 16”, and 24” framing, and both run long panels with no joints to seal. Neither one needs glue, caulk, or adhesive — ever. And both do the job you’re after: keeping the area beneath your deck dry and handing you back square footage you can use for storage, entertaining, or simply enjoying.
In other words, whichever system you choose, you’re getting the same rubber-membrane quality and the same dry space below. The difference is how — and when — it gets installed.
Talking TOPSIDE®
TOPSIDE® installs on top of your deck joists, before the decking boards go down. Because it captures water before it ever reaches the framing, it’s the system of choice for new construction and major deck renovations — and it’s our most popular product for good reason.
Here’s what sets it apart. First, it keeps your framing dry. By managing water above the joists, TOPSIDE® protects the entire structure from the kind of moisture exposure that leads to rot and deterioration down the road.
Second, it works with gravity, not against it — contractors install it from above rather than wrestling panels overhead, which means simpler installation, less labor, and a more efficient job.
Third, TOPSIDE® leaves the bottoms of your joists open and ready, so if a finished ceiling is anywhere in your future plans, the framing is already set up for it.
Finally, TOPSIDE® simplifies flashing where the deck meets your home and installs cleanly over support beams, keeping project design straightforward.
Understanding UNDERSIDE®
UNDERSIDE® takes the opposite approach: it installs beneath the deck framing, catching water that slips through the surface boards. It’s purpose-built for one job a lot of homeowners need help with — creating dry space below a deck that’s already standing.
Its biggest advantage is right there. Because UNDERSIDE® can often be installed without removing your existing deck boards, it’s a practical, low-disruption way to add drainage to a deck you’re otherwise happy with. This makes it a natural fit for retrofit projects, and especially for homeowners who want to turn the space below into dry storage for lawn equipment, bikes, firewood, seasonal décor, and everything else that needs a roof over its head. You say you want more than storage? No worries. Our UNDERSIDE® Recessed profile fits up into the bays between your joists, so you can add a finished under deck ceiling later — complete with lighting, fans, and other comforts of home. The door to a true outdoor room stays open.
Which System Is Right for You?
It comes down to your project.
Both systems work, and both have earned their place in the lineup. But TOPSIDE® remains the solution contractors reach for again and again. It keeps framing dry, installs with gravity, and leaves the door open for a finished ceiling. That said, UNDERSIDE® isn’t a runner-up. For existing decks and storage-focused projects, it’s often exactly the right tool — no demolition required. The best drainage system isn’t the one with the most features. It’s the one that fits your deck, your budget, and your goals.
TOPSIDE® and UNDERSIDE® are both proven ways to create dry, usable space beneath an elevated deck. For new builds and major renovations, TOPSIDE® typically gets the nod thanks to its framing protection and easier installation. For existing decks, UNDERSIDE® reclaims the space below without disturbing the surface above. Pick the side that fits your project — either way, DEK Drain has you covered.
Reach out to our team today to learn more and get started.
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FAQs
What is the difference between TOPSIDE® and UNDERSIDE® drainage systems?
TOPSIDE® installs above the joists before the decking goes down, managing water before it reaches the framing. UNDERSIDE® installs beneath the joists and is most often used to add drainage to an existing deck.
Which system is DEK Drain’s most popular?
TOPSIDE®. Its framing protection, gravity-assisted installation, and finished-ceiling readiness make it the preferred choice for most new builds and renovations.
Is UNDERSIDE® still a good option?
Absolutely. For existing decks and storage-focused projects, UNDERSIDE® is often the perfect fit — and it can usually be installed without removing your deck boards.
Can both systems create usable space beneath a deck?
Yes. Both are designed to create dry, functional space below an elevated deck.
Which system is better for a finished ceiling?
For existing decks, the UNDERSIDE® Recessed profile is the finished-ceiling option — it recesses into the joist bays so you can add a ceiling with lighting and fans later. UNDERSIDE® can also support a finished ceiling with some additional framing.