TL;DR
The right DEK Drain® system depends on where your deck is in its life. Building new, or replacing your boards? TOPSIDE® installs above the joists and keeps your framing dry from day one. Already have a deck you’d rather not tear up? UNDERSIDE® installs from below to reclaim the space without disturbing the surface. And if you’re building now but not ready to finish the space yet, the Dog Ear Joist Flashing System lets you plan ahead. Here’s how to match the system to your situation.
It all starts with your deck.
Drainage decisions, that is.
Maybe you’re drawing up plans for a brand-new one. Maybe you’ve had the same deck for 15 years and simply want the soggy mess underneath to stop.
Those are two very different starting points, and DEK Drain® has a system built for each.
Here’s how to figure out which one fits where you are.
If You’re Building New (or Replacing Your Decking)
When the joists are exposed and the deck boards aren’t down yet, you’re in the ideal position to install TOPSIDE® — DEK Drain’s most popular system, and for good reason.
TOPSIDE® panels attach to the top of the joists before the decking goes on, capturing rainwater before it ever reaches the framing. That timing is the whole advantage: instead of managing water after it’s already run across your beams and joists, you keep the structure dry from the very first storm. The panels come pre-cut for 12”, 16”, and 24” framing — one per bay, with no waste — and install cleanly from above. And because they leave the bottoms of the joists open, your deck is ready for a finished ceiling whenever you decide you want one.
If a new build or a full re-deck is on your horizon, TOPSIDE® is the system to plan around.
If You Already Have a Deck
Tearing up a perfectly good deck surface just to add drainage underneath is a tough sell — and with UNDERSIDE®, you usually don’t have to.
UNDERSIDE® installs beneath your existing joists, catching the water that slips through the deck boards and diverting it away from the space below. Because the system works from underneath, it can often be installed without removing your decking at all — which makes it a practical, low-disruption way to reclaim the area under a deck you’re otherwise happy with. It’s custom-fabricated from the same thick rubber membrane as TOPSIDE® and pre-cut to your deck’s measurements, so it fits your structure rather than the other way around. Whether you want dry storage or a true outdoor room, UNDERSIDE® turns wasted space into usable space without a demolition project.
One exception worth noting: if you’re already planning to replace your decking boards, TOPSIDE® becomes an option for your existing deck, too. With the boards off, you can install it on top of the joists and gain that same keeps-the-framing-dry advantage a new build enjoys. In other words, an existing deck doesn’t automatically rule out our popular TOPSIDE® system — it just depends on whether your boards are staying or going.
If You’re Building Now but Finishing Later
Not everyone is ready to commit to a full system on day one — and that’s exactly what our dog ear joist flashing system is for.
Built from the same materials as our TOPSIDE® and UNDERSIDE® systems, the joint flashing system protects your deck’s frame against the elements now, while setting you up to add an UNDERSIDE® system later — without having to take up the deck boards to do it. For homeowners who want to protect the framing today and keep their options open for tomorrow, it’s a smart way to future-proof the investment.
Think About How You’ll Use the Space
Your deck’s stage of life points you toward the right system, but how you plan to use the space below is worth considering, too. If your goal is simply dry storage for lawn equipment, bikes, and seasonal gear, either system delivers. If you’re dreaming bigger — a finished ceiling with recessed lighting and a fan, an all-weather lounge, a genuine outdoor room — keep that in mind as you choose. TOPSIDE® leaves your joists ready for a ceiling, and the UNDERSIDE® Recessed system is purpose-built to accommodate one.
The Bottom Line
Matching a DEK Drain® system to your project is refreshingly simple. Building new or replacing your boards? Reach for TOPSIDE®. Protecting a deck you already have? UNDERSIDE® is your answer. Building now but finishing later? Dog Ear flashing keeps the door open. Whichever path fits, you’re getting the same durable rubber membrane and the same fully transferable limited lifetime warranty — the only one of its kind in the industry.
Not sure which system fits your deck? Reach out to our team at 1-866-DEK-DRAIN (335-3724), and we’ll help you find the right fit.
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FAQs
I’m building a new deck. Which system should I use?
TOPSIDE®. It installs above the joists before your deck boards go down, so it keeps your framing dry from the start and sets you up for a finished ceiling later if you want one.
Can I add drainage to my existing deck without removing the boards?
In most cases, yes — that’s what UNDERSIDE® is designed for. It installs beneath the joists to reclaim the space below without disturbing the deck surface.
What if I’m building now but not ready to finish the space underneath?
The Dog Ear Joist Flashing System protects your framing now and lets you add an UNDERSIDE® system later without taking up the deck boards — a good way to plan ahead.
Do the systems differ in quality or warranty?
Nope. TOPSIDE® and UNDERSIDE® use the same rubber membrane and carry the same fully transferable, non-prorated limited lifetime warranty. The right choice comes down to your deck, not a trade-off in protection.
Can I get a finished ceiling either way?
You bet. TOPSIDE® leaves the joists open and ready for a ceiling, and the UNDERSIDE® Recessed system is built to support one, complete with lighting and fans.