Buyer's Guide

Under-Deck Drainage Systems Compared: How to Choose the Right One

If you are shopping for an under-deck drainage system, you have probably noticed they do not all do the same thing. Some protect the deck framing; most do not. Some install as part of the deck; others bolt on underneath. This guide compares every type of under-deck drainage system on what it actually protects, how it installs, how it looks from below, and how it is warrantied, so you can pick the right one for your deck instead of the first one you found.

The two families of under-deck drainage

Every under-deck drainage system on the market belongs to one of two families, defined by where the waterproofing sits relative to the joists.

Above-joist (also called over-joist or topside) systems put the waterproofing on top of the joists, between the deck boards. Water drains off the surface before it reaches the framing. This family keeps both the joists and the space below dry, and it is installed as the deck is built.

Below-joist systems put the waterproofing under the joists. Water passes through the deck boards, gets caught by trays, panels, or a membrane hung beneath the framing, and is routed to a gutter. This family keeps the space below dry but leaves the joists wet, and it can be retrofitted to an existing deck.

Type 1: Above-joist integrated drainage

How it installs: A flexible TPE seal goes between every deck board as the deck is built, so the walking surface itself becomes watertight. Water sheds off the top into a gutter and downspout.

What it protects: Joists, beams, ledger, hangers, fasteners, and the space below, all dry for the life of the deck.

Looks from below: A clean, fully open ceiling. No trays, panels, or visible components.

Best for: New construction, where keeping the framing dry and the ceiling open both matter. This is how AmeriDex is built. See the AmeriDex install.

Type 2: Below-joist tray drainage

How it installs: Interlocking plastic trays or troughs are hung under the joists on a slope and taped at the seams, feeding a gutter at the low edge. Retrofits onto an existing deck.

What it protects: The space directly below. The joists and beams keep getting wet on every storm.

Looks from below: Visible trays unless a separate ceiling is added on top, which is an extra cost and trade.

Best for: Existing decks where the boards are staying put and the goal is a dry patio underneath.

Type 3: Below-joist panel or membrane drainage

How it installs: Sloped vinyl panels or a stapled membrane trough system is mounted to the underside of the joists and drained to a gutter. Also a retrofit.

What it protects: The space below, similar to trays. The framing still gets wet.

Looks from below: A panel ceiling, often off-white, with visible seams. Cleaner than bare trays but still a separate added layer.

Best for: Existing decks where a finished-looking ceiling matters more than protecting the framing.

Side-by-side comparison

How to choose

• New deck: Choose an above-joist integrated system. It is the only family that protects the framing, keeps the space below dry, and leaves an open ceiling, all with one trade and one warranty.
• Existing deck, finished look wanted: A below-joist panel system gives you a ceiling, with the trade-off that the joists keep getting wet.
• Existing deck, budget retrofit: A below-joist tray system is the most economical way to get a dry space below.

The factor most comparisons miss: the framing

Almost every under-deck drainage comparison focuses on whether the space below stays dry, and on that measure all of these systems work. The factor that separates them is the deck framing. Below-joist systems, by definition, sit beneath the joists, so the joists and beams keep getting wet for the life of the deck. Pressure-treated lumber tolerates that, but the preservative leaches out, fasteners corrode, and the ledger-to-house connection sits in a wet zone. An above-joist system is the only family that moves the waterproofing above the framing, which is why it is worth specifying on any deck you are building from scratch. See the full above-joist vs below-joist breakdown.

Frequently asked questions

What types of under-deck drainage system are there?

Under-deck drainage systems fall into two families. Above-joist (also called over-joist or topside) systems seal the deck surface so water drains off the top before reaching the framing. Below-joist systems hang trays, panels, or a membrane under the joists to catch water after it drips through the boards. Within the below-joist family, the common designs are interlocking trays, sloped vinyl panels, and stapled membrane troughs. Above-joist is the only type that keeps the joists themselves dry.

What is the best under-deck drainage system?

There is no single best system for every deck. For a new build, an above-joist integrated system is the strongest choice because it protects the framing and the under-deck space at the same time, installs as part of the deck, and leaves a clean open ceiling. For an existing deck you are keeping, a below-joist tray or panel system is the practical choice because it can be retrofitted without removing the deck boards.

What is the difference between an above-joist and a below-joist drainage system?

An above-joist (over-joist) drainage system puts the waterproofing on top of the joists, so water never touches the framing. A below-joist drainage system puts trays or panels under the joists, so the framing still gets wet on every storm but the space below stays dry. Above-joist protects more and looks cleaner; below-joist is the only option that can be added to an existing deck.

Can any under-deck drainage system be added to an existing deck?

Below-joist systems can. They are designed to retrofit under the joists of a deck that already has its boards installed. Above-joist systems cannot be added later because the seal is installed between the boards as the deck is built; adding one means removing the existing boards. If you are building new, that is when an above-joist system like AmeriDex is specified.

Do under-deck drainage systems keep the joists dry?

Only above-joist systems do. Because their waterproofing layer sits on top of the joists, the framing never gets wet. Below-joist tray, panel, and membrane systems sit beneath the joists, so they keep the space below dry but leave the joists and beams exposed to water on every storm. AmeriDex is an above-joist system and keeps the joists dry for the life of the deck.

Related reading: Above-joist vs below-joist comparison · Above-joist deck drainage explained · How to protect under your deck from water · Deck rain shield & rain guard options · What to put under a deck for a dry space · How the AmeriDex Dryspace system works