Under Deck Drainage System for Existing Decks: Easy Guide

Hello guys, welcome back to my blog. Today I am going to talk about under deck drainage system for existing decks, because so many people have a deck with wasted, wet space underneath and don’t know what to do about it. If you already have a deck and you are tired of that muddy patch below it every time it rains, this blog is for you.

If you ask me, adding a drainage system under an old deck is one of the best upgrades you can do. You basically get a whole new dry room for free, and you also protect your deck’s frame from rot. I went through a lot of real talk from homeowners and builders on Reddit to bring you honest info, not just sales talk.

Quick Facts Table

Let me give you a quick table first, because I always like tables. It is easy to understand everything at one glance.

Point	Above-Joist System	Below-Joist System
Deck boards removal	Yes, needed	No, not needed
Best for	New or existing decks with time for full job	Quick retrofit on existing decks
Moisture risk	Very low, water stopped at top	Higher if airflow is poor
Cost per sq ft	Around $6 to $8 material, more with labor	Usually cheaper, more DIY friendly
Good for	Long term protection of joists	Fast fix without big renovation

Why Your Old Deck Needs This

An under deck drainage system for existing decks basically catches rain water as it falls through the deck boards, and sends it away through a gutter, instead of just dripping on your head and turning the ground into mud. This also stops water from sitting on your wood joists again and again.

Builder of a few decades here, but have never used an under deck drainage system in my entire career, one Redditor said before finally deciding to add one to his own home over a walkout deck, and honestly this shows even experienced builders often skip it until they really need it, r/Decks. I think a lot of people just don’t know this option exist until they see it somewhere.

Above-Joist Systems: What Reddit Says

Above-joist systems sit on top of the joists, under the deck boards. Water hits this layer first and never touches the wood frame at all.

Whatever you do, make it an above the joist system, one builder wrote, adding that below the joist just traps moisture and causes rot, r/Decks. I think this is honest advice from someone who actually builds decks for living, so it is worth listening to.

Another person explained the real reason why this matters so much, saying the big benefit of these systems vs an under mounted one is that it protects the framing, r/Decks. If you plan to use the space under your deck as a patio or storage, this extra protection is worth the effort in my opinion.

If you like doing things yourself, one Redditor shared a smart trick, saying Dr Decks makes his own with EPDM pond liner and it seems like the best way to do it because you can customize it, r/Decks. This is a good option if you want to save some money and don’t mind a weekend project.

Below-Joist Systems: Easier for Existing Decks

Now if removing your whole deck floor sounds like too much work, below-joist systems are made exactly for this situation.

There are plenty of systems designed to be installed under a deck with the deck boards already installed, one Redditor pointed out, and another added that Underdeck Umbrella is a between joist system where you don’t have to remove the deck boards to install it, calling it a simple DIY that can save a ton of money, r/Decks. This is honestly a big deal for anyone who don’t want to touch their existing deck boards.

For a cheaper DIY route, some people just build their own version. You can DIY a system using metal or plastic roofing panels but it’s still going to be a bit of a pain to build and clean, one Redditor said about using corrugated panels screwed under the joists, r/Decks. So it saves money, but you have to be ready to put in the work.

But it is not all easy, below-joist systems also have a downside. The type that mount under the joists seem like they would trap moisture if or when the gutters get clogged up, someone warned on Reddit, r/Decks. This is why picking the right system and keeping it clean really matters, more on that below.

How Installation Works on an Existing Deck

For an under deck drainage system for existing decks, the install process depends on which type you pick. According to Trex RainEscape’s own guide for retrofit jobs, undermount panels attach right under the joists and don’t need any deck board removal, which makes them a lot more DIY friendly for older decks. Overmount systems need the deck boards, railings, and any pergola removed first, but once that is done, two people can finish around 70 square feet in just one hour.

I think this is good news for homeowners, because it means you don’t always need a full deck tear-down to get a working drainage system.

Cost Table

Cost is always a big question, so here is a simple table based on data from Angi’s cost guide on under-deck drainage.

Deck Size	Square Footage	Estimated Cost Range
12×12	144 sq ft	$3,030 – $6,200
12×20	240 sq ft	$5,040 – $10,320
16×20	320 sq ft	$6,720 – $13,760
20×20	400 sq ft	$8,400 – $17,200

Materials alone for kits like Trex RainEscape run around $6 to $8 per square foot, but labor can add a lot on top of that, specially for above-joist jobs. If you go the DIY route with a below-joist kit, you can save a good chunk of this labor cost.

Don’t Forget Maintenance

Whatever system you pick, you cannot just install it and forget about it forever.

I would highly advise figuring out a system to enable it to come off relatively easy for cleaning, otherwise it will fail with dirt and grime within a year or so, one Redditor warned about below-joist panels, r/Decks. I think this one tip alone can save you a lot of headache later.

Airflow also matters a lot, specially for below-joist systems. If you have proper water flow, and leave some space for airflow, and your joists are pressure treated you should be fine, someone explained on r/HomeImprovement. So basically, don’t seal everything up too tight, your deck frame also needs to breathe a little.

For bigger or more complex above-joist installs, hiring a certified installer is also a smart move. If I were to choose I would go with the Trex Rain Escape system, and I would also have a certified installer install it, one experienced Redditor shared, r/Decks. If your budget allows it, I agree this is the safer route for long term protection.

FAQ on Under Deck Drainage Systems

Q1: What is the best under deck drainage system for existing decks? Above-joist systems give the best protection, but below-joist or between-joist kits like Underdeck Umbrella are easier and cheaper for a deck you already have.

Q2: Do I need to remove my deck boards to add drainage? Only for above-joist systems. Below-joist and undermount systems attach from below and don’t need board removal.

Q3: How much does an under deck drainage system cost? Based on real data, most homeowners pay between $3,000 and $17,000 depending on deck size, material and labor.

Q4: Can I install an under deck drainage system myself? Yes, specially below-joist and undermount kits, these are made to be DIY friendly. Above-joist systems are more complex and I think hiring a pro is smarter there.

Q5: Will an under deck drainage system trap moisture and cause rot? It can, if airflow is bad or the gutters get clogged. Keep it clean and make sure there is proper airflow so this don’t happen.

Final Words

So guys, this was my simple guide on under deck drainage system for existing decks. I have tried to cover above-joist and below-joist options, real cost, installation and maintenance tips, using honest opinions from Reddit builders and homeowners. None of this is very hard to understand, you just have to pick the system that fits your deck and budget. Keep on visiting my blog for more simple home improvement tips like this.

Under Deck Drainage System for Existing Decks: My Simple Guide