Underneath your deck is an often-overlooked part of the deck, but what is you put under a deck can affect the longevity of the deck and decking. For it creates the environment in which the joists exist along with the underside of the decking.
On top of that, what grows and lives under the deck affects the look of the deck. If the deck is overgrown by weeds, it doesn’t matter its design. The deck will be ugly and unusable.
Which raises the question, should you put plastic under the deck to prevent weeds from growing under the deck?
No, putting plastic under a deck is not good for the deck. Plastic under the deck will trap moisture, increase rot and attract insects to your deck. Landscape fabric is a better alternative for stopping weeds from growing under the deck while allowing water to drain.
That’s the short answer. Never put plastic under a deck except if its part of a dry deck system drainage system. In which the liner is installed to collect water and divert it away from the deck. But that’s a different conversation about creating a dry space under the deck.
Let’s focus on three areas off ground cover under lower decks in regards to plastic.
- Why plastic under a deck is not good for the deck.
- A better alternative to plastic under a deck.
- Best practise in installing ground-covering under a deck.
3 Reasons Why Plastic Under a Deck is Bad.
Drainage! The single biggest reason why plastic under a deck is not good, is drainage. Plastic is not porous and will trap water under the deck.
This trapped water under the deck has several adverse effects.
Fungi needs moisture to thrive. With the additional water under the deck, increasing the speed in which the wood will rot. Shortening the life of your deck.
Wood needs to have a moisture content of less than 19%, or it will rot. That is why all lumber is dried to that level before leaving the mill.
But plastic under the deck will trap water, raising the moisture level. Increase wood rot under the deck.
#2, Stagnant Water Stinks
Water is good. Rain and ruining water over your deck will not damage the deck. It’s natural and fine. It’s the pooling stagnant water that’s the problem.
Not only does trapped water under the deck increase moisture, accelerating rot.
Stagnant water is a breeding grown for bacteria and fungi.
Remember that picture in your Junior High science textbook, with that creature climbing out of the cesspool explaining the origin of life in evolution. No, that is not going to happen under your deck with plastic. But trapped stale water is an ideal environment for bacteria to breed and grow.
Worse case, this bacteria transfers over to the wood of your deck, destroying it.
Meanwhile, stagnant water stinks. As the bacteria break down, whatever is sitting in the trapped water.
Making your beautiful outdoor oasis smell like a swamp. Beautiful in nature. Not so much in your backyard.
#3, Stagnant Water Increase Annoying Insects
Mosquitoes the most annoying of all insects, buzzing around your head and biting you.
Most mosquitoes only travel a few hundred feet from where they hatch. Meaning, most of the mosquitoes buzzing around your deck came from your yard. Those blood-sucking pests, were born and raise at your home.
But mosquito’s larva need water to live, right?
Your yard is not a swamp. Where are the mosquitoes coming from?
Under your deck, from the water trapped on the plastic.
Mosquito young need about two weeks to develop in water. Water sitting on plastic under the deck for two weeks hatches a new patch of annoying mosquitoes.
It doesn’t seem long, but shaded under your deck. The plastic preventing the water from draining provides enough time. And voila, you have a mosquito nursery.
Drain the water. Fewer mosquitoes on the deck.
There is a simple solution to deck rot, smells and insects. Stop water pooling with landscape fabric.
Landscape Fabric is Better than Plastic Under a Deck
The difference between landscape fabric and plastic is drainage. Landscape fabric is porous, allowing water to drain. Drain the water from under the deck. Problems gone.
But let’s not forget the reason for putting plastic under the deck, weed control. Weeds stinking up through the decking is ugly. And pulling weeds out from between deck boards is next to impossible. You try to pull them. You only get the top few inches, and tomorrow they are back again.
Landscape fabric prevents and kills weeds under a deck in three ways.
Weeds need three things sunlight, nutrients from the soil and water. Remove any one of these three, and the weeds die.
#1. Weeds need sunlight to grow. Remove the sunlight. The weeds die. It’s that simple. Landscape fabric kills weeds by preventing the weeds from getting sunlight to grow.
By covering the ground with fabric, you have created a dark space where weeds cannot grow. Even if the light gets under the deck, and it should, the weeds in the ground under fabric cannot get light.
#2. Landscape fabric prevents weed seeds from rooting in the ground, preventing the seeds from getting nutrients from the soil.
So even with fresh seeds blowing under the deck, the seeds will quickly die as they cannot take root.
It is especially important here that there is no organic material on top of the landscape fabric. If the weed seeds can sprout on top of the fabric, the weed’s root will puncture through the fabric. But more on that later.
#3. Landscape fabric doesn’t pool water. The advantage fabric has over poly. The water drains preventing all those problems for your deck.
But water, not pooling, is also helpful for weed control.
If you love gardening, which I do. You know that often it is helpful to help the seeds germinate by soaking them in water a few days before planting. Giving the seeds a little boost, helping them to germinate. Increasing the number of seeds sprout and grow.
The same can happen under your deck. A little pooling water, collecting weed seeds can increasing germination. Then it’s just a numbers game. If a hundred weed seeds germination instead of five. How many more weeds will be poking through the deck?
Let the water drain. You will have less weed gemination, less smell, fewer mosquitoes, and less rot. Do I need to continue?
Landscape fabrics porous design is better than plastic under the deck as it drains the water eliminating the many problems that pooling water brings.
This is one thing that I appreciate about ECOgardener Weed Barrier it is designed to let water through. Which not all landscape fabric does. Some landscape fabric is non-woven, trapping water the same as plastic. The very thing we don’t want to do under a deck.
I also like the handy size, either 3’ or 4’ wide. This allows easier one person installation with more control when the wind gusts. Also, less cutting when you come up to a low beam or concrete pile.
Now you are convinced, how should the fabric be installed?
How to Lay Landscape Fabric Under a Deck
1. Roll out fabric under the deck area.
Ideally, this is done before deck construction, as it is much easier. My standard practice is to dig and pour all the deck footing. Rake out the extra dirt making level ground for the landscape fabric.
If the deck is really low with little room between the ground and the underside of the beam, it is a good idea to place the deck beams first encase you need to dig a trench for the beam. Way easier before the landscape fabric is in place.
I explain more about low deck beams in another article. Click here if you want to install a low deck beam better.
2. Make sure seams are overlapped
Overlapping the fabric is essential to preventing weeds. One inch is the minimum. I like between three and eight inches (75-200mm). Not overlapping the fabric creates the potential of growing your weeds in nice neat rows under the deck. Cute but not good.
Only full coverage stops weeds. In landscaping beds, gardeners will often cover the ground with landscape fabric to minimize weeds but cut a hole in the fabric allowing the plant to grow. Holes in the fabric can let weeds to do the same.
3. Pin the landscape fabric to the ground to prevent movement
Pinning fabric under the deck is essential if you are not covering the fabric. As without pins, the fabric will blow around. Becoming meaningless as the ground is no longer covered.
Hoople spikes are 6” long with sharp points making pushing through fabric easier. They are also rust resistant ensuring that they will continue to hold down the fabric for years to come. To order pins to hold down your fabric click here.
If you are going to cover the fabric under the deck, pinning still helps to keep it in place as you work. The pins holding the fabric down as you place the gravel and rake it out.
4. Cover with non-organic material
Non-organic material is critical, as I alluded to earlier. Fabric can hold down the plant and block the sun, but roots are a different story. God designed the plant’s roots to burrow through the soil. They can snake around rocks to find the water and even break concrete as the snake through a crack. Once in, there is no stopping them.
Fabric is no match for weed roots.
I have had to repair the unfortunate mistake of landscape fabric covered with soil growing weeds. The fabric stops plants, not roots. If the weeds can take root above the fabric, they will puncture the fabric and continue to grow.
Therefore, the fabric needs to be covered with non-organic material. Often this is washed rock or gravel.
Washed is vital as road crush that includes sand, and small particles can hold just enough water to allow the seeds to germinate and take root.
If you want something different then washed gravel, there are alternatives. In another article, I discuss nine additional under deck ground cover material. If you want to learn more, click here.
Recently I have even used old sidewalk block laid on the fabric to hold it down under the deck. Not beautiful, but it did keep the fabric in place under the deck behind the skirting.
In Conclusion of Should You Use Plastic Under a Deck
Putting plastic under your deck is wrong because it prevents water from draining. The trapped water on the plastic encourages deck rot, creates a foul odour on the deck and provides a breeding ground for annoying insects on your deck.
Using landscape fabric allowing water to drain under the deck. Removing the problems that plastic under the deck creates. Making your deck more enjoyable and lasting longer.
While still stopping weeds from growing under the deck. The whole reason for covering the ground under the deck. Weed Control.
The good news is landscape fabric costs about the same as heavy-duty plastic but is better for under your deck.