What to Put Under Deck for Moisture Resistance

- June 14, 2024

Imagine relaxing on your deck, enjoying a serene evening as the sun sets. While the surface of your deck is essential, what lies beneath is equally crucial. Protecting against moisture, weeds, and other unwanted problems ensures your deck’s longevity and safeguards your home’s foundation. Let’s explore what to put under deck for moisture resistance and how different materials can make a difference.

What to Put Under Deck for Moisture Resistance

Moisture barrier options for deck floors include wet/liquid membranes and rolled/sheet membranes, both of which can aid in waterproofing beneath the deck. Most builders favor the more effective rolled waterproofing since it can provide a more uniform moisture barrier surface compared to the less expensive wet membranes.

Mulch, Sand, Soil, Gravel? How To Choose What You Put Under Your New Deck

When planning a new deck, there are numerous decisions to make, from selecting between wood and composite decking to choosing deck board colors and the overall design. However, an often-overlooked but important question is what to place underneath your deck.

There are several options for under-deck materials, such as gravel, mulch, sand, and soil. But which one is the best choice for you? In this blog, we’ll explore the pros and cons of each material to help you make an informed decision for your deck project.

Read more: How to Build a Four Season Room on a Deck

What Are the Pros and Cons of Different Under Deck Materials?

• Gravel Under Decking

The Pros of Gravel Under Decking

Excellent for Drainage: Gravel is a top choice for ensuring high drainage levels. Since gravel doesn’t absorb moisture, water beneath the deck flows away instead of soaking into the material, reducing the risk of dampness. Less moisture also means fewer mosquitoes around your deck.

Weed Prevention Benefits: Gravel is highly effective in preventing weeds. Plants, including weeds, thrive in moist environments. With gravel, the lack of moisture inhibits weed growth under the deck, preventing them from damaging the decking. This also helps deter rodents and other small animals from taking up residence beneath your deck.

Doubles as Storage Space: If you’re looking to utilize the space under your deck, gravel is an excellent option. Especially with a high deck, the area underneath can serve as a useful storage space. Gravel’s inability to hold moisture provides a relatively dry storage solution. For even better protection, consider using Dexerdry, a patented above-floor joist flange gutter system that creates a virtually dry space under your deck by preventing water penetration.

Non-Flammable: Gravel is also non-combustible, making it a safe choice for areas near homes or other buildings. Using non-flammable materials is crucial for safety, and gravel meets this requirement effectively.

Visually Appealing: Gravel under a deck offers an aesthetically pleasing, clean look. With various types of gravel available, you can choose the perfect style to complement your deck’s appearance.

The Cons of Gravel Under Decking

It Can Be Costly: Gravel tends to be one of the pricier choices for deck underlayment. While you can find budget-friendly options such as plain crushed rock, these still often cost more than alternatives like mulch.

Also, if you’re aiming for a particular aesthetic, you might opt for premium gravel types, which can be even more expensive.
Mulch Under Decking

The Pros of Mulch Under Decking

Inexpensive Option: Mulch is usually a budget-friendly material for use under your deck. If cost is your primary concern, mulch is likely your best choice.

Low Flammability: Contrary to popular belief, wood mulch is not highly flammable. For the least flammable wood mulch, select coarse materials. Fine wood chips and rubber mulch are among the most flammable options available.

The Cons of Mulch Under Decking

Retains Moisture: Mulch tends to hold a significant amount of moisture, which can result in mold, mildew, or fungi developing under your deck. This damp environment may also attract insects, rodents, and other small animals. Excessive moisture can cause wooden decking or non-capped decking to rot.
Placing a fabric layer beneath the mulch can mitigate this issue somewhat, though not entirely. Using rubber mulch instead of wood mulch can also help reduce moisture retention and the associated problems.

Weeds May Grow Through the Mulch Layer: The moisture-retaining properties of mulch can encourage the growth of plants, like weeds. While weeds might not directly harm your deck, they can cause staining due to their organic matter. To maintain the visual appeal of your deck, it’s important to manage weed growth. Adding a fabric layer and/or using rubber mulch can effectively deter weeds and minimize the risk of staining on your deck surface.

Needs to Be Replaced Regularly: Although mulch is inexpensive, it decomposes quickly and will need to be replaced regularly. This increases the lifetime cost of your chosen under-deck material and the replacement process is time-consuming.
Rubber mulch, in contrast, decomposes much slower than wood mulch, reducing the frequency of replacements needed.
Sand or Soil Under Decking
Sand and soil offer predominantly the same benefits and similar issues.

The Pros of Sand or Soil Under Decking

Affordable: Like mulch, sand and soil are cost-effective materials to use under a deck. They are readily available, so finding a supplier for sand or soil should be easy if you choose these options.

Non-Flammable: If you’re worried about your under-deck material being flammable, sand is a safe choice. It’s non-combustible and won’t burn, eliminating any fire hazard. Similarly, soil is also not very flammable due to its high moisture content.
Suitable for Flat Decking: Sand and soil work well for flat decks, such as decking paths, rather than raised deck areas. They can be placed beneath the decking boards, providing a protective layer between the ground and the deck boards.

The Cons of Sand or Soil Under Decking

Lack of Visual Appeal: If the area beneath your deck is visible, using sand or soil may not be as visually pleasing as other materials like gravel. Typically, sand or soil is chosen only when the space under the deck is not visible.

Attraction to Insects and Animals: Sand and soil can attract insects, rodents, and small animals. While these pests are more commonly drawn to wooden decks, our composite and PVC decking materials are designed to resist insect damage. With our residential decking options, you get a durable, pest-resistant solution that reduces the risk of damage, ensuring long-lasting performance and peace of mind.
Moreover, certain creatures like mice can pose health risks and may attract predatory animals such as snakes if they nest or burrow in the soil or sand.

Water Retention: Although sand and soil can dry out quickly, they retain moisture if exposed to continuous wetness, such as during heavy rainfall. This moisture can promote mold, mildew, and weed growth on your decking.

Erosion Concerns: Sand can erode rapidly when exposed to the elements, requiring frequent replacement or topping up, which can be time-consuming and costly. Soil also erodes, although typically not as quickly as sand.

Plant Growth: Sand and soil encourage plant growth, similar to mulch. Placing a fabric layer beneath these materials can reduce the chances of weeds and other plants growing through and near your decking.

What is the Best Material to Put Under a Deck?

Gravel is typically the top choice for the area beneath a raised deck due to its numerous advantages. While it may cost a bit more than alternatives, it offers superior benefits. It’s non-flammable, doesn’t retain water, and is less likely to attract plants, insects, or small animals. Gravel also comes in various colors and textures, enhancing its aesthetic appeal.

If budget is a concern, coarse wood mulch can be a decent alternative, although it has more drawbacks compared to gravel. Rubber mulch, while less prone to water retention issues, carries a higher risk of flammability than wood mulch.

Sand and soil are viable options if the deck is used for pathways or flat surfaces, but they’re generally not recommended for use under raised decks due to various limitations.

• Put a Drainage System Under the Deck

When uncertain about what to place under a deck to handle heavy moisture, opting for a drainage system proves beneficial. Installing such a system post-deck construction can be challenging due to limited space. It’s advisable to integrate the drainage system during deck preparation, if feasible.

To prep the ground beneath the deck for the drainage system, installing troughs to channel water away to another part of the property is necessary. A French drain might be needed to divert water from under the deck to a location farther from the home’s foundation.

One perk of using a drainage system is the additional space it creates for storing kids’ toys or lawn equipment. With the drainage system in place, concerns about these items being in water puddles are alleviated.

• Put Landscape Fabric Under the Deck

Landscape fabric serves as a lightweight yet effective moisture barrier beneath a deck, particularly in areas where standing water isn’t common. It stabilizes soil, preventing weed growth and maintaining the area’s aesthetic appeal.

For optimal performance, cover the landscape fabric with a layer of gravel or rocks. Without this covering, dirt may accumulate on the fabric’s surface, potentially leading to weed growth. Additionally, rocks improve the visual aspect compared to using fabric alone.

During installation, consider using treated wood landscape timbers as a perimeter barrier. This allows you to attach the fabric securely to the wood, preventing displacement. The wood barrier also helps keep the gravel in place, minimizing debris on your lawn.

For the fabric itself, opt for woven stabilization fabric. This type promotes gradual water seepage into the soil below, ensuring proper drainage. In contrast, plastic landscaping material can hinder water seepage, leading to water accumulation under the deck or runoff into the yard.

• Put Concrete Under the Deck

While it can be costly, there are individuals who opt for a concrete slab beneath their raised deck. This choice is advantageous because it prevents erosion even if the deck allows substantial rainwater to pass through to the ground below. Properly graded concrete ensures efficient water flow away from the home’s foundation and into the yard. Moreover, if you plan to install an outdoor kitchen beneath the deck, a concrete slab provides a solid foundation for your kitchen equipment.

What Do You Put Under Raised Decks?

Before you start building your new deck, remember to consider what lies underneath it. Neglecting this area could lead to a messy, neglected space right outside your door. You’ve likely seen how things can turn out when you don’t plan ahead—mud, weeds, and an unsightly mess.

Instead of just using the space under your deck for storage or leaving it unused, think about turning it into something attractive and useful. With a bit of creativity and careful planning, you can transform this area into a beautiful and valuable part of your home.

How do I prepare the ground under the deck?

Before placing any materials under the deck to control moisture, it’s essential to prepare the ground properly. Level the soil beneath the deck to ensure it slopes away from the house. A slope of at least six inches for every 10 feet is recommended. Avoid creating gaps between the soil and the foundation to prevent water from accumulating. After leveling the ground under the deck, you can proceed to add the desired materials on top.

Final Words

As you embark on your deck project, remember that what lies beneath is just as important as what’s above. Choosing the right material for moisture protection ensures not just the longevity of your deck but also the integrity of your home’s foundation. Consider factors like drainage, weed prevention, and visual appeal when deciding between gravel, mulch, sand, soil, or even a drainage system, landscape fabric, or concrete. And don’t forget about deck waterproofing to add an extra layer of protection against moisture damage. With careful planning and the right materials, you can create a beautiful and functional outdoor space that stands the test of time. Happy decking!

Frequently Asked Question

1. What can I put under my deck to keep it dry?

Moisture barrier options for deck floors include wet/liquid and rolled/sheet membranes, which can bothh can help waterproof under the deck. Most builders prefer the more effective rolled waterproofing because they can create a more even moisture barrier surface than less costly wet membranes.

2. How to reduce moisture under a deck?

Make sure your deck is equipped with effective drainage systems to divert water away from the structure. This can involve fitting gutters, downspouts, or under-deck drainage systems. Adequate drainage helps avoid water accumulation or infiltration into the deck, minimizing the chance of mold development.

3. What is the best material to put under a deck?

Gravel stands out as the optimal choice for the space under a raised deck. Despite being slightly pricier compared to alternative materials, it offers numerous advantages. It is non-flammable, does not hold moisture, and is less prone to attracting vegetation, bugs, or small animals.

4. Should I put a vapor barrier under my deck?

Protecting your deck against moisture penetration and buildup can prevent structural damage from mold and corrosion, reducing the need for frequent maintenance and repairs. Installing a moisture barrier for your deck is essential to preserving the value of your home or building.

5. Should I hire someone to prepare the ground under the deck?

While this project is suitable for DIY enthusiasts, many opt to enlist a contractor for grading the space beneath the deck and installing suitable materials. Hiring a local deck builder is a common choice for this task, as they can offer guidance on the ideal materials for under-deck installation.

6. How do I get rid of moisture under my elevated deck?

For an elevated deck, consider adding a deck drainage system. Designed to capture and divert water, these systems protect a deck’s substructure from moisture while creating dry space beneath the deck usable for storage or additional living area.

7. What should I put under the deck to prevent weeds?

To prevent weeds under a deck, consider using landscaping fabric or plastic sheeting as a barrier, followed by a layer of gravel or coarse wood mulch. This helps inhibit weed growth and maintains a tidy appearance.