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

When planning a new deck, there are many choices to make, from choosing between wood and composite decking to deciding on deck board colours and the overall deck design. However, one important question that’s often overlooked is what to put under your deck.

When deciding what material to put underneath your new deck, there are a few options, like gravel, mulch, sand, and soil. But which should you go for? In this article, we’ll cover the pros and cons of each under-deck material so that you can make the best decision for your deck project.  


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

Before discussing which is the best overall choice, let’s cover the benefits and issues associated with some of the most common materials to put under a deck. 

Gravel Under Decking

The Pros of Gravel

  • Good for Drainage

Gravel is a great choice if you want a high level of drainage. As gravel doesn’t absorb moisture, any water under the deck will run off instead of soaking into the material below the deck, potentially causing dampness. Less moisture also reduces the chance of mosquitoes around your deck. 

  • Helps with Weed Prevention 

Because plants, including weeds, will thrive when moisture is present, gravel is excellent for weed prevention. If weeds aren’t present below the deck, they won’t be able to grow up through the decking, which can cause damage. A lack of plant life under the deck also helps deter rodents and other small animals. 

  • Doubles as a Storage Space

Gravel is an excellent option if you’re looking to make use of the space under your deck. If you have a high deck, you’ll especially want to think about using the area underneath the decking. 

As the gravel won’t hold moisture, it can provide a relatively dry storage solution for your belongings. However, for even greater protection from water and the elements, you may consider using Dexerdry, a patented above floor joist flange gutter system. Dexerdry creates a virtual dry space under your deck, preventing water from penetrating the deck’s surface.  

  • Not Flammable 

Another benefit of gravel is that it isn’t combustible. Having non-flammable materials near homes or other buildings is highly important and gravel is one of the best under-deck options for meeting this requirement. 

  • Aesthetically Pleasing 

Gravel under a deck is quite aesthetically pleasing as well, giving a clean look. You can also choose from many types of gravel to get the exact look you want. 

The Cons of Gravel

  • It Can Be Costly

Gravel is one of the more expensive options when looking at what you can put under a deck. You can find affordable options, like plain crushed rock, but even this can cost more than other options, like mulch. 

Of course, you can also choose from quite highly-priced gravel options if you’re wanting a specific type of gravel to give a certain aesthetic look. 

Mulch Under Decking

The Pros of Mulch

  • Inexpensive 

Mulch is generally an inexpensive material to opt for under your deck. Therefore, if you’re looking for the cheapest option, mulch will likely be the best choice. 

  • Not Highly Flammable 

While you may think a wood mulch would be flammable, it’s actually not overly combustible. When choosing the least flammable type of wood mulch, opt for coarse materials. Fine wood chips and rubber mulch are the most flammable options when looking at mulch materials. 

mulching garden beds with pine bark pieces
The Cons of Mulch

  • Retains Moisture 

Mulch will retain a lot of moisture, which can lead to mold, mildew, or fungi growth occurring under your deck. Insects and even rodents and other small creatures may be drawn to the area due to moisture as well. Additional moisture could also cause your deck to rot if using wooden decking or decking that is not capped on all four sides.

Adding a fabric layer below the mulch can help reduce this issue, however, not completely. Opting for rubber mulch instead of wood mulch can also reduce moisture retention and associated problems. 

  • Weeds May Grow through the Mulch Layer

The moisture retention capabilities of mulch can attract plants, like weeds, to grow. While weeds themselves may not directly damage your deck, they can potentially cause staining due to the presence of organic matter. It’s important to address weed growth to maintain the aesthetic appeal of your deck. Adding a fabric layer and/or using rubber mulch can be effective in deterring weed growth and minimizing the chances of staining on your deck surface.

  • Needs to be Replaced Regularly     

While inexpensive, mulch decomposes quickly and will, therefore, need to be replaced regularly. This will add to the lifetime cost of your chosen under deck material, and replacing the mulch is time consuming. 

As with the last two issues, rubber mulch will suffer from quick decomposition less so than wood mulch, meaning you’ll need to replace rubber mulch less frequently.   

Sand or Soil Under Decking

Sand and soil offer predominantly the same benefits and similar issues. 

The Pros of Sand or Soil
 
  • Inexpensive

As with mulch, sand and soil are relatively inexpensive materials to put under a deck. They’re also both widely available, so you should have no problems finding a supplier if you opt for sand or soil under your deck.

  • Not Flammable 

If you’re concerned about your under-deck material being flammable, that’s not something to worry about with sand. This material isn’t combustible and can’t burn, so is unlikely to cause any fire hazard. Soil is also not very flammable, in part because it retains so much moisture. 

  • Can Be Used Under Flat Decking

Sand and soil are good options if you’re creating a flat deck, like a decking path, as opposed to a raised deck area. They can be placed underneath the decking boards as a protective area, between the ground and the deck boards. 

The Cons of Sand or Soil

  • Not Visually Appealing

If you can see the material under your deck, sand and soil are not as aesthetically pleasing as other materials, like gravel. Generally, sand or soil will only be considered if the space under the decking cannot be seen. 

  • Attracts Insects and Animals

Insects, rodents, and other small animals may be attracted to materials like soil and sand beneath your deck. While they are generally more attracted to wooden decks, it’s important to note that our composite and PVC decking materials are designed to be resistant to insect damage. With our decking options, you can enjoy a durable and pest-resistant solution that minimizes the risk of damage, providing long-lasting performance and peace of mind.

Additionally, other creatures, like mice, can cause health hazards and could even attract predatory animals, such as snakes, if they manage to nest or burrow in the soil or sand.  

  • Retains Water 

While sand and soil can dry out quickly if they get wet, both will retain a lot of moisture if they continuously get wet, for example, during a period with lots of rainfall. 

This moisture can encourage mold and mildew to grow on your decking and can create an environment for damaging plants like weeds to thrive.  

  • Can Erode Quickly

Sand can erode quickly if it’s open to the elements (for example, when used under a raised deck), meaning you’ll need to replace the sand frequently or at least top it up. This takes time and money. Soil will also erode, although generally not at the same rate as sand.

  • Plants May Grow Through the Material 

Sand, and especially soil, will encourage plant growth. As with mulch, placing a fabric layer beneath these materials can lower the chance of plants, such as weeds, getting through the material and near your decking.   

What is the Best Material to Put Under a Deck?

In general, gravel is the best material to use under a raised deck. While a little more expensive than some other options, it provides the most benefits. 

It’s not flammable, doesn’t retain water, and is less likely to attract plants, insects, and small creatures. Gravel is also aesthetically pleasing and can be purchased in a range of colours and textures. 

Coarse wood mulch can be a good choice if you want to save some money, although it comes with more issues than gravel. Rubber mulch is less prone to issues like water retention, but is more potentially flammable than coarse wood mulch.

Lastly, sand and soil could work if you’re using decking to create a path or other flat surface, but is generally not considered a good choice under any type of raised deck. 

Types of Gravel for Deck Projects

If you’ve decided that gravel is the best material to put under your new deck, you may be wondering which type of gravel to get. The type you choose will most likely be based on your budget and style preferences. The following are some of the most popular options for gravel to put under a deck. 

  • Crushed Stone: this is often crushed limestone. It comes in a naturally neutral colour and is relatively inexpensive. Crushed stone is available in various colours if you’re looking for a certain style, but these unique hues will be more expensive than standard crushed stone. 

This material can also have jagged edges, so may not be the best choice if you’ll be walking over the area (even when wearing footwear).   

  • River Rock: this material has a smoother texture with rounded edges, making it better for an area that will be walked on. It’s also aesthetically pleasing and is used a lot for landscaping. However, it is more expensive than crushed stone.  
  • Pea Gravel: if you want a small version of river rock, opt for pea gravel. This material has a rounded edge and is easy to walk on. Do note, that due to its smaller size, pea gravel retains water a little more than the above two options and won’t be the best choice if you’re placing the gravel on a sloped area. 

In Conclusion

Deciding what to put underneath your deck doesn’t have to be challenging. This article should have helped you to compare the most common materials to put under a deck, allowing you to make the best decision for your project. If you want to learn more about the best decking materials and answer some more frequently asked questions, explore more of the TruNorth website. We have resources to help you through every step of your new decking project. You can also find an array of inspiration for your upcoming deck installation by visiting our Get Inspired page