Should You Put Gravel Under a Deck? Here’s the Answer

A deck is a great way to increase your outdoor space and have a level surface upon which to sit and recline. Ensuring that it will last, though, begins with the ground underneath it. Should you put mulch, sand, soil, plastic, or gravel under deck structures?

Companies that install decks recommend using washed gravel or crushed stone under the deck. The clean stone allows precipitation to drain, prevents plant growth, and moisture away from the wood members. It protects the ground from erosion and the wood from rot.

In this article, we’ll explain what is the best rock for under deck structures, how to prepare the ground, calculate the amount of stone, and how to place and spread it for optimum drainage. By the end of the read, you have a better understanding of why gravel should be under your deck.

Why Should You Put Gravel Under a Deck?

Constructing a deck of any size usually begins with the footings and ground preparation. Preparing the ground before building your deck is much easier and less back bending than doing it after construction. There are many benefits to having gravel under your deck.

Improve Drainage

Grading the ground away from buildings will help prevent water from pooling. Gravel doesn’t absorb moisture, so water tends to follow the slope away from the deck and nearby structures. Water collecting under the deck creates smelly dampness, attracts insects like mosquitoes, and also accelerates wood rot, shortening the life of your investment.

Reduce Runoff Erosion

Gravel doesn’t suck-up moisture but slows water movement and minimizes runoff erosion. Other mediums under a deck can retain moisture, become saturated, and increase the possibility of erosion.

Prevent Weeds

Plants prefer moisture and soil to grow. Placing landscape fabric, garden cloth, or plastic on the ground with a good layer of gravel reduces the likelihood of weeds growing up between the deck boards. It also makes it more difficult for rodents and other critters to make a home or latrine under the deck.

Protect Footings

Placing and compacting gravel under a deck can decrease moisture damage to wooden posts by moving water away, preventing rot. Gravel under deck footings also keeps water from pooling under them and causing damage to deck support.

Visually Attractive

A mud-hole or weed bed under a deck is not very attractive. A layer of gravel hides ground cloth or plastic, dirt, prevents weeds, and provides a clean and uniform look. Using colored gravel or different sized aggregate can further improve aesthetics.

Better Storage Space

Gravel under a deck can also turn a muddy area into a great storage space, especially if the deck is elevated. The gravel helps keep the area dry so that items won’t rot or rust. Seasonal items, ladders, children’s toys, even firewood, can be stored there instead of elsewhere.

What Is the Best Material to Put Under a Deck?

Decks that allow precipitation to drip through between boards can collect moisture underneath, which can cause rot. Different materials can be used under decks; however, the amount of precipitation should be taken into account when making a choice. A concrete pad, bricks, or pavers, are expensive options, which leaves soil, sand, mulch, plastic, or gravel as affordable choices.

Soil

Exposed soil may absorb moisture, or harden and be susceptible to erosion. Animals and weeds also enjoy the shade and use it as a lavatory or a place to nest or burrow.

Sand

Sand is similar to soil in that it absorbs moisture and can erode easily. It is also easy for insects, rodents, and weeds to take up residence or put down roots.

Mulch

Wood chips, sawdust, or mulch under a deck in a dry climate may work, although insects, reptiles, and rodents also enjoy it. In damp or wet climates, mulch may have difficulty drying out, which can lead to mold, mildew, and fungi growth. The moisture can also accelerate wood rot in the deck.

Plastic

Plastic and landscape cloth are options that can be used alone, or with another ground cover. They minimize erosion, prevent moisture collection, and may keep critters and weeds out. Unfortunately, they look and work better with another material holding it in place. Soil, sand, and mulch have the same issues when used over plastic or weed membrane as they do without it. Plastic has a shorter lifespan than garden cloth. Plastic exposed to the elements will crack and become brittle. If it is buried under mulch or gravel, it may last up to 10 years.

Gravel

A layer of crushed gravel, with or without plastic or weed barrier cloth, is the best choice for using under a deck. It sheds moisture instead of absorbing it, keeps the area under the deck drier, and won’t decay. The stone looks good but isn’t as inviting to critters or insects, nor is it easy for plants to put down roots. It also provides a clean area to store yard items, toys, and even tools.

What Kind of Rock Goes Under Deck?

Selecting the type of aggregate to put under a deck depends on your location, how visible the stone will be, and the budget. Different colors are available, with some being local in origin, and others being imported. Crushed stone is the least expensive. Leaves and organic debris should be cleared off the gravel seasonally to once a year using a leaf blower to prevent moisture, insects, and seeds from finding a home.

Crushed Stone

There are different grades of crushed gravel available, with 3/4″ to 1” being the best gravel sizes. However, it is important to specify clean or washed gravel. Gravel dust and sand or dirt can collect and prevent moisture from percolating through and away from the deck. Crushed stone has rough edges that may catch leaves and other debris, which should be removed to prevent organic debris from piling up and causing rot.

River Rock

River rock has smooth rounded surfaces and commonly ranges between 1/2″ to 8” but can be ordered in larger diameters. The rounded edges are less likely to catch debris and, depending on their size, can be easier to walk or crawl over. The two most common sizes of smooth stone for use under a deck are 1/2×7/8” and 5/8×1-1/8”, although some people prefer larger stone.

Pea Gravel

Pea gravel is pea size, between 1/8” and 3/8” and is smooth-edged making it easier to walk and crawl on. It also doesn’t catch leaves or debris easily. Although the pea sized stones allow moisture and precipitation to percolate through, they do retain moisture longer than the larger aggregates.

Egg Rock

Egg rock is an egg-shaped river rock or machine washed stone between 1” and 2” in size. The elongated rocks are smooth surfaced and less likely to collect leaves or debris. The non-uniform shape, though, is less forgiving when crawling over them.

How Much Gravel Do You Put Under a Deck?

The depth of crushed gravel placed on the weed membrane cloth or plastic under your deck needs to be thick enough to cover and hold the barrier in place. It should also be thick enough to prevent weeds from taking root and keep water from displacing it. A thin layer is also easily displaced by critters or people moving around or placing items underneath the deck – if it is elevated.

An inch of gravel may hold the garden cloth down but won’t prevent seeds from taking root. Using 3 to 4 inches of clean or washed 3/4″ gravel under deck; it is the best rock for under deck use. The deeper stone provides a deep enough layer to allow dust to settle through but keep seeds from rooting. The few that do take root should be easy to remove as there is no soil base.

To calculate the amount of crushed stone, multiply the length of the deck by the width, and then multiply by the desired thickness. This calculation provides the cubic footage of gravel. Dividing that total by 27 (the number of cubic feet in a cubic yard) will give you the cubic yardage required.

Using 4” (0.33 of a foot) of gravel under a 10-foot by 12-foot deck for example:

10’ x 12’ = 120sqft x 0.33 = 39.6 cubic feet ÷ 27 = 1.467 or 1.5 cubic yards of gravel.

The total weight is about 2 tons, so shop around for the best deal.

How to Prepare Ground Under Deck

Preparing the ground under a deck before it is built is much easier than afterward. Ensure the ground slopes away from buildings. Any soil or dirt added to improve the slope should be well tamped, so it doesn’t settle and change the direction of water flow later on. Rake it out, so the grade is smooth and even.

Mound and tamp around support posts to prevent water pooling and damage. Roll out the landscape fabric. The barrier should go up adjoining walls 1-inch higher than the planned gravel depth. Use some gravel or rocks to hold it in place.

How to Install Gravel Under Deck

With the landscape material down, move the gravel into place. Use a 2×4 or several 4-inch rocks to help get the desired thickness of gravel. A wheelbarrow, garden rake or hoe, shovel, and level are the usual tools unless you have access to machinery.

If using a wheelbarrow, place a board under the tongue or wheel guard when tipping to protect the fabric and not create an indent for water to pool in. Once most of the gravel is dumped into place, use a shovel and rake to level and spread it around. Move the thickness guides (rocks or 2×4) to ensure consistent thickness. The surface of the gravel should follow the lay of the ground.

Spread the gravel beyond the edges of the weed barrier to prevent it from lifting, and to make it more aesthetically appealing. A border or garden may also be used to protect the edge or improve the look.

4 years later:

Keep the gravel under the deck free of leaves and other organic material. Using a leaf blower or leaf rake when necessary may help prevent weeds and moisture damage.

Conclusion

Preventing weeds, insects, and wood rot are all good reasons to prevent water from pooling under your deck. Sand, soil, and mulch are options; stone is better. The best gravel for use under a deck is crushed stone. It prevents erosion, weeds, keeps moisture away from the underside of the deck, and looks much better than mud or weeds.

Hopefully, you have a better understanding of why you should put gravel under your deck and why it is the better choice. If you found the article interesting or helpful, please share it with others. Your suggestions and comments are always appreciated.