How to Build an Underground Room in Your Backyard

Whether you’re living out a childhood dream of having a secret fort or want to be prepared in case of an emergency, learning how to build an underground room in your backyard is an exciting project. Although not all backyards are ideal for digging an underground room, if you have the right conditions and follow the steps below, you can create a neat and safe space under your yard! 

Step One: Prepare and Plan

Before you choose where to build an underground room in your backyard, you’ll need to make sure that you consider a few factors.

First and foremost, make sure that you’re choosing a spot that is legally in your property and that you have permission to build an underground room there.

If you have any doubts, call your local government in order to check for utility lines where you’ll be digging. You’ll want to be 100% positive that where you plan to dig is away from any gas, sewer, or electrical lines. 

For more information on digging in your backyard, read this article.

Check for Debris

In addition to the advice above, you also want to look for debris on the surface. Any areas with lots of rocks and tree roots are going to make digging a very difficult task.

Avoid any areas that are muddy or where water tends to pile up, as this could cause flooding later on. Unfortunately, if you live in a flood zone, you should avoid creating an underground room altogether to be safe.

Test out different areas by digging up a small amount of dirt to see what’s underneath. Ideally, the space you choose should have good drainage, be away from anything sharp, and be mainly made up of the earth. 

Determine How Big You Want Your Room to Be

Are you looking to create a simpler fort for emergencies only? Or maybe you want a space with a little more wiggle room. 

To build a simple underground room in your backyard, starting with a 3×3 foot pit is a good rule of thumb.

If you’re looking to create a room with a little more space, aim for a 5×5 foot pit.

While you may want it large enough to walk around in, digging more than 6 feet down can be dangerous. The deeper you dig, the more risk there is for the room to collapse, which could harm you or others inside. 

To further avoid walls from collapsing, make sure not to dig deeper than you dig across. Keep the ratio equal at a minimum.

Map it Out

Once you have an idea of how big you want your underground room to be, start by mapping it out. This will allow you to visualize the final product as well as catch any potential structural issues. 

Make your map once you’ve picked a location so that you have a good idea of any areas that you’ll need to work around, such as tree roots. 

Use pegs or markers to map out the exact dimensions you’ll be using.

Sit inside the “box” to see how it feels before you start to dig. This will give you an idea of how much arm and legroom you’ll have once the project is finished.

Step Two: Start Digging

Be Safe

Even if you take all of the proper precautions, there is still a chance of an emergency. Though we hope you’ll be able to go through the process seamlessly, it’s better to be safe than sorry. 

Prior to digging, make sure always to tell someone where you are. Even once you have the room dug up, someone should always know where to find you.

If you have any friends or family that are able to help you with this task, even better! Not only will it be safer, but you’ll be able to get through the digging process more quickly. 

Begin to Dig

Now that you have an idea of what your room will look like and feel like, you can get started digging. Begin by using the shovel to skim the top of the soil within the dimensions you’ve chosen.

Double-check your measurements, and then go ahead and start digging out your room. Dig evenly the best you can, and check the dimensions throughout the process to ensure you don’t dig too far off of your original plan. 

The digging process may take a couple of days, especially if you’re doing this project alone. Make sure to make preparations as necessary with this in mind. 

In order to protect your work, make sure to cover the hole with a tarp overnight. Use rocks or stakes to hold the tarp in place and give you peace of mind that it will stay untouched.

If there’s a chance someone could walk near the pit you’ve dug, it’s a good idea to add something to warn them. For example, add small flags in the four corners or use the dirt you’ve dug out to create small “walls” around the area. 

Before finishing for the day, make sure you have a way to enter and exit for the next day safely. 

Slope the Walls

In order to keep your room safe from collapsing, you’ll want to shape the walls so that they are a bit wider and more open than the floor.

Skim off some dirt around the top of the room while standing above it. Carefully work your way down, and aim to make the top of the room 6″ wider than the base. 

Use a small shovel to scrape the walls down to ensure each wall angles out. 

Create Cubby Holes and Shelves 

Rather than just having a small underground box, you can add small shelves in your room. This way, it’s easier to store items such as flashlights or glowsticks for light.

Avoid putting any candles or anything involving fire inside of your underground room. Having a fire in a small space could result in too much carbon monoxide, and potentially cause someone to suffocate. There’s also a chance that it could cause debris to fall and injure someone. 

To create shelving, slowly scoop out dirt in the areas you like with your hands or a small shovel. Pat the area to compact the dirt and create small “boxes” within the wall. 

Create a Way to Get in and Out

One of the most important things you’ll need to remember as you build the underground room in your backyard is having a way to get in and out. 

You could create small steps similarly to how you create shelving. 

For an easier method, simply use cinderblocks to create stairs. If doing this, make sure that the dirt is compact around all of the blocks by at least an inch thick to cover sharp corners and edges.

Finish Up the Walls

You’ll want the walls of your fort to be somewhat smooth and clear of any rocks or tree roots. To keep your room as strong as possible, pat the walls down with either the flat side of a shovel or your hands.

If patting down the walls with your hands, be sure to wear gloves. Keep pressing and compacting until the walls don’t seem crumbly and are smooth to the touch. 

For added stability, you could use plywood to line your fort walls. If doing this, you’ll want to make sure the plywood is flush with the walls starting with the base of the fort.

Drive two posts down at each corner of the room and then screw plywood edges to the posts to create the siding. The posts should touch on one of the corners to create a small box of space when looking from above. 

Furnish Your Room

At this point in the process, you can begin making your underground room cozier. A good place to start finding items is a thrift shop near you.

Grab something for seating, such as small stools and tables, if there is room for them. Use a blanket you find at the thrift store or one that you don’t care about for the floor. 

When you’re finished spending time in the room, be sure to remove any items with fabric to keep them from getting moldy. This includes clothing, blankets, cushions, pillows, etc.

Step Three: Add the Roof

Option One: Use Plywood or Wooden Planks

One option to use as a roof for your underground room is a piece of plywood. It should be at least a few inches larger than the room on each side. 

To create a handle, drill a hole in the plywood to tie a cord or rope through. Once secured, you can use the rope to lift up the roof when you’re ready to head into your fort. 

Option Two: Use a Tarp

Using a tarp is a great method to keep your underground room safe from rain and moisture. 

Either tie the tarp to nearby trees or by driving steaks into the ground to secure the tarp. Make sure that you pull the tarp taught to create a sturdy roof. 

To create an A-frame roof with your tarp, tie a string taught over the fort. Drape the tarp over the string, and secure the four corners into the ground.

No matter which method you use, it’s a good idea to place some sort of marker to prevent anyone from accidentally falling in.

Option Three: Build an A-frame Roof

You can build an A-frame over your room to make it slightly above-ground. This can help protect it from the weather. 

You’ll start with three initial logs for the A-frame. One piece of wood will be held up on an end at the intersection of two other shorter logs that will create an “A” shape.  

The pit will be directly in the center of the triangle shape of your roof. Then, place sticks parallel to the other two short logs all the way down the long piece of wood. 

Once finished, you can pack in a light layer of mud to create your roof. Then you can add leaves, pine cones, or any other natural material to camouflage. 

Option Four: Build a Lean-To Roof

To create the lean-to roof, you’ll start by hammering two poles into the ground hovering slightly over the room. A board will be held up by the two poles, creating a slanted roof on the ground.

You can also nail sticks to the peak of the roof, gradually using smaller sticks until you reach the bottom. Another option is to pack with a layer of mud, covered with pine needles, leaves, clay or other natural materials.

Final Thoughts

Building your own secret underground room in your backyard can be a big project to take on, but rewarding nonetheless. As long as you properly learn how to build an underground room in your backyard and follow all the precautions, you’ll end up with a safe and cozy space.