How to Build a Deck on a Slope

Lead Image
  • 20-40 hours
  • Advanced
  • 15,000-25,000
What You'll Need
Post hole digger
Tape measure
Deck posts
Deck beams
Three-inch nails or deck screws
2x2 lumber
2x4 lumber
Lag screws
Stain or paint
Eye protection

Sloping ground presents a variety of challenges when you are building a deck. Since the ground is not flat, building a level deck requires accounting for the severity of the grade the land slopes to make sure your construction is stable. In fact, building a deck on sloped land creates a living and recreational space because, without a flat surface, this type of property is almost impossible to use. An attractive, usable deck will not only enhance your lifestyle but will also increase property values.

Step 1 - Dig the Foundation Holes

Dig one-foot diameter holes at the top of the slope that will hold supports for one side of your deck. Make sure these are deep enough by digging until you find solid soil.

Step 2 - Place Deck Poles

Place a deck pole in each hole allowing them to extend at least a foot higher than where you plan the deck to sit. The tops of the poles will be cut to fit later once all poles are set and the deck height is finally determined.

If your deck requires a railing, you can make the posts the height of your railing to eliminate the need for end posts later.

Step 3 - Fill in the Holes

Mix enough concrete to fill each hole. Use a level to check to make sure each post is straight while the concrete sets.

Step 4 - Mark Second Side

placing a post in a hole in the ground

Once the first side of support posts is installed, use string attached to the first poles extending this across the surface area to where you wish to place the opposite posts. Mark the spot where the new set of poles will go.

Step 5 - Add Bottom Posts

At the lower end of the sloped deck area, once again dig two post holes where you previously marked. Install two additional posts holding the string attached to the other side high enough up to make sure the posts are long enough to support the deck once leveled. Fill these holes with concrete making sure they are level as the concrete sets.

If you are extending your deck all the way to the bottom of the slope and setting the second set of posts on level ground, there is no extra planning needed. However, if the footings are being set in sloped ground, a good rule of thumb to make sure your footings will not slide away with the surface of the soil is to set the widest part of the concrete seven feet from the slope. This will likely require a deep hole and much more concrete than your other footings, but in average soil conditions, this should make your deck secure.

For more severe grades and unstable soil conditions, you will need to consult a professional.

Step 6 - Mark Where the Floor Goes

Check to make sure your string attached between the posts is level, and then use it to mark each post, indicating where the deck platform will attach.

Step 7 - Frame the Deck

Frame both sides of the deck by installing 6x2-inch beams (in a wood of your choice) using three-inch nails or deck screws.

Step 8 - Attach Floor Beams and Floorboards

installing boards onto a deck

Install floor beams to your deck frame using three-inch nails. Local building codes for decks may specify proper spacing for these floor beams, so be sure to check first before attaching.

Once these support beams are in place, lay deck boards across the beams, attaching with nails or deck screws.

Step 9 - Add Railing

Most decks above ground level will also require a railing for safety. Railing height and baluster width and spacing are often regulated by building codes, it is best to check first.

For the easiest railing solution, you can actually buy a premade railing system at many home improvement stores. If you would like to build it yourself, however, you can use 2x4 and 2x2 lumber for a basic design.

A standard railing height is between 36 and 42 inches, so cut your balusters to the proper height. Then, cut your 2x4 top rail pieces. If the distances around your deck are too long for a single piece of 2x4 to span, you will need to segment the rail and install a 2x2 picket for each seam to support the bearing.

Attach all of the balusters to the outer edge of the decking using lag screws, allowing no more than nine inches between them and testing them with a level to be sure they are all plumb as you go. Then, attach the top rail to the pickets and the support posts using deck screws.

Step 10 - Add Wood Finish

Choose an exterior wood stain or paint to finish your new deck. Sandpaper any and all rough areas before coating with paint or stain, and allow dry time between each coat as you apply it.

Once the paint or stain is dry, add polyurethane to protect the finish further. Leave it to dry and sand the surface lightly between coats. Since the surface area is rather large, consider renting professional sanding equipment to get the smooth surface necessary for successful finishing.