If you’re building a low deck or wooden patio next to a concrete foundation, you can solidify the deck frame by mounting it to the foundation.
Mounting a deck frame to concrete eliminates the need for post holes. It also eliminates the need for additional concrete next to the foundation, making for an easier building process.
This brief tutorial will take you step-by-step through mounting the outer part of a deck frame to a concrete foundation, including mounting a frame to a curved concrete step.
Materials Needed to Attach a Deck to Concrete
The first and most important tool you’ll need to mount the deck frame to the concrete foundation is a rotary hammer drill. A rotary hammer drill produces a pounding force, making it an excellent tool for drilling into (or through) concrete.
Other materials you’ll need include pressure-treated lumber (this example uses 2×6 pressure-treated lumber), a level, wedge anchors, a hammer, a ratchet, and a clamp. You’ll also need a miter saw for making your lumber cuts. And you might find a chalk line helpful to mark your level line.
How to Determine What Wedge Anchor Size You Need
In this example, we use 1/2″ wedge anchors, 4-1/4” long.
To calculate wedge anchor length, you’ll need 2-1/2” of the anchor into the concrete itself, then whatever length will go through your lumber (in this case, the true width of a 2×6, which is 1-1/2”), plus 1/4″ for the anchor washer and nut.
You’ll need to drill about 1/4″ deeper than 2-1/2” into the concrete, as per wedge anchor instructions.
Because it’s more accurate to place the 2×6 and drill through it into the concrete (better to align them precisely when they’re drilled together), we measured and marked the drill bit at 4-1/2” with colorful tape. Using this measurement gives us a little extra space, just in case.
How to Mount a Deck Frame to Concrete Foundation
Step 1: Determine Height Placement for Your Frame
Determine the height placement of your 2×6 pressure-treated frame piece. Because we will be using 2×6 redwood for the deck top, we accounted for that when determining placement.
We used a scrap piece of 2x to determine where the top of the frame should hit the concrete and marked along the bottom of the scrap 2×6 with a pencil.
Here you can see the pencil guideline for the top of the framing lumber.
Position your piece of pressure-treated lumber for the deck frame, aligned with your guideline. Use a level to make sure it’s straight.
Step 2: Drill Through the Lumber Into the Concrete
Have one or two people hold the frame board in place. The rotary hammer drill will vibrate the lumber and move it out of place if you’re not careful. Drill through the lumber and into the concrete with your rotary hammer drill up to the point you’ve marked on the drill bit.
Step 3: Install the Anchor
Slide the washer onto your wedge anchor, then thread the nut onto the anchor to hold the washer in place.
Hammer the wedge anchor into the frame board and concrete while holding the frame board in place.
Hammer the wedge anchor all the way into the board. You might have to unscrew the nut a bit as you get closer to finishing up with the hammer. Remember, you only want about 1/4″ of the wedge anchor exposed. After you unscrew the nut a little, hammer in the anchor a bit more.
Once the top of the wedge anchor is flush with the outer edge of the nut, you’re ready to tighten the anchor into place.
Use a ratchet to tighten down on the anchor nut.
The lumber might bend inward slightly as you tighten; this is okay. Don’t worry about cranking it down too tightly, though. You don’t want to damage or weaken your frame board.
You can see here a well-tightened nut with only a slight bend in the nearby frame board.
Step 4: Drill the Remaining Holes
Keep the frame board level and hold it tightly in place while you drill the other holes. A good rule of thumb is to place an anchor every few feet.
Cut and butt additional frame boards up next to the first one, if needed.
Be sure to keep the boards level, even at the connection points. You can mount the boards up to about 6” in from the end of the board.
Continue that way for straight foundation mounts, and you’ll be on your way to an easy deck frame job. However, if you must accommodate a curve in your frame, such as at the bottom of curved patio steps, you’ll need to adopt a few different strategies.
Step 5: Cut Smaller Boards If You’re Working Around a Curve
First, cut smaller boards that will allow you to work around the curve. Each board should extend about 4”-6” past the curve on each side.
You’ll want two bolts per board to keep them securely mounted and level.
Notice that these boards don’t sit flush against the curve at all points (which is, of course, because they’re mounted onto curved concrete). However, they are stable because we installed the wedge anchors where the curve starts to separate from the board’s back face.
Use a clamp to hold the pieces level. Because they are smaller boards, everything (including fingers and faces) gets much closer to the drill bit, which can make everyone nervous. The clamp helps to keep things in place, although you’ll still need to use some muscle to keep the board where it belongs.
Keep the pressure-treated lumber boards level with the steps, and work your way around the curve, marking where to cut each board length, piece by piece.
You’ll notice the ends of these frame boards touch but are not flush – this is fine for a deck frame. The boards are secure, with their double wedge anchors into the concrete, to provide plenty of support for the deck frame.
You’ve just mounted your deck frame to the concrete foundation on a straight plane and/or curved sides. Now you can begin to visualize the final results.