How to Use Concrete Deck Blocks for Deck Posts

Building a deck can be one of the most satisfying DIY projects and choosing a fast and effective foundation solution—like concrete deck blocks—makes it even more approachable. These blocks offer a simple, cost-effective way for handy homeowners to support their decks without the hassle of digging deep footings. While they aren’t ideal for every situation, I’ve seen concrete deck blocks save time and provide a solid base for many smaller deck projects over the years.

In this post, I’ll walk you through everything you need to know about using concrete deck blocks, so you can confidently get your project off the ground—literally!

What are Concrete Deck Blocks?

Concrete deck blocks are precast concrete footings used to provide support for beams. They can also hold posts that are stabilized by a surrounding structure. These blocks are commonly used for everything from sheds and gazebos to walkways and—my personal favorite—ground-level decks.

Deck blocks feature slots designed to cradle framing lumber. For simple ground-level deck designs, the beams rest directly in the blocks, held in place by the weight of the deck structure above. Unlike traditional poured footings, they rest directly on the ground, making them a quicker and easier option for low structures.

Are Concrete Deck Blocks Right for Your Project?

Concrete deck blocks may be a great choice if your planned deck is:

• Under 10–12 feet wide and up to 8–10 feet in length
• A simple rectangular or square layout
• At ground level on firm, well-drained soil
• Has no railing

For small, ground-level decks with minimal loads, you’ll typically place deck blocks just at the corners. The beams fit securely into the blocks’ slots, forming a solid frame for the joists and decking above.

Before you decide if deck blocks are the right choice, keep these important factors in mind:

• Soil Stability: Deck blocks rely on stable ground for support. They work best on compacted, stable soil but are not a good choice on loose or shifting soil or areas with heavy freeze-thaw cycles.
• Deck Post Support: Due to lateral load concerns and limited stability, I do not recommend using concrete deck blocks to support deck posts, including through-mount railing posts. Railing posts (and posts for elevated decks) require a more stable base. For both safety and code compliance, I always choose poured, in-ground concrete footings when deck posts are involved.

Finally, while concrete deck blocks can work well for small, low decks, larger decks or those with heavier loads require additional blocks to distribute weight evenly. Blocks can be spaced 4–6 feet apart along the center or at intermediate points between the corners. However, I find the complexity of planning and leveling numerous blocks can make for a challenging installation. For larger decks, I again recommend poured footings.

How to Use Concrete Deck Blocks for Support

If your project and soil conditions are compatible with concrete deck blocks, here’s the step-by-step process I recommend for your small square or rectangular ground-level deck:

1. Prepare the Site

Start by clearing and grading the area where your deck will be built to prevent shifting or uneven blocks and create a stable foundation for your deck.

Create a slight drainage slope across the entire deck area to prevent standing water that can lead to wood rot or soil erosion under the deck. Aim for a slope of about 1/8 inch per foot, sloping away from your house or one edge of the deck.

Next, set up stakes at the corners of your deck layout. From there, you can either stretch a string between the stakes or—my preference—a handy laser level. If you’re using a string, manually adjust it to your desired deck height and slope. If you’re using a laser level, set it at one corner and adjust the beam accordingly.

Evaluate the ground. You want it to align with your reference line. Adjust by removing excess soil or adding fill as needed. At this point, I like to add 2-4 inches of gravel across the deck area for drainage, then use a tamper to compact the gravel to create a solid base. If you’re not adding gravel, just tamp down the soil.

2. Set the Blocks
   Lay out your deck blocks, placing them at the corners you marked with your stakes. Position blocks at the same elevation so they are level with each other, using a little gravel or leveling sand underneath if needed.
3. Place the Beams
   Place your beams into the block slots. For a standard floating or ground-level deck, the beams rest in the blocks and meet at a 90-degree angle within the block slot. Aim for an intersection without significant gaps, with the beams butting up against each other. They’ll be held in place by the weight of the deck structure as you build.
4. Double-Check for Level
   Finally, use a level across the tops of your placed beams to ensure those at the same elevation are level with each other and verify that your drainage slope is maintained. If needed, add shims in the block slots under the beams to adjust your frame.

Now you have a stable, even foundation for attaching your joists and decking boards.

Advantages and Disadvantages of Concrete Deck Blocks

Concrete deck blocks offer various pros and cons depending on your deck’s requirements.

Concrete Deck Block Pros:

• Quick Installation: The blocks are precast and set on the surface; no concrete mixing, pouring, or curing is required.
• No Digging: Since deck blocks sit above ground, there’s no need to dig.
• Cost-Effective: Concrete deck blocks are typically cheaper than other footing options. For smaller, simple, ground-level rectangular decks, four blocks placed at the corners are usually sufficient.

Concrete Deck Block Cons:

• Limited to Lower Decks: Concrete deck blocks work best for ground-level decks; higher decks need deeper, more stable foundations.
• Soil Stability: The deck blocks need stable ground; they aren’t suitable for use on clay or loose soil.
• Shifting Potential: Concrete blocks can be susceptible to shifting, especially in areas with freeze-thaw cycles.
• Not Ideal for Irregular Shapes: Blocks are designed for rectangular layouts, making them a less effective choice for decks with chamfered corners, notches, or varying dimensions.
• Not Ideal for Larger Decks: Larger dimensions can make it difficult to evenly distribute weight using concrete blocks.

What Weight Can a Concrete Deck Block Support?

A common question I get is, “How much weight can these blocks actually handle?” Concrete deck blocks can usually support between 1,000 and 1,500 pounds. For most modest ground-level deck projects, I find this weight range is sufficient, but for larger or taller decks, you’ll need to consider buried footing options.

Always account for the weight of the decking materials, furniture, and people your deck will support. If you’re unsure about weight limits, local building regulations, or whether buried footings are required, find a builder to assist.

How to Determine the Spacing of Concrete Deck Blocks

I suggest limiting the use of concrete deck blocks to the smaller, low ground-level decks I’ve described here: those under 10–12 feet wide and up to 8–10 feet long. For these simple, rectangular designs, deck blocks are typically placed at the four corners. This method works well for small decks that don’t need additional central support.

If you’re determined to use concrete blocks for a larger deck, the spacing between them can vary depending on the deck’s size and load requirement, but anywhere from 4 to 6 feet apart may be necessary for adequate support and to prevent sagging. However, remember that as the deck size increases, concrete deck blocks are a less ideal footing choice: I still advocate for buried, poured concrete footings as the best choice for the structural integrity and safety of larger decks.

Always consult local building codes for specific requirements based on your deck’s size and design.

Concrete Deck Blocks: A Solid Choice for Simple Projects

Concrete deck blocks can be a smart choice to save time and money while offering a simple footing solution for your deck. While it’s appealing to avoid digging holes and pouring concrete, keep the sweet spot of concrete deck footings in mind: small, ground-level decks that don’t require railings. Remember, as your deck size and design grow, so does the need for more robust, alternate foundation solutions. Be sure to check and adhere to your local building codes, and happy building!