Building a deck can be daunting, but it doesn’t have to be.
Using concrete deck blocks is an easy and fast way to construct the foundation of your deck.
Deck blocks are preformed concrete piers placed directly on the ground instead of digging out footings or using a deck pier system.
Proving stability for the deck structure without the mess of mixing and pouring concrete for a traditional footing.
What are Concrete Deck Blocks?
Deck blocks are essentially a simple version of precast foundations. When used they must adhere to the same regulations and codes that apply to regular footings. For example, they must have an adequate bearing area and should sit at least 12 inches below the ground – or below local frost depth. They cannot simply be installed on the dirt without proper ground preparation.
Making the deck block base as important as the block itself but more on the latter.
Pros & Cons of Concrete Deck Blocks
- A single person typically has no difficulty in managing the deck blocks.
- Don’t require messy mixing of concrete on site.
- Less work to install than poured concrete footings
- No hard beam connection to the deck blocks. They can be lifted off.
- The blocks often do not contain reinforcement metal (rebar) therefore can be easily cracked.
- Cannot be used for decks attached to a house
What is the Difference Between Precast Deck Blocks and Poured Concrete Footings (Piers)
The biggest difference is concrete mess.
With Precast blocks, there is no mixing and placing of concrete. Then waiting for the concrete to set up before you can build off of them.
But his ease of use and speed of installation comes at the price of versatility.
With poured concrete, you can build whatever size of concrete footing you need. Both in width as required by the soil and deck size.
Plus, the height as required by the frost line. Which is often deeper than 12″, the average height of a deck block.
In my region, our deck footings must be 4′ below grade. Yes, you place the deck block 4′ below grade and stack 4′ of concrete blocks on top of it. But it is much simpler to dig in a 4′ soon tube, and pour concrete into it than do the same with concrete blocks.
You can also improve the durability of the concrete pier by adding rebar to provide tensile strength. Not as important for vertical strength, but if you are building off a larger deck pad footing, this will make a world of difference.
Finally, anchoring can be better achieved with a poured concrete footing. Once the concrete is poured but before it sets, installing a post saddle will not only help the post base from rotting but also provide a positive connection ensuring the post stays in place.
There is an old saying in construction.
“There is Quality, Speed, and Cost. But you can only have 2.”
Deck blocks give you speed and cost but is lacking in quality.
Poured concrete provides quality at a reasonable price, but it will take longer.
When Should You Use Deck Blocks?
Knowing all this, when are deck blocks the better solution for supporting a deck?
When Deck Blocks Are a Good Idea
- When building a deck over tree roots
- Low Frost Levels where extra height is not required
- Temporary Decks
When Deck Blocks Are a Bad Idea
- On Soft or Unsettled ground
- Decks Attached to a House
- Elevated Decks (not enough Lateral Support)
- Deck Pergola or Roof
For more information on Are Deck Blocks a Good or Bad Idea?, click the link.
How to Install Concrete Deck Blocks
Most deck blocks have a 12″ (300mm) base and are 8″ (200mm) high. Some have a cavity on the top to hold a 4×4 or 6×6 post. Others also have notches allowing you to run a single-ply beam material over the deck block.
Plan out your deck and its required support to determine how many deck blocks you will need. Sketching out a simple plan on a scrap piece of paper helps to make it clear where to place each block and how many you will need.
It’s the base under the deck blocks that makes all the difference.
Good base, good support.
- Dig out the topsoil and level the ground where the deck block will sit. It is essential to remove all organic material as this will make the block unstable.
- Place about 2″ (50mm) of non-organic drain material in the hole and compact. This is to provide drainage under the deck block to minimize heaving. The water draining away and the space between the rocks provided room for the ice to form without moving the block.
- Place the deck block and level with a two-foot or torpedo level. Add or remove base material to level the block as required. Make sure to compact before setting the block to minimize shifting later.
- Now that was easy!
How Much Weight Can A Concrete Deck Block Hold?
The size of deck block base is going to make a huge difference here.
- 10″ deck block with a 0.69 sqft base can support 1380 lbs.
- 12″ deck block with a 1 sqft base can support 2000 lbs.
How Many Deck Blocks Do I Need?
For a 12′ x12′ deck you would need (8) 10″ deck blocks or (6) 12″.
But this is going to be hugely affected by the soil under the deck blocks. Compacted gravel can support more weight than clay. And loam, the dirt that the grass grows in, is never a good idea to install deck blocks on.
For a more in-depth calculation of the number of deck blocks required for a floating deck, click the link.
How Far Apart Can You Place Concrete Deck Blocks?
For most decks, deck blocks can be placed at 6-8 foot intervals apart. But for longer joists spans or softer soils, 4 feet apart may be better to ensure stability and durability.
Equal spacing will provide the best support. So, its best to do the calculation of home many deck blocks your deck needs. Then, equally space them under the deck.
In most cases, this will be less than 8′ apart but more than 4′.
What Do You Put Under Deck Blocks?
You want something that is stable once installed but will not trap water.
Which makes compacted gravel ideal.
It can easily be compacted to provide a level but strong base under the deck block. But even more important, it allows water to flow through.
If there is any chance of frost, water trapped under the deck block will freeze and cause heaving of the deck. Not good.
Even if the risk of freezing is zero. Trapped water under a deck block softens the ground weakening the support. Causing the deck block to sink over time.
Yes, 2″ (50mm) of non-organic drain material under a deck block is a must.