Calculating Out How Many Deck Blocks for a Floating Deck


I am amazed by how many deck blocks people use when building a floating deck. With deck blocks under each joist or even three blocks per joist. Which is generally overkill. Simply this is way too many deck blocks for a floating deck. To figure out how many deck-blocks a floating deck needs, you need to step back and simplify a few things. Then you will know exactly how many deck-blocks you need for a floating deck. Regardless of what size the floating deckOpens in a new tab. is.

The number of deck blocks needed for a floating deck is based on how heavy the deck is, divided by how much weight each deck block can support. Arranging the deck blocks to distribute the weight over each deck block evenly as not to exceed the weight limit of any single deck block can hold.

Unfortunately, knowing this does not really help very much. A few more questions need to be answered to determine how many deck-blocks the deck will need. Primarily these floating deck questions.

  1. What much weight can a floating deck hold?
  2. How much wieght can a deck block hold?
  3. How do deck blocks need to be arranged to support a floating deck?

I am going to go over these three questions to answer how many deck-blocks a floating deck needs. Once we have the answers, you will know how many deck blocks any floating deck needs. Building to the safe side of deck construction without installing an excess number of deck blocks that are not required.

If you want to jump ahead to the answer without explanation. Here is a quick reference of deck size, weight and number of deck blocks each deck needs.          

Deck DimensionSquare Feet  of DeckLive Weight People & Furniture (40lbs sq’)Total Weight Supported by Deck Blocks (50lbs sq’)Number of Deck Blocks 12″ x12″ Supporting 2000 lbs
8×8642560 lbs3200 lbs4
8×10803200 lbs4000 lbs4
10×101004000 lbs5000 lbs4
10×121204800 lbs6000 lbs6
12×121445760 lbs7200 lbs6
12×161807200 lbs9000 lbs8
10×202008000 lbs10000 lbs8
12×202409600 lbs12000 lbs10
16×1625610240 lbs12800 lbs8
14×2028011200 lbs14000 lbs10
12×2428811520 lbs14400 lbs12
16×2032012800 lbs16000 lbs10
20×2040016000lbs20000 lbs12
Number of Deck Blocks Based On Deck Size

Unfortunately, this only gives you enough information to be dangerous. Without knowing how I came up with the number of blocks for each deck, you could arrange them incorrectly. Or not understand why some decks have more blocks than the total weight needs. For example, a 10 x 10 floating deck needs four-deck blocks. Four deck blocks can hold 8 000lbs, almost twice the support necessary for a 10 x10 deck. But you do not want to install less than.

An even better example is a 10×12 floating deck. A 10 x12 floating deck needs six deck blocks. Six deck blocks can hold over 12 000 lbs, twice as much as the deck’s designed weight. Why not only four deck blocks? Or why not three deck blocks matching the deck’s weight with how much the deck can hold?

That is what you will be able to answer and be better able to determine how many deck blocks to build for your next floating deck.

How Much Does a Floating Deck Weigh?

Better grab the bathroom scale. You have a lot of boards to weigh. This could take a while.

Just kidding, no weigh scale required.

In construction, we can often work off assumed values. For decks, the total accepted weight is 50 lbs per square foot. That’s for every square foot of deck the deck load with furniture and people can hold up a minimum of 50 lbs. Split into two categories, Live and Dead Weight.

Deadweight is kind of how it sounds. The static weight of the deck material averaging 10 lbs per square foot. Combining the beams’ weight, joists, decking and everything else needed to make your deck, a deck. This is the smaller of the two numbers but is an essential part of the equation.

Live weight is why you build a deck. The moveable parts of what makes a deck great. This includes deck furnitureOpens in a new tab., umbrellas, drinks and people. Don’t forget people, the essential part of every deck. The majority of the decks calculated weight calculated at 40 lbs per square foot, but the most important. Being live, moving around the deck and is the hardest to calculate for firmly. As people walk around the deck, their weight also moves from one deck board to another. From one end of the deck to the other. One-minute holding hundreds of pounds, the next nothing but joists and decking.

The good news is rarely will you ever load a deck up to the designed weight. A 10 x10 deck, by design, can hold 20 full-grown men. But you will never want to have that many guests on your deck. When entertaining on a deckOpens in a new tab., the max amount of people you want is one per 20 per square feet of deck. Limiting a 10 x10 deck to five people. A much better experience but way below the weight limit.

How Much Weight Can a Deck Block Hold?

The size of the deck footprint and base material under it will make a big difference here. The bigger the base of the deck block, the more it can hold. The firmer the base material under the blockOpens in a new tab., compounding the amount of weight it can hold.

For simple math, a deck block with a square foot base can support 2 000lbs. That’s a deck block with a 12″ by 12″ base set on gravel or sand for drainage. Compacted over hard clay ground. There are other deck block options. To read and see other options, read Figuring Out How Much Weight a Floating Deck Can HoldOpens in a new tab.? But for simplicity, we will assume clay dirt under the deck and use one-foot deck blocks.

But even more important than how much weight each deck block can support is.

How Much Deck Can A Deck Block Hold?

What I mean is if we support the deck from either a drop beam on opposite ends or double up the rim joist to act as a flush beam holding the joists between.  How far apart can the deck blocks be to support the joist spanning between?

A deck block with a one-foot base can support 2 000 lbs of deck. A floating deck should be built to support 50 lbs per square foot.

2000 ÷ 50 = 40

A single deck block can support 40 sq’ of deck.

According to Canadian Wood CouncilOpens in a new tab. the maximum beam span between supports, the deck blocks is 8′ (2.4m), and the minimum is 4′ (1.2m) according to the residential deck guide. So, the max distance between deck blocks is 8′ (2.4m), and there is no need to have them closer than 4′ (1.2m). What determines how close to install the deck blocks is the joist’s length supported by the beam.

Feet Between Deck BlocksMax Joist Length/ Depth of Deck
420’0″
516’0″
613’3″
711’5″
810’0″
Deck Block Spacing

The longer the joists’ span or the deck’s depth, the closer the deck blocks need to be.

For example, if the deck is 12′ (3.6m) deep, the deck blocks can be spaced no more than 6′ (1.8m) apart. Yes, technically, they can be spaced six feet and a few inches, but practically, there is little value fighting for a few inches and makes the math easier if you round.

Okay, but why does a 10 x 10 deck only needs four-deck blocks when the max span is 8′ (3m)? Good question. Which brings us to block placement.

Arranging Deck Block for Maximum Effectiveness Under a Floating Deck?

To start laying out the deck blocks. Put a block under each of the four corners of the deck.

The problem with that arrangement is the corner deck block only holds half of its beam span potential. If you slid the block towards the centre, it would support more of the beam. But there are limits to how much you can overhang the beam.

1.5 x Beams Nominal Height= Max Deck Beam Overhang

With a 2×8 deck beam, 8″ x1.5= 12″. The deck beams can be moved 12″ (0.3m) closer to the centre. With each deck block supporting 5′ (1.5m) of the beam. A quarter of the deck each.

Not only does this decrease the amount of deck block needed, but it also makes the beam stronger.

Another reason I like overhanging the beam. And I do this both for floating deck blocks and deck footings is it gives you a little play in construction. If you find its out of square after framing the deck, you can slide the beam and frame over a little. Correcting the deck by squaring it without moving the deck block or having the beam sit incorrectly on it. You will notice when one beam overhanging 3/8″ (10mm) and the other flush but not the same 3/8″ (10mm) difference over a foot. This is why for all floating decks, plan to set the deck block 1′ (0.3m) in from the edge of the deck. It works and looks better.

Once you have established your outside block location, measure the distance between the blocks and divide by the number that is closes to the joist’s span.

Equation

Now you know the number of deck blocks. To figure out the distance between the blocks. Take the number of blocks, add one and divide the distance between the outside blocks again. This gives you the centre number between blocks. Or the distance of the space between the blocks.

Finding the Distance Between Deck Blocks for a Floating Deck; Example

You want to build a big floating deckOpens in a new tab., Twenty feet wide and twelve feet deep. A good size deck for barbequing and relaxing with friends. Let’s work through the equation of how many deck blocks you will need.

  1. 20-2=18 The deck’s width minus the one-foot beam overhang on each side.
  2. 18÷6=3 Six in the maximum distance between deck blocks to support joist spanning twelve feet. You need 5 deck blocks per beam. Two for the edges, plus three for the middle.
  3. 2×5=10 A floating deck less than fourteen feet deep needs two beams. One on each end, more if you use smaller joists but let’s assume you use larger joists like 2×8. A deeper deck will need larger joists or closer spacing. For this deck, you will need to purchase a minimum of 10 deck blocks.
  4. 18’0″ ÷4=4’6″ Number of fielddeck blocks plus one for the number of spaces between the deck blocks. Divide the distance between the outside deck blocks, and you have the centre spacing for your deck blocks.

This is easier with an imperial calculator, but even without one, you can get an idea. If the number was 4.4, you know it’s not 4’6″ that would be half, and it’s bigger than a quarter, 3″ so, somewhere between the two and you are close enough for a deck block. If your eye can see the difference, then adjust, but otherwise, you have a little play.  Remember, in theory, you only need eight blocks but have two extra for beam strength.  

Conclusion: to How Many Deck Blocks Does a Floating Deck Need

Two primary things will determine how many deck blocks your floating deck will need.

  • Floating Deck Size
  • Size and Placement of Deck Blocks

Once you know those two, you can figure out how many blocks your deck needs. It will require a little math and some sketching to understand placement. But at ten dollars and a half hour per block to install a few minutes figuring out the right number will save you money and time. Time you can spend enjoying your deck. And money buying for buying a comfy deck chairOpens in a new tab..  

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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