Figuring Out How Much Weight a Floating Deck Can Hold?

A floating deck out in the back yard, to set up a couple of chairs to soak up the sun. Invite a few friends over. Sit out of the dirt while enjoying a few drinks together. But how much weight can a floating deck support?

A floating deck designed by code can hold a minimum of 50 lbs per square foot. Calculated as 20% being dead weight, the mass of the deck itself. The other 80% or 40 lbs is the weight you can have on the deck, people, furniture, dogs, etc. A 12’x12′ (144sq’) deck properly supported can hold 5 760lbs on it.

Proper support is the key to how much a deck can hold. It does not matter what size of floating deckOpens in a new tab. you build. All floating decks, in fact, all decks built to code, can support no less than 40 lbs per square foot of deck.

A simple 12’x12′ floating deck can hold 28 full-grown men, each drinking a 1-litre bottle of coke without damaging the deck. Pretty impressive. Not sure how much coke will be spilled being so close, but the deck can hold them. The Deck is strong enough but only if the deck is supported correctly.

Which is really the crux of how much a deck can hold. What is holding the deck? What is under the deck, supporting the deck? Even if that deck is built to code, or even exceeding code with 2×10 joist, 12″ (300mm) on centre. If that floating deck is only supported by four plastic straws sitting on leaves, the deck will collapse. It’s that simple.

 Also, how that weight is distributed on the deck matters. If all 28 of those men decide to do a cheerleader pyramid stand in the corner of the deck, it will fail. The deck and all 28 men will fall, spilling their cokes and who knows what else all over the yard. A real no, no! Weight needs to be spread evenly over a deck’s surface to support its design weight fully.

There are several ways to support a floating deck. Before we breakdown the best floating footings and arrangements, let’s go over a few more details to consider about the deck. 

How Much Can Your Floating Deck Hold?

I have not been to your yard, and if I have, thanks for having me over. But I am not an engineer nor claim to be one. These are general calculations based on averages with assumed values. If you want to know the exact amount of weight that your floating deck can support, you will need to hire an engineer to calculate how much your deck can hold based on soil bearing capacity tests.

If that is more than you what to do, these are some quick calculations to give you an idea of how much weight your deck can hold. Or better yet, how to build your floating deckOpens in a new tab. to support the maximum weight.

Before inviting your 27 friends over for a deck partyOpens in a new tab., a general rule with construction and safety is the rule of three. Build your deck to hold three times more than the expected weight. Only invite9 of your friends for your next deck party. You and your deck will live to tell about it. Or build a bigger deck. Or ask your engineer friend to put down the chips and figure out exactly what your deck can support. Then finish up with the guest list.

A Stable Base Increases How Much a Weight Floating Deck Can Hold

The most critical element to how much a deck can hold is the base materialOpens in a new tab. under it. Your hand sinks into a pillow but is held by a rock. The same is true with a floating deckOpens in a new tab.. A deck built on stable, strong base material will hold much more weight than one floating on the grass.

Base MaterialBearing Capacity per square foot
Gravel, or Sandy gravel5,000 lbs (2268 kg)
Sand3,000 lbs (1360 kg)
Clay, sandy clay, silty clay, and clayey silt2,000 lbs (907 kg)

Source: Table 401.4.1; CABO One- and Two- Family Dwelling Code; 1995.

Gravel or sand are the best base materials under a floating footings because of ease to compact during installation, drainage of water and compression strength.

“One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it won’t settle to any significant degree.”

Brent AndersonOpens in a new tab.

Few of us will use such a large drain rock under a deck footing. More common under a deck block is ¾” (20mm) or smaller. Using the same material under the footing as we ordered to spread over the landscape fabric for weed control under the deckOpens in a new tab.. The smaller rock will require more work to compact. Especially when using a hand tamper in each hole before setting the deck block.

If the gravel base is fully compacted and is as deep as the concrete footing is wide, you can use the higher load-bearing capacity.

But we are better to assume that the gravel is only to replace the unstable topsoil that was removed before setting the deck block. Gravel provides excellent drainage but may not change our load-bearing capacity. Instead, calculating that the deck block is resting on clay, which can support 2,000 lbs (907 kg) per square foot of footing. If the deck is actually stronger than our math, excellent, if not. No harm, no foul.

Quick math, a 144 sq’ deck needs 4 sq’ of deck blocks on clay to hold it intended weight (7 200lbs).  More precisely, 3.6 sq’ of deck blocks evenly disturbed under the deck, supporting each member of the deck. Here are a few options for support a floating deck.

Deck SupportBase FootprintSupported Weight# for 12×12 Deck
6″ x6″ Treated Post0.25 sqft500 lbs15
10″ Patio Deck Block0.69 sqft1380 lbs6 better 8
12″ Patio Deck Block1.00 sqft2000 lbs4 beams require 6
16-in Bullnose Concrete Block1.78 sqft3 560 lbs3 beams require 6
23″ x23″ Concrete Patio Stone3.86 sqft7720 lbs?
30”x 24” Rectangular Patio Stone5.0 sqft10 000 lbs?

Of course, placing a single rectangle patio block under a 12′ x 12′ deck will not work. To properly support the deck, the support and load must be spread out. Holding the entire deck up. All four corners and everything in between need to be supported. That is where and why the selection of deck blocks, beams and placement matter.

Its footings must entirely support a floating deck’s weight as it cannot be attached to the house. All decks connected to a house must be supported by frost footings, not floating. Therefore, it is safe to assume two beams, one on each end of the deck. With 2×8 joist 16″ (400mm) centre spanning over the dropped beams supporting the decking and everyone on the deck.

If you wish to learn more, Richard Bergman has a very in-depth article titled DECK LOAD CAPACITY AND TRIBUTARY AREA: HOW THEY RELATEOpens in a new tab., going to great details while getting nerdy with math about supporting a deck. Explaining the minute details for supporting a deck. For keeners, worth the read.

Here are a few floating footing options and their bearing capacity and advantages.

6×6 Deck Post Holding a Floating Deck

A 6×6 installed vertically bearing on the ground has little value. With such a low weight capacity, you will need a 6×6 post every foot and half.

As an alternative, you can replace the deck beam with a 6×6 post-treated for ground contact. Trench out the entire length where the beam will go and fill with compacted gravel. The gravel draining away water, preserving the post while making a stable base to support the beam. Being continuously supported it can support up to 8′ (2.4m) of deck. The maximum span of dimensional deck joists.  

A good option for a low floating deck.

If the floating deck will be higher than a 6×6 beam’s height, you will need some larger footing options.

10″ Patio Deck Block Holding a Floating Deck    

Now, this is the go-to deck support for a floating deck and rightfully so. Its 10″ (250mm) footprint can hold 1380 lbs, roughly 30 sq’ of deck. Ideally suited for a 12′ x12′ deck. Three deck blocks holding each beam under the deck.

As Richard explains, the weight of a deck is not all evenly distributed. A little manipulation of the deck blocks placement will be required to relieve the middle block and prevent it from being overloaded and sinking. If we overhang the beam 1′ (0.3m) on each side, the span of the beam held by the middle block is reduced to 5′ (1.5m), 30 sq’ of deck. In theory, this is still a little too much but very close.

Meaning you either need to increase the middle deck block footprint or add an additional block. Bringing the number of required deck blocks to 8, not six. Spacing them every 3’6″ (1.016m) while overhanging the beam 12″ (300mm) on the ends.

12″ Patio Deck Block Holding a Floating Deck    

A couple of inches makes a huge difference for how much a deck block can hold. In theory, using deck blocks with a one-foot base, you will only need four to support the deck’s weight.

But the beam cannot span 12′ (3.6m) or even 10′ (3m), but the maximum deck beam span is limited to 8′ (2.4m) or less. Requiring three 12″ deck blocks hold each beam of the deck. The single most significant advantage is with the middle of the beam. A larger deck block can support the additional weight of the centre of the deck. Not requiring any additional support for a 12’x12′ deck.

A larger deck with 16′ (4.8m) long joists will require a larger footing or more. Suitable for smaller decks, but not for a large floating deck.

16″ x 4″ Thick Concrete Block Holding a Floating Deck

This is more than a 16″ (400mm) square concrete patio block. The thickness is 4″ (100mm), which is essential for the concrete not to crack under the weight of the deck post. The deck post needs to be a minimum 6″ to distribute the weight over the deck block. But in Calgary, where I build decks, this is the requirement for all deck beams anyway.

The large footprint, significantly increasing how much weight the deck can hold. Over seven times what a deck post can by itself and over double what a 10″ (250mm) deck block can hold.

Meaning six, 16″ (400mm) square concrete block 4″ (100mm) thick not only support a 12′ x12′ but can support a deck 16′ (4.8m) deep spaced 8′ (2.4m) the maximum span for a deck beam’s footings. For a 12′ (4.8m) deep deck, the beam’s limitations make them overkill unless the ground is super soft. Not a bad thing but not required.

Concrete Patio Stones Holding a Floating Deck

I have seen both 30″ x24″ sidewalk blocks and the smaller 2-foot square block under floating decks. With their large footprints, I get why they are used. But their thickness makes the weight they can hold deceiving.

Being less than 2″ (50mm) thick, the potential of the deck post cracking the concrete is high. Concrete is only as strong as it is thick. As a general rule transferring the weight twice as much as it is thick.

So, for a patio block, 2″ (50m) thick is adding 2″ (50mm) on each side of the deck post. You are not holding the deck post with the full width of the concrete but 4″ (100mm) more than the post. Only 10″ (250mm) for a 6×6 deck post. No bigger than your smallest deck block. Not only is this a small footprint, but the large span of the block increases the potential of cracking from upward pressure on the sides while being compressed by the deck post in the middle.

In short, it looks like a good idea, but that’s it. It looks like it will hold a lot but is too thin to do much. Stick to deck blocks.

Conclusion of How Much A Floating Deck Can Hold

All decks, including floating decks, should be designed and framed to hold 50 lbs per square foot. Ten of those are the deck itself, and the other forty is for furniture and friends on the deck. But the critical determining factor of how much a floating deck can hold is often overlooked because it’s hidden under the deck. Even a floating deck has footings. It’s the footing’s size and how much they can support, determining how much your deck can hold.

Ryan Nickel

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

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