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Composite Floor Decks Explained: How They Work



V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com


Composite Floor Deck

Composite floor decking is a technique that uses metal floor sheets to create a solid deck that strengthens floor frames without adding large amounts of weight to the frame. There are many different variations of composite decks, each with its own merits. These include 1.5″, 2″, 3″, and deep decks.

Traditionally, strengthening a floor would mean increasing its self-weight, which would, in turn, create more loading problems to deal with.

The extra weight added to the floor supports ends up affecting the steel frames of the building.

Thus, it is crucial to make floors as efficient as possible with minimal effects on the frame structure.

Engineers have figured out a way to do exactly that by creating and implementing composite floor decking.

This article aims to serve as a complete guide for composite floor decking by explaining its benefits, types, and many other aspects.

What is Composite Floor Decking?

Composite floor decking is a method of utilizing composite steel floors to increase the strength of floor frames in a building.

The main advantage is that by using this technique, the extra weight is kept to a minimum, and so the initial design of the floors is not compromised.

There are a lot of variations in the metal decking design, but not all of them are suitable for every scenario.

Composite floors can have different lengths, gauges, and profile types, each of which is engineered for specific slabs and has a particular value of strength.

Steel in Composite Floor Decks
Steel in Composite Floor Decks

How is Composite Floor Decking Achieved?

Composite floor decks use metal floor decks with a unique uniformly spaced embossment pattern in their flutes.

This pattern facilitates the bonding between the concrete and the metal floor plate.

As the concrete cures, the metal decking and the concrete unify to become a single deck. This is known as a composite deck.

A composite deck thus has a higher strength output than the individual two components that make it up.

The embossment pattern is a critical factor in determining the strength of the composite deck and is called many names.

Difference Between Composite Floor Decking and Roof Decking

The main difference between the roof and the floor deck is that the floor deck is made to function with structural concrete while the roof deck is not.

Therefore, floor deck metal plates have embossment patterns on them, which help them bond to the concrete, and the roof decks do not.

Roof decks are used in securing roofs that do not have to bear any significant external loads.

Whereas floor decks must carry multiple variations of live and dead loads.

Floor decks can be categorized as composite decks, while roof decks can be categorized as steel-form decks.

Composite Decking vs Steel Form Decking

Steel form decking and composite decking are two separate decking methods that are often confused with one another.

The primary difference between the two is that in steel-form decking, the concrete does not bond with the steel deck.

Whereas in composite decking, the concrete develops a bond with the steel deck due to the embossment pattern on the flutes of the steel.

Steel form decking does not add to the strength of the floor, whereas composite decking adds significant strength to the floor system.

Types of Composite Floor Decking

Composite floor decks are primarily of three types based on the depth of their panels. These depths include variations of 1.5”, 2”, and 3”.

The depth and the gauge of the panels determine the strength of the deck.

The strength of the deck is directly proportional to the depth of its panel. A deeper panel will be stronger than a shallow one.

Therefore, a 3” panel will be significantly stronger than a 1.5” panel.

1.5” Composite Floor Deck

This is a low-profile deck used for making smaller and lighter slabs that can have a thickness ranging from 3.5” to 7.5”.

It has a depth of 1.5” and a width of 3’. It is also known as a B floor deck system or B FormLok.

It has the shallowest profile out of all the composite floor decks and forms the shortest spans.

2” Composite Floor Deck

This floor deck allows for medium-ranged spans and thicker concrete slabs to be made as compared to 1.5” composite decks.

The deck itself is 2” deep and 3’ wide and allows the slabs to have a thickness of 4” to 8”.

It is also known as a W2 FormLok.

3” Composite Floor Deck

This deck looks similar to a 2” deck but is 1” higher.

This extra depth allows for more concrete to be poured in, which makes thicker and stronger slabs.

It allows for the longest spans to be constructed and is known as W3 FormLok.

Types of Composite Floor Decks
Types of Composite Floor Decks

Deep Deck

1.5”,2”, and 3” composite decks make up more than 90% of all the composite deck systems, but there are some other types known as deep decks.

These can have a depth of 4.5”, 6”, or 7.5”. These decks are not commonly used.

They are required when the slab spanning conditions are not met by the 3” composite slabs and longer spans are required.

Deep decks are costly and are not available easily.

Thickness and Finishing of Composite Floor Decks

Composite floor decks have 4-gauge (thickness) sizes of 22″, 20″, 18″, and 16″.

The lower the gauge size, the stronger the composite deck.

Galvanized G60 is the most common type of floor deck finishing that is used in the industry.

It is easily available and hence constitutes more than 90% of all composite floor deck finishings.

Another finishing used is the Galvanized G90 which is not as readily available and has a lead period of three to five weeks.

G90 has 50% more galvanization than G60 and is used in highly corrosive environments such as near oceans.

The third finishing used on composite decking is the phosphatized and prime-painted finishing.

This finishing is also not readily available and has to be ordered in large volumes to be cost-effective.

The upper side of the metal is phosphatized, while the underside is painted.

This type of finishing is used in large-scale projects, such as high-rise buildings.

It also has a lead period of three to five weeks.

Benefits of Using Composite Metal Floor Decking

Composite metal decking offers greater value to floor systems than its alternatives.

The biggest benefit of using composite floor decks is that it add significant strength and stability to the floors.

This strength is the result of the bond between the concrete and the embossments on the metal deck.

The second benefit is that it is lightweight and doesn’t add to the load on the floor.

Thus the columns and the foundations do not have to deal with extra-large values of uncalibrated floor loads.

If you use other methods to increase floor strength, then you will have to account for extra loads in multiple areas of the building.

Lastly, composite floor decking can be cost-effective. It takes $1 per square foot to add FormLok embossment on the metal decks.

Which Metal Decking Should You Choose for Your Project?

You cannot decide the type of metal decking to use by yourself.

Usually, an engineer will tell you the type and size of metal decking that is required in your building.

He will draw up metal decking specifications based on code requirements.

Alternatives Ways to Increase Floor Strength

There are alternatives available that can be used instead of composite floor decking or in addition to composite floor decking to increase floor strength.

One way is to use heavier gauge metal decking in the floor system. Another is to increase the size of the secondary beams in the system.

Adding more rebars in the slabs also increases the strength of the floor significantly.

Additionally, the deeper the metal deck used, the stronger it will behave under loads.

Lastly, increasing the thickness of the poured concrete will raise the strength of the floor.

But the reason these options are avoided is that most of these methods end up adding additional weight onto the floor system.

This added weight then needs to be supported by the connecting beams, columns, and footings of the steel frames.

The problem grows when a building has multiple stories, which consequently end up adding even more weight to the columns and foundation.

The total sum of the loads thus being transferred onto the foundation can increase significantly by using these alternative methods.

This can cause the foundation to fail if the loads are not carefully calibrated beforehand.

Cost of Composite Floor Deck

The cost of metal decking depends on five factors.

This includes the cost of steel, the gauge of the decking, the type of metal deck, the finishing used, and the size or scope of the project. 

The cost of steel is the most influencing factor out of all the ones mentioned. It keeps fluctuating and should be checked every 30 days.

Custom cutting and delivery are also factors that might need to be considered while buying composite floor decks.

The cost of decking per square foot is listed in the table below.

GaugeDeck Weight (PSF)Price Per Square Foot
221.9$6.00 – $7.50
202.3$7.00 – $9.00
182.9$9.50 – $10.75
163.5$11.00 – $12.50

Price of Composite Floor Decks

What should be the Thickness of the Concrete in a Composite Deck?

The minimum thickness of concrete should at least be 2”, aside from the depth of the metal deck.

So, for example, if you have a 1.5” metal deck, then the total thickness of the composite deck after pouring concrete should be 3.5”.

Generally, the thickness of the slab used in the industry ranges from 3.5” to 7.5”

2" Composite Floor Deck
2″ Composite Floor Deck

What Type of Concrete Should Be Used in a Composite Floor Deck System?

There are normally two types of structural concrete that are used in composite decks.

One is the normal-weight concrete which weighs between 145 lbs to 155 lbs per cubic foot.

The other is the lightweight concrete which weighs about 110 lbs per cubic foot.

An engineer determines the type of concrete that your project should use.


Composite floor decks use metal decks that bond with the concrete to create solid and strong floor slabs.

Using this technique increases the strength of the floor without inducing extra loads on the floor system.

Composite floor decks are generally available in three sizes, based on their depth. These include 1.5”, 2”, and 3” composite decks.

Typically, G60 galvanized finishing is used on composite floor decks.

The cost of these decks varies based on their sizes, finishing, and the cost of steel, among other factors.

Utilizing composite floor decks is an excellent way to enhance the strength of floor frames in buildings without worrying about any repercussions. 

Frequently Asked Questions (FAQs)

Can you install composite decking over dirt?

Usually, composite decking is not directly placed on the ground.

This is because when laid in direct contact with dirt, it becomes susceptible to moisture damage and pest actions.

Composite decking requires some spacing between the ground to provide room for ventilation and for maximum soil containment efficiency.

However, there are cases where composite decking is placed directly on the ground.

What are the Demerits of Using Composite Floor Decks?

Composite floor decks are highly durable, but this makes them very expensive.

On average, composite materials cost 15 to 20% more than treated lumber.

Composite floors are not as attractive as fully wooden floors, which give off a smoother and sleeker finishing look.

Do composite decks need to be sloped?

Composite decks are installed with little to no slope.

The slope is usually provided if the deck has the potential to be exposed to moisture more frequently.

Sloping helps shed off the water, which may otherwise accumulate on the deck and start deteriorating the deck surface.

If a composite deck is not directly attached to any structure, then it is recommended to provide a certain slope in the deck.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com

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