The appeal of wood is undeniable as it has tremendous aesthetic appeal, is environmentally friendly, and is sustainable. Thus, it is no wonder many people consider utilizing wood beams for their ceilings. However, wood’s combustible nature can make you wonder whether wooden ceilings are a fire hazard.
Wooden ceilings are not a fire hazard depending on the type you use. Hardwoods and engineered wood burn extremely slowly, thanks to their exceptional density, making them incredibly fire-resistant. Fire-retardant wood is also not a fire hazard since it has been treated to limit the fire spread rate.
Read on to learn more about what happens to wooden ceilings in a fire and the ideal wood types for a fire-resistant wooden ceiling.
What Happens To Wooden Ceilings in a Fire?
The beauty of wood, timber, or lumber is that it burns in a predictable fashion. In fact, wood does not burn; it combusts (more on that later).
The rate of wood combustion depends on a few factors, namely:
- Wood type – different wood types have unique densities, affecting their flammability
- Fire-retardant treatment – fire-retardant woods have chemicals that slow down the burn rate
- Environmental conditions – the amount of oxygen and secondary fuels in the environment determine the burn rate
Nevertheless, as mentioned, all lumber follows a predictable pattern when exposed to fire.
When wooden ceilings catch fire, the wood starts breaking down in a process known as pyrolysis. Pyrolysis occurs in three main stages, which we’ll discuss better in the next section.
However, the wood breakdown process due to fire starts with dehydration, which allows the wood to burn properly. The dehydration process then decomposes the outermost layer of the wood, leading to the release of gases such as carbon monoxide and carbon dioxide.
The resulting coat of char acts as an insulator, significantly reducing the burn rate and thus protecting the inner layers of the wood. Consequently, the wooden ceiling retains its load-bearing capacity for a longer time, allowing for intervention by firefighters.
The above is a summarized version of what you would expect when a wooden ceiling catches fire. However, I think an in-depth breakdown of wood pyrolysis is crucial since it will allow you to make an informed decision when assessing the fire risks of different ceiling types.
Wood Pyrolysis – How Wood Burns
To reiterate, wood does not burn; it decomposes to produce heat, gases, and unburnt carbon compounds (ash). Burning and decomposition are both forms of combustion, with the fuel source being the main difference between the two.
Burning involves the combustion of highly-flammable fuels, such as propane, to produce light, heat, and carbon dioxide. In burning, the fuel typically goes through complete decomposition due to its high flammability.
In contrast, decomposition involves raising the temperature of a material with low flammability, such as wood, to produce heat. However, since wood is solid, it must first go through a series of steps before it burns readily, acting as fuel to the fire.
All wood species have the same composition: carbon, hydrogen, and oxygen. The carbon in wood consists of lignin, cellulose, and hemicellulose. These compounds decompose at different temperatures, explaining why wood pyrolysis occurs in three main stages.
Stage One of Wood Pyrolysis
This stage involves the dehydration of wood to allow for efficient combustion. All wood contains moisture, explaining why you are advised to use seasoned firewood in bonfires or fire pits. Wood seasoning is the process of dehydrating lumber to make it easier to burn.
As you can imagine, we don’t use seasoned timber in construction since we don’t want the structure to burn easily.
As such, since the wood used in ceilings has a significant amount of moisture in it, the first step of pyrolysis will involve attempting to dehydrate the lumber completely. Complete dehydration occurs at around 320 degrees Fahrenheit (160 degrees Celcius).
When temperatures get to 392 degrees Fahrenheit (200 degrees Celcius), the hemicellulose component of wood begins to decompose, releasing carbon monoxide and carbon dioxide.
Stage Two of Wood Pyrolysis
This is where the wood starts to combust more rapidly, producing heat and unburnt volatile gases, such as methane.
Stage two of wood pyrolysis typically occurs between 536 and 608 degrees Fahrenheit (280 and 320 degrees Celcius). However, this will depend on the type of wood since wood experiences decomposition at different rates depending on density.
Nevertheless, this is when charring occurs—charring results from the incomplete decomposition of the wood’s cellulose. The char consists of unburnt solids covering the burning wood’s surface.
Char formation is the main reason wooden ceilings can remain stable for extended periods while the structure is on fire. The char acts as an insulator, preventing the flames from accessing the wood’s inner layers, thus reducing the rate of decomposition significantly.
Stage Three of Wood Pyrolysis
This stage occurs when temperatures exceed 608 degrees Fahrenheit (320 degrees Celcius). Here, the remaining carbon component – lignin – begins to decompose. If the fire is not dealt with effectively at this stage, your wooden ceiling might collapse due to losing its structural integrity.
Fire-Resistant Wooden Ceilings
As you can see, wood burns predictably. Therefore, if you can control the rate of decomposition, you can have a wooden ceiling that can withstand high temperatures for prolonged periods before collapsing.
Ideally, the wood used in ceilings should be specially chosen for its fire resistance, i.e., decomposes slowly.
You typically have three options when choosing fire-resistant lumber for your ceiling:
- Engineered wood, such as Cross Laminated Timber (CLT)
- Fire-retardant wood
Let us look at each of these options in a little more detail.
Using Hardwoods for Wooden Ceilings
Hardwood species, such as mahogany, oak, and azobe, are called so for a reason; they are exceptionally dense.
Most hardwoods are much denser than softwoods because they take a lot longer to mature.
Hardwood trees can take over 150 years to mature, whereas softwoods rarely need more than 40 years. Consequently, hardwoods have a more compact cell structure, resulting in greater density and, thus, higher strength and durability.
As you can imagine, the compact nature of hardwood lumber means that it decomposes extremely slowly. Moreover, its superior sturdiness makes hardwood timber a good candidate for load-bearing structures, such as ceilings.
However, their benefits notwithstanding, hardwoods are notoriously challenging to work with, explaining why they are only used for exterior structures. Hardwoods are also costly.
Using Cross Laminated Timber for Wooden Ceilings
Cross-laminated timber (CLT) is an excellent choice for wooden ceilings. CLT is a type of engineered wood made from stacking several layers of solid-sawn timber and gluing them together.
The idea behind CLT is to take advantage of the greater structural rigidity resulting from combining the individual structural integrities of several pieces of timber.
One of the benefits of this design is its inherent fire resistance. As discussed earlier, the density of wood determines its ability to remain stable when exposed to fire. Since CLT consists of several layers of lumber, it is thick by design, allowing it to resist decomposition for lengthy periods.
Each layer of lumber in CLT burns individually, meaning the decomposition process will take a lot longer since the fire has to decompose one layer entirely before moving on to the next.
Consequently, CLT wooden ceilings can maintain their load-bearing capacity for extended periods under extreme temperatures.
Another benefit of using CLT is its customizability; you can choose the number of layers you want your CLT ceiling to have.
Using Fire-Retardant Wood for Wooden Ceilings
Fire-retardant wood refers to lumber that has been treated with fire-retardant chemicals to increase its fire resistance.
There are two ways to make fire-retardant wood:
- Using a high-pressure system to infuse fire-retardant chemicals deep into the wood’s internal structures
- Applying a coat of fire retardants on the wood’s surface
It is crucial to note that the International Building Code (IBC) does not consider fire-retardant coatings safe. According to the IBC, for lumber to be considered fire-retardant, it must be impregnated with fire-retardant chemicals via a high-pressure system.
Therefore, only use pressure-treated fire-retardant wood for your ceiling.
Fire retardants are typically proprietary chemicals that stall the spread of flames and encourage charring. They do that by neutralizing volatile gases produced by wood when it’s undergoing combustion.
Additionally, fire-retardant wood actively resists ignition, thus increasing the time it takes for the wood to decompose significantly.
Perhaps the greatest selling point of fire-retardant wood is that it must be tested by third-party labs and awarded a certificate of clearance before it is released to consumers. Third-party testing ensures that the wood actually meets the required standards to be considered fire-resistant.
Therefore, you can take all doubt out of the equation by simply using fire-retardant wood, as it has been specifically designed to resist fire. But to be on the safe side, you should do your due diligence to ensure that you buy third-party tested (and certified) fire retardant wood.
If you’re interested in building safety, you may also want to know if you can use drywall anchors in wood. Take a look at my guide on this subject. [Can You Use Drywall Anchors in Wood?]
You can enjoy the beauty and warmth of a wooden ceiling without worrying about it being a fire hazard. However, you must use the correct wood. Softwood is not advisable for use on ceilings as they lack the compact cell structure that hardwood has.
Wooden ceilings should be made of fire-resistant lumber to ensure they retain their load-bearing capacity for prolonged periods when on fire. The ideal wood for ceilings should also be challenging to ignite, leave alone combust.