Ceiling joists play a critical role in the structural systems of homes. And, there is a lot of confusion surrounding the function of these particular joists.
Logically, we could ask a contractor what the difference between a ceiling joist and a floor joist is. The answers you will receive may all be different. And, they are all probably correct. But, this also shows us the “gray area” around floor joist vs. ceiling joist.
So, what exactly is the function of a ceiling joist? How is it different from other structural framing members? Is it different from a floor joist? Are they used in attics with trusses and rafters?
If these are questions you are seeking answers to, read on.
We will give you our opinion on the subject from a structural engineering point of view.
What is a Ceiling Joist-
Ceiling joists are the horizontal structural members that span your ceiling and are responsible for transferring roof loads to vertical members. Without these members, your home would cave in.
You can think of homes and buildings like the game Jenga; when loads are unbalanced, structures fall down. Thankfully, structural engineers ensure this doesn’t happen.
They analyze lumber type, lumber grades, board sizes, live and dead loads, distance spanned and joist spacing while performing detailed calculations to design the perfect ceiling joist system for the project at hand.
Once the framing is finished, ceiling joists are installed by a contractor via spanning them in between walls and securing them with wall plates. Structural engineers determine how and where each ceiling joist is placed to ensure it meets safety standards and building codes.
Note: Be sure to check into your area’s building regulations. Below is an example of attic construction regulations (including ceiling joist construction) from the 2020 Minnesota Residential Building Code.
How far apart should ceiling joists be spaced?
Again, it’s the structural engineer that decides how far apart each ceiling joist is spaced. Since each framing member is part of a larger picture, the engineer must use a “whole home” approach to designing the bones of the building.
Now, there are some general guidelines that many homes adhere to when it comes to ceiling joist spacing.
A typical room, either 10×15’ or 10×17’, using a standard 2×6-inch lumber (supports 5’x8’ sheetrock) would most likely be spaced 16 inches on center, meaning 16 inches apart.
Larger rooms, such as great rooms, will require different spacing and lumber. You could expect a room spanning 16-20 feet to use 2×10-inch lumber and space joists 24-inches on center. A room spanning greater than 20 feet (when running parallel to the rafters) may need 12-inch joist spacing and 2×10’ lumber such as douglas, hemlock fir or southern pine.
In general, the larger the room span length, the more load carrying capacity that is needed. This means larger lumber or shorter joist spacing. Look into your area’s building regulations and be sure to consult a structural engineer. Click here to learn more about joist span regulations in Minnesota.
What size ceiling joist for a 20 foot span?
So, there is no “one size fits all” answer for this question. Every single project is unique. Factors like snow load, materials used, building requirements and other project conditions must be factored in.
This is why we have architects and engineers. The architects create the blueprint, the engineers make it work. To know if your new construction build needs 2×10 lumber spaced at 24” on center for that 20’ span (which could be common), get your structural engineer involved. This should be dictated on your building plans.
The other benefit to this is that structural engineers will build for what you need. They won’t “over build” and drive material and labor costs up.
How is a ceiling joist different than a floor joist?
Since this question seems to have a lot of grey area surrounding it, we asked our engineers what they thought. This is what they said.
“Floor joists and ceiling joists are very similar. Think of it like this, floor joists not only support the floor, but will also support the ceiling, while a ceiling joist may not support a floor.”
So, what does this mean?
Well, floor joists are located underneath any floor you walk on. This may include main floors of a home or building and possibly the attic floor IF it was designed to support a living space.
Not all attics were designed to support dead (building materials) AND live loads (people and furniture).In contrast, ceiling joists are installed to frame in a ceiling and support drywall underneath, not support the live loads that a floor joist would hold.
They are typically found between the top floor and the attic. Unless initially designed for, attics are not designed to have a floor.
What are the primary functions of a ceiling joist?
We know that ceiling joists are responsible for transferring loads from roof to vertical members like wall framing. Without ceiling joists, your home’s walls could crumble from the pressure of the roof. Beyond this very important task, they also have a few other functions.
Ceiling joists help to hold the walls of a room together. Without ceilings joists your walls may bow outwards because of roof load pressure.
Ceiling joists provide a structure to attach a ceiling to. At the basic level, ceiling joists provide something solid for a drywall (or other) ceiling to be attached to. The ceiling joist will be designed to carry any ceiling load.
Remember, if your attic is to be used as a living or storage space, let your structural engineer know. This is really important information for them. The joists will need to be beefed up to accommodate the additional live and dead loads.
Can I cut a ceiling joist?
There are circumstances that may require cutting or notching a ceiling joist.
Some of these reasons could be:
- Running electrical wiring
- Plumbing pipes
- HVAC ductwork or vents
- Fire sprinkler systems
- Data or low voltage cabling
- Retrofitting or renovation work
When renovations happen, access through joists may be needed to connect new systems to existing infrastructure.
We get it.
However, it is important to realize that anytime an alteration is made, the wood fibers lose their ability to withstand compression and tension. What does this mean? Well, it could lead to a saggy ceiling, joist failure, ceiling cracks, or door sticks.
We recommend that you always reach out to a structural engineer prior to making any structural changes including joist notching and boring. There are standardized regulations and guidelines for how big and what locations can be notched or bored.
Beyond this, homes are unique and your structural engineer’s instructions for boring/notching on your home may vary beyond traditional guidelines/regulations. This all depends upon how your home’s structural support system was designed.
That said, listed below are some basic guidelines in compliance with the International Residential Code (remember, every circumstance is unique so don’t try this without a licensed structural engineer’s opinion):
- Always avoid making notches in the middle third of the joist (holes are acceptable here)
- Notches should never be larger than ⅓ of the joist’s depth
- Holes cannot be within 2 inches of the joist’s edges (top & bottom)
- All holes or notches must be more than 2 inches away from each other
- Notches have to be less than ⅙ the joist depth.
- End notches must be less than ¼ the joist depth
- Notch length must be less than ⅓ the joist depth
- Avoid square/rectangular cutouts, cuts and notches.
- Large beams, anything over 4 inches thick, can only have notches at the ends of the beam
Click here to learn more.
How to find a Ceiling Joist –
Above we discussed common spacing for ceiling joists is 16 or 24 inches on center. This knowledge makes it fairly easy to locate joists.
Here are the steps:
- Grab a tape measure and place the end in the corner of the room. Lengthen the tape measure to 16 inches first.
- Knock lightly on the ceiling (at 16 inches) and listen to the sound. A hollow sound indicates empty space while a solid sound indicates a joist. If you are uncomfortable with your judgement, you can use a stud finder here.
- If you are not finding a joist at 16 inches, lengthen your measuring tape to 24 inches from the corner of the room. Repeat the steps above.
Note: Sometimes ceiling joists are exposed, making them easy to locate.
How does a ceiling joist differ from a beam?
Beams and joists are both horizontal structural members that extend over long areas, right? What makes them different?
Beams are a home’s main structural support system and are held up by walls, a foundation or posts.
Because they are responsible for carrying most of the home’s weight, they are typically quite large in size and can be made of steel, engineered wood or other types of lumber. Engineers are responsible for sizing the beams to ensure they are strong enough to carry all the loads above them.
Note: Remember the material the beam is made with comes into play. Some materials are stronger than others meaning the beam may not appear dramatically larger than joists.
For instance, steel i-beams are often used in modern construction as the main support beam in a basement. Because steel is much stronger than wood in both compression and tension, it can support heavier loads and longer spans with a smaller cross-section.
This is why there is no head room in the basements of old homes. Steel i-beams were not being utilized yet, so wood beams were used to support massive loads in the home. This meant the beams had to be quite large which did take away some serious ceiling height.
Engineer’s place beams under the most important load bearing areas of a home. For example, it’s essential to have a support underneath your second story rooms. If load bearing walls are not placed on the first floor to support the second floor, a beam will be necessary.
Joists, on the other hand, are much smaller than beams and there are a lot more of them. Technically, they could be considered a type of beam although they will never be found alone the way beams are. Joists are spaced evenly and consistently, typically 16” or 24” apart across a broad surface.
They will either rest on top of a beam or be connected to the side of a beam. In the case of ceiling joists, they help to transfer loads from the roof to vertical members, prevent walls from crumbling and also provide a structure to attach the ceiling to. Floor joists are built to support the floor and any live/dead loads that will be placed on them.
Are ceiling joists used in attics with trusses and attics with rafters?
The simple answer, yes. However, their function could differ slightly. For example, an attic with trusses does not leave much room for usable storage space and certainly not for livable space.
With that being said, the ceiling joists will have to carry less weight than some attics with rafters. This is because attics with rafters are more likely to be converted to a living space and the ceiling joists would have to account for those live loads hence a beefier joist.
If you have an attic with trusses, you probably won’t be opening your attic up any time soon. I say this because it’s extremely expensive from an engineering perspective to alter trusses. It’s also very labor intensive and time consuming.
It can be done. It just comes down to if you want to spend the money to do it.
Pro Tip- Don’t ever try to cut into a roof truss without first consulting a structural engineer. We’ve seen the damages and it’s not pretty or cheap to fix.
Attics with rafters offer more flexibility. The attic space could be turned into a master bedroom or storage area. This is the same for garage conversions. Because of this, structural engineers would ensure the ceiling joists were designed to account for the additional live and dead loads.
Typically older homes use rafters. Most modern homes use trusses simply because they are cheaper, very efficient and easy to install. However, there are times when rafters have to be used. For example, mountainous areas may present a challenge for transportation of trusses. It may only be possible to ship rafters in. Of course, if a homeowner plans to use the attic as a living space, rafters would be the best option.
Can ceiling joists hold weight?
As you know by this point in the blog, ceiling joists are designed to hold up ceiling materials (drywall & lights) and help to stabilize walls.
They were not meant to carry additional loads unlike floor joists.
If you plan to hang something from a ceiling joist such as: punching bags, hammocks, gym equipment, or water tanks, consult with a structural engineer.
There’s a solution to make your vision work, you just have to make sure the structural support system can handle the task you’re throwing at it.
A structural engineer can design a simple and functional solution, such as reinforcing your ceiling joists, to ensure no structural damage occurs long term from your desired weight modifications.
Can a ceiling joist hold a punching bag?
Now this question is a bit random, but we’ve gotten asked it so many times that we thought it should be included here.
Your ceiling joists may be capable of holding the weight of a punching bag.
If you read the section above, you probably already know what we are going to say. Ceiling joists were designed to carry the weight of the ceiling and not to carry live loads like people or furniture.
Therefore, if you’d like to hang a punching bag, you must consult with a structural engineer to determine if your ceiling joists need to be “beefed up” or reinforced.
A few things to consider are the long term effects of the punching bag jolting and banging on your ceiling joist. This can weaken the framing member. If you do not appropriately install the punching bag it could cause structural issues with the joist.
Our firm was asked to structurally engineer an existing commercial space that was intended to be used as a gym. The existing ceiling joists were not adequately engineered to support multiple punching bags. We performed the inspection, analysis and engineering necessary to “beef up” the joists and make them structurally sound.
Whenever you increase loads in a building whether that’s by hanging a punching bag, installing granite counter tops, adding a large fish tank or other, call a structural engineer first. It’s always more affordable to analyze it on the front end than it is to repair it on the back end.
Well, that’s it. We hope you have a better understanding of all that is ceiling joists after reading this blog. If you have any specific engineering questions about your ceiling joists, please don’t hesitate to reach out to us at 763-544-3355.
