Houses and other similar structures are made up of many different parts that make up the whole. There’s the foundation, the floors, the walls, the ceiling joists, the roof. What is a ceiling joist you ask?
Ceiling joists are horizontal structural components that usually run parallel to the rafters. They run from exterior support wall to exterior support wall, transfer the roof load to the walls, and prevent the walls from spreading. They fully span the distance but may be supported by beams or interior walls. They also have an aesthetic element in that they form the ceiling.
In this guide, we’ll explain what a ceiling joist is, what they look like, are made of, and why they run in a specific direction. We’ll discuss load-bearing aspects, different types of ceiling joists, layout, span, spacing, and size. Plus, we’ll identify how to find them using a stud finder or magnet, and finish off with a comparison of ceiling joists vs floor joists.
What Is A Ceiling Joist In Construction?
Constructing buildings involves age-old practices that have proven to work, and have been improved through engineering, modern technology, and building codes to be safer. Foundations support the whole structure while walls carry the floors and roof loads and transfer them to the foundation. The roof covers the whole and keeps out the weather. Ceiling joists in construction are an integral part of the structure and help tie the roof and walls together to keep everything from collapsing.
Ceiling joists are typically horizontal wooden structural members that are fastened to and span from exterior support wall to the opposing exterior support wall. They commonly run parallel to the main rafters, transfer roof loads to supporting walls, and help prevent the walls from spreading out or inward at the top.
They are often supported by interior walls or a central support beam and form the framework to which the ceiling is attached. They are typically required by building codes for a structure to be deemed habitable.
In residential construction, a ceiling joist often serves as a rafter tie. A rafter tie is fastened to and spans between opposing sloping rafters no more than a third of the horizontal distance up from where the rafters rest on the walls.
The ties keep the roof loads from splaying and flattening the rafters under normal loads. The ceiling joists rest on the opposing support walls and are fastened to both the top plates and the rafters to prevent spreading or being forced inward.
Engineered trusses are premade triangles that form the sloping roof deck and the ceiling. They typically have 2-by webbing material that connects the rafter and ceiling joist components and helps transfer the loads to the support walls. Premade and engineered means they are more accurately made, faster to erect, and often a money saver overall.
Ceiling joists may be designed to support only the ceiling, to support lightweight attic storage, or to support an attic living space. Different building code requirements must be adhered to depending on what the ceiling joists will be supporting. While the underside of floor joists also forms the framing to which a ceiling is attached, their primary role is different and has differing code requirements.
What Do Ceiling Joists Look Like?
Ceiling joists are usually hidden from view by the finished ceiling but may be left exposed for aesthetic reasons. They commonly look like stud wall framing laid horizontally across the tops of the wall.
The horizontal joists usually run parallel to each other and the rafters above. Spanning across the open space above and between the walls, they are fastened to the top plates to help hold the walls vertical and transfer the roof loads to the walls.
Ceiling joists form the framing to which the ceiling finish is attached. The joists may be one continuous board or be several lengths that overlap and are fastened at the ends to span the distance between opposing support walls.
While the ceiling joists help keep walls vertical, they also tie opposing rafter ends together. Thus, preventing them from spreading and flattening under loads and pushing the walls in or outward, causing the structure to collapse.
Ceiling Joist Material
Ceiling joists are commonly made out of softwood dimensional lumber but may be made from engineered wood, metal, or even reinforced concrete. Today, they typically are 2×4 or 2×6 lumber but may be larger dimensions depending on span, purpose, aesthetics, and the age of the building.
Ceiling joists that are left exposed are often larger in cross-sectional dimensions, rough-cut, adzed, or planed and sanded smooth, and may be oak, maple, cedar, pine, or other wood species.
Which Way Do Ceiling Joists Run?
Rafters and trusses usually run across the narrower width of a building. This typically means the ceiling joists run the same way as they tie the rafter ends together. However, irregular-shaped structures or those with mixed rooflines often have ceiling joists that run in different directions.
A hipped roof often has ceiling joists that are all parallel to each other, but some will have the joists under the hip ends running perpendicular to those in the middle of the structure. Gable and valley roofs on ‘L’ or ‘T’ shaped structures often have ceiling joists over the different sections running in opposing directions. Hexagonal, octagonal, and circular structures may have ceiling joists that run parallel to each other or follow the direction of the rafters and are spoke-like.
Are Ceiling Joists Load-bearing?
Flat, low slope, or high sloped roofs, whether rafter or truss, typically have a member or component referred to as the ceiling joist or tie. The ceiling joists help support the roof material weight and external roof loads, and fasten and hold the interior and exterior walls vertical, helping to prevent spreading and shaking or racking. They help transfer those loads to the walls, and thus to the foundation.
The term joist, in construction, usually means a horizontal load-bearing structural member, so ceiling joists are load-bearing.
Ceiling joists form the framework to which ceiling material is attached and also create the attic floor. As such, they must support the weight of the ceiling and any insulation, plus anything stored or people moving or living in the attic surface above.
The live and dead load capacity of ceiling joists commonly depends on its span and lumber dimension, plus its planned use – uninhabited and without storage, uninhabited with some storage, and inhabited living space.
Types Of Ceiling Joists
Wandering through different homes, whether physically, online, or in print formats, you’ve probably noticed some ceilings have exposed ceiling framing and others don’t. Exposed ceiling joists and those hidden behind ceiling finishes are the two main types of ceiling joists. They typically serve the same structural purpose but have aesthetic attributes better suited for certain styles of construction.
Exposed ceiling joists provide a rugged, rustic, or historical look to a ceiling. Renovated warehouses, distilleries, other old buildings, and even some modern construction leave the planks exposed for a rustic or industrial look. The ceiling joists function as designed, just provide an openness or modern vibe that is aesthetically pleasing to some.
One drawback and one our ancestors noted, is the exposed ceiling structure allows dust, dirt, and creepy-crawlies to fall down onto everything underneath. The exposed wood is also a dust magnet as it is often unfinished and rough-cut lumber.
It is also difficult to paint or seal, although modern paint spraying systems help. Another potential drawback is sound movement and echo, so although it may look great, it may not be ideal.
Hidden ceiling joists are used for their intended purpose and are covered with drywall, lathe and plaster, T&G lumber, or other material to provide a smooth, uniform finish.
The ceiling material also hides insulation, electrical, HVAC, and plumbing works, and is easier to paint or finish for a uniform look. It also provides another barrier to dust, dirt, and bugs. Those who prefer the exposed look, often add faux beams or joists to the finished surface of the hidden joists, giving them the best of both aesthetic choices.
Ceiling Joist Layout
Ceiling joists typically run parallel to the rafters, so perpendicular to the ridgeline of the structure. That means they commonly span the narrower dimension or width of the building. Hipped roofs, gable and valley roofs on ‘L’ or ‘T’ shaped structures, and hexagonal, octagonal, and circular structures are the exceptions. A quick look in the attic to see the rafter or truss direction should identify the layout.
Ceiling joists for rafter roofs are commonly laid across the top plates parallel and with the same spacing as the rafters, so every 12”, 16”, 19.2”, or 24”. The top plates are marked wherever ceiling joists cross to assist in alignment and fastened when placed. If the ceiling joist isn’t long enough to span the full distance, then it is spliced or lapped by at least 6” at a beam or interior wall – making the wall a load-bearing structure.
Roof truss construction connects the rafters and ceiling joist members with structural webbing, making roof construction easier. It also means that most interior walls below that ceiling aren’t load-bearing, so wall removal or placement is also easier. The ceiling joist layout will usually be the same as that of the truss layout too.
The underside of floor joists often has ceilings attached to them, and some people refer to them as ceiling joists. Their primary function, though, is floor support. So, if you are attempting to determine their layout and direction, it may be more difficult.
Floor joists often run parallel across the narrower dimension of a room, with different rooms having ‘ceiling joists’ running perpendicular to those over other rooms.
In homes built prior to 1970, the ceiling joists layout may be similar to the floor joists layout and span the shortest distance between walls. I’ve seen ceiling joists laid out in what looked like a checkerboard pattern that left me scratching my head trying to understand. In this situation, the ceiling joists weren’t structural to the roof; they just held up the ceiling.
Ceiling Joist Span
Ceiling joists, according to the IRC-2021, span across the structure either continuously or are fastened where they meet at the top plate of an interior wall. Spans that require more than one board to span the distance are spliced or lapped at least 6” where they cross over the top plate of an interior wall, making that wall a load-bearing wall. In some situations, the splice or lap occurs at a beam to increase the size of the open space underneath.
The span depends upon the species, grade, width, and height of the board being used for the ceiling span, and the spacing between the parallel ceiling joists. Plus, the live and dead loads it must carry, and how the space above it will be used – uninhabitable attics with or without storage space, or as habitable living space.
Based on all factors, a 2×4 ceiling joist can have unsupported spans of 5’-5” to 13’-2”, a 2×6 between 7’-11” and 20’-8”, while 2x8s and 2x10s can span 10’-0” or 12’-3” respectively, to more than 26’.
What Is The Standard Ceiling Joist Spacing?
Ceiling joists may be spaced 12”, 16”, 19.2”, 24”, or 48” apart. However, 16” O.C. (on center) spacing is the most common spacing today, with 24” being the runner up. Much depends on the distances being spanned and loads the joists must carry, but 16” spacing is adequate for most residential ceilings with limited or no storage.
Using 16” spacing also means there are four fastening points for 4-foot-wide ceiling material vs three with 24” spacing, resulting in less sag over time. While ceiling panels oriented along their 8-foot length will have seven at 16” O.C. vs five at 24” O.C. It takes more fasteners and more time to fasten a 4×8 ceiling panel to 16” spaced ceiling joists versus those spaced at 24” centers, which some argue is good and others that it is a waste – personally, I prefer 16” O.C. as it also matches up with most wall stud spacing too.
Ceiling Joists Size
Ceiling joist sizing is addressed in Section 8 of the International Residential Building Code (IRC). In the most recent version, IRC 2021, R802.5.1 identifies that ceiling joists need to be sized based on Tables R802.5.1 (1 and 2), or using AWC ST JR. Joist sizing depends on the wood species, grade, span, spacing, and the live and dead load values.
Typically, either 2x4s or 2x6s are used for ceiling joists as they span between 5’-5” to 13’-2”, and 7’-11” and 20’-8” respectively depending on the variables. Some builders prefer a uniform size and others will mix and match to fit the room sizes and reduce costs. However, for longer spans, 2x8s, 10s, and 12s may be used, or other engineered lumber or even metal joists or beams.
The size of the room the ceiling joist traverses also tends to be a determining variable. The joists need to reach from top plate to top plate and overlap with the next plank to form a continuous connection from exterior wall to opposing exterior wall. So narrower rooms may use smaller dimension lumber than rooms requiring longer spans. It is best to check with a Structural Engineer when determining ceiling joist sizing.
Ceiling Joist vs Floor Joist
Ceiling joists typically support only the weight of the ceiling material, insulation, and possibly some storage. The joists are commonly located 8’ to 10’ above the floor, and below the roof, to form the attic space. Their main function is to keep the walls vertical so they don’t lean in or outward, and to prevent the rafters from spreading and collapsing.
Ceiling joists support between 5 and 10 psf of dead load and 10 to 20 psf of live loads. The exception is if they are designed to support living space or heavier storage, in which case they must meet floor joist requirements. Ceiling joists are usually horizontal, but in some situations, they may be angled and have a slope.
Floor joists often support ceiling material and insulation, but their main purpose is to support living space, so they are rated for live loads of 30 or 40 psf or more. Floor joists are usually greater in dimension or depth too, as they have to support greater loads. Additionally, floor joists are supposed to be laid horizontally or level, although time and poor workmanship can result in floors that aren’t.
How To Find Ceiling Joist
The easiest way to find the ceiling joists is to look up. If they are exposed, they’re easy to find. If they’re hidden, there are some options that will help locate them. Climbing up into the attic with a flashlight and tape measure might illuminate their location and identify their spacing. It should be noted though, that ceiling joists that overlap don’t necessarily align.
Instead of climbing up into the attic, grab a chair or step ladder (safety first!), tape measure, and pencil. Stand on the ladder so you can reach the ceiling. Tap along the ceiling and listen to the sound. The open space between joists will make a hollow sound and striking a joist a more solid sound. Tapping along the solid-sounding space will hopefully identify the direction of the joists
If there is a light fixture, tap around it to locate the joist to which it is attached, provided it’s not fastened to blocking. Alternatively, loosen the fixture and lower it to see if it’s possible to visually determine the attachment location and verify if it is a joist. Measuring and tapping may help locate a parallel ceiling joist.
Measure over 12” and tap again, if the sound is hollow, move to 16”, if it still sounds hollow try 19.2” and 24”. Insulation between the ceiling joists can also make it difficult to hear a hollow sound. Hammering a fine finishing nail is an alternative to tapping, but can result in a perforated ceiling! If the tapping method doesn’t work, try a stud finder or magnet.
With Stud Finder
Rent, buy or borrow a good quality stud finder or download an app and follow the instructions to locate the ceiling joists.
A quality stud finder can differentiate between insulation and framing to locate studs.
So, using it on the ceiling is no different, but use a step ladder to make sweeping it across the ceiling easier.
Tap a fine finishing nail into the ceiling where the stud finder indicates a joist is located to verify location and accuracy.
Use a strong magnet to locate the heads of screws or nails that hold the drywall in place. Stand on a ladder and begin at a wall or corner. Slowly sweep or glide the magnet over the ceiling.
Fasteners are typically 7” to 8” apart, so use a pencil to mark the location and direction the joists travel and the spacing between joists based on the distances. To prevent marking the ceiling finish, put the magnet in a plastic bag. It will still allow the magnet to stick to the metal fastener but not scratch or mark the ceiling.
Ceiling joists provide both a functional and aesthetic purpose. They are typically horizontal structural members used to keep walls vertical and to help keep rafters from spreading, causing structural failure and collapse.
They fasten to the top plates of interior and exterior walls to hold them level and prevent them from leaning in or outward. Additionally, they provide the framing to which ceiling material can be fastened. Hopefully, you have a better understanding of what a ceiling joist is, what it does, and how to find them.