Selecting building materials is a key part of planning, designing, and building anything. Familiarity with different structural materials can improve the speed and cost of a build and affect design variables. Floor trusses vs floor joists are a case in point. Both are used to support floors, but they also impact important design variables. If you’re wondering what the differences are and which is better for your build, we’re here to help!
The wider and deeper the truss or joist, the further it can span. Floor trusses have an open web design of triangle-forming vertical and diagonal struts between the top and bottom flange. They are stronger and can span further than dimensional lumber and I-joists. Floor joists are solid between the top and bottom surfaces of the support, so have to be drilled or cut for plumbing or electrical services.
In this guide, we’ll explain what floor trusses and floor joists are, identify the different types of each, and discuss their differences. We’ll review truss and joist sizes commonly used in residential construction, look at the advantages and disadvantages of each, and which is better. Our aim is to provide you with the information to make the best decisions for your project.
Contents (Jump to Topic)
- What Are Floor Trusses?
- Types of Floor Trusses
- What Are Floor Joists?
- Types of Floor Joists
- Floor Trusses vs Floor Joists: Key Points
- What Is the Difference Between Floor Trusses and Floor Joists?
- What Are Standard Floor Truss Sizes?
- Floor Joist Size in Residential Construction
- Advantages and Disadvantages of Floor Trusses
- Pros and Cons of Floor Joists
What Are Floor Trusses?
Floor trusses are designed, engineered, and manufactured floor support members. They have diagonal and vertical components that form triangles sandwiched between horizontal top and bottom chords or rails. The triangle webbing components are commonly fastened to the rails using metal connector plates, but may also include adhesives and nails too.
Typically manufactured from readily available 2×3 or 2×4 lumber, open web trusses are an efficient and economical use of materials. The top chord provides a wider support surface for subfloor sheeting and the bottom for fastening ceiling finishes. The webbing of triangles within the two rails is engineered to transfer the loads to bearing points.
Trusses are stronger and able to span further than standard dimensional lumber or I-joists. Another type is steel floor trusses which are similar and have an open webbing of triangle-forming struts or sections between the top and bottom wood or steel flanges.
Floor trusses offer four common bearing designs for different load distributions. There are top-chord bearing and bottom-chord bearing trusses, mid-block bearing trusses, and bottom-chord bearing trusses with trimmable ends. Top chord trusses have a double top plate at the ends so they can be recessed or supported on beams or walls.
Bottom chord trusses are designed to rest on bearing walls and those with trimmable ends can have up to 6” cut off at each end. Mid-block trusses raise the top chord higher above bearing points. The different designs make it easier to match existing floor heights and allow for greater design variables.
Floor trusses can span further and require fewer intermediate supports, reducing construction costs and creating larger open spaces. The open webbing also makes it easier to install electrical and communication cables, plumbing, and HVAC. Additionally, they can accommodate loads, such as heavy slate or marble tiles, kitchen islands, pianos, marble, quartz, or concrete countertops, and high traffic.
Types of Floor Trusses
There are different types of floor trusses based on the pattern or configuration of the web members. Each type has specific strengths for bridging or spanning distances between bearing points. They all use smaller dimensional lumber to maximize strength and minimize material and labor costs, as well as reduce the weight of the truss. The two most commonly used in residential construction are the Pratt truss and the Warren truss systems.
Pratt trusses were originally used for railway bridges and have been in use since 1844. The placement of diagonal and vertical web members within the top and bottom chords can be adjusted in the design and engineering phase to support greater vertical loads over different spans.
The vertical components are in compression and the diagonal pieces are in tension, allowing vertical forces to transfer to bearing points. The Pratt configuration makes an effective and efficient floor truss design.
Warren trusses were first used around the same time as the Pratt design and for the same purpose. This truss design utilizes equilateral triangles instead of vertical and diagonal components to evenly spread loads. So, it is better for evenly distributed loads than point or concentrated loads, such as a piano or island.
What Are Floor Joists?
A floor joist is a horizontal structural member carried on foundation walls and other structural points or beams. It is used to form and carry floors, support other structural components, and transfer loads to the foundation.
Depending on the material used for floor joists, they can span different distances to or from foundation walls, intermediate beams, or other support points. Joists are solid structural members that transfer loads to vertical supports or walls.
Floor joists form floors and are typically larger dimensional lumber, engineered lumber, or metal. Joists distribute structural loads and applied loads. When wood fibers at the top of wooden joists are compressed by loads, those at the bottom go into tension, helping to evenly transfer or distribute loads.
The distance a joist can span directly affects the dimensions of the open space created underneath them. The species of wood, grade, and dimension, plus the type of joist and span requirement, and other load and design factors, all affect the span. Joists are commonly spaced 12”, 16”, or 24” on-center (OC) apart.
Types of Floor Joists
There are different types of floor joists available on the market. Some span further, carry greater loads, squeak or creak less under loads, or cost less. Knowing what is available can be a big help. Some common choices are dimensional lumber, LVL joists, I-joists, and steel joists.
Softwood trees that are cut, milled, and planed into dimensional lumber are the least expensive joist option. Dimensional lumber bending strength is determined by wood species, grade of lumber, and the width from top-to-bottom, which combined affect the span, spacing, and permissible loads. Most building codes require #2 grade or better for use as floor joists, but since Select and #1 are more expensive, #2 is more commonly used.
Southern pine and Douglas fir have the greatest bending strength of common dimensional softwood lumber. They can span further than hemlock, spruce, and other pines and firs with medium bending strength. Common dimensional lumber used for floor joists are 2×6, 2×8, 2×10, and 2×12.
A 2×12 has double the wood of a 2×6 and can span about 80% further. The greater span could reduce costs for intermediate support beams and increase span distances for open spaces. However, it also costs almost 3-times as much, so it’s important to calculate savings based on span vs additional support points.
Laminated Veneer Lumber or LVL for short is an engineered wood often used as an alternative to lumber joists. LVL joists have a horizontal top and bottom flange made of LVL lumber, like thick plywood. So, it has a cross-sectional ‘I’ profile. The vertical web is typically plywood (LVL) or OSB, resulting in a straight, strong joist.
The depth of the web and width of the flanges affects its strength and usually determine its span potential. An LVL beam is commonly thicker and made of multiple thin layers or plies of veneer, and doesn’t have a top and bottom flange. Narrow LVL beams can also be used as joists in a similar manner to dimensional lumber too.
Cold-formed steel (CFS) joists used in single-family residential construction come in a variety of different configurations. Commonly cold-formed from a single band or strip of heavy gauge steel to look like an I-joist. The web can be punched to allow HVAC, plumbing, and cables to pass through. Steel joists are very strong, support heavy loads, are insect and mold resistant, and don’t creak like wooden joists.
I-joists typically have a top and bottom 2×3 or 2×4 horizontal flange separated by a vertically oriented strip of plywood or OSB web, forming a cross-sectional ‘I’ profile. Each flange has a grooved side into which the web is inserted and fastened.
The flanges may be made of solid wood lengths, shorter finger-jointed lengths, or laminated wood. They are engineered to span further than dimensional lumber and are a more economical use of wood. The wider flange also makes fastening sub-floor sheathing and ceiling finishes easier.
Floor Trusses vs Floor Joists: Key Points
Floor trusses and floor joists span between support walls and/or beams to support and create floors and open spaces. However, they differ in how they are designed and engineered to perform their structural purpose. The table below compares floor joists and trusses to demonstrate the similarities and differences based on key points.
|Floor Trusses||Floor Joists|
|Design||Engineered with a top and bottom chord with an open web of triangle-forming vertical and/or diagonal struts.||Tested or engineered members that are solid or closed between the top and bottom.|
|Material||2x3 or 2x4 dimensional lumber with barbed steel connector plates fastening wooden struts or all metal struts.||-2x6 through 2x12 dimensional lumber of #2 grade or better
-finger jointed 2x3 or 2x4 or LVL, OSB, and adhesive
-LVL, plywood or OSB, and adhesive
-18, 16, 14, or 12 gauge cold-formed steel
|Complexity||Higher complexity of design, engineering, and manufacturing||Simple to more complex requiring engineered design specs|
|Load-Bearing Capacity||Engineered and manufactured to support all necessary loads from 35psf to 60psf or more||Milled or engineered and manufactured to support all necessary loads from 35psf to 60psf or more|
|Strength||Determined by the depth and width of lumber used and the geometry and spacing of the struts, plus the metal connector plates.||-Dimensional lumber strength depends on the depth, width, wood species, and grade of lumber.
-LVL and I-joists strength depend on depth and width, and the quality of adhesives used.
-CFS strength depends on depth and width, and the gauge of metal used.
|Stability||Greater stability.||Good to great stability.|
|Spacing||12”, 16”, 19.2”, or 24” OC with 16” and 24” being most common.||12”, 16”, 19.2”, or 24” OC with 12” and 16” most common for some types of joists, and 16” and 24” for others.|
|Span||Depends on a multitude of factors, but can span up to 60 feet.||-Dimensional lumber up to 25’-7”
-Depending on all factors, LVL up to 60 feet, I-joists up to 40 feet, and CFS joists up to 30 feet.
|Expertise||Engineered design and layout, and fabricated by skilled carpenters.||Select wood species, grade, and dimension as per drawings, so reasonably simple. I-joists, LVL joists, and CFS joists require more expertise to manufacture and select.|
|Installation||Installation by professionals or skilled DIYers.||Installation by skilled DIYers or professionals.|
|Best Uses||Best to create large open spaces, run utilities, heated floors, support kitchen islands, heavy floor and countertop finishes, and for high traffic locations.||Dimensional lumber is easy on the budget, readily available, and easy to install. LVL and I-joists both create larger open spaces and support heavier loads. Steel joists are ideal for damp areas or those susceptible to insect, termite, and rodent damage.|
|Cost||Floor truss costs depend on numerous variables and range from $7 to $18 a linear foot.||-Dimensional lumber, depending on all variables, ranges from $1.40 to $3.25 per linear foot.
-I-joists range from $5 to $10 or more a linear foot.
-LVL floor joists range from $5 to $24 a linear foot.
-CFS joists run $5 to $20 a linear foot.
What Is the Difference Between Floor Trusses and Floor Joists?
Floor trusses and joists are used to span distances and form a key part of the framework upon which floors and structures are built. Their differences affect important design, engineering, and construction variables, so awareness of those differences is helpful.
Floor trusses are designed using horizontal top and bottom chords that sandwich an open web of triangle-forming diagonal and vertical members, all fastened together using steel plates. They are designed to support greater weight over longer spans and transfer loads to vertical support components.
Floor joists tend to be closed between the top and bottom and span shorter distances. Solid wood joists are an age-old application of timber that supports loads over set distances based on wood species and dimensions, and a lot of testing. LVL and I-joists are designed, engineered, and manufactured to support loads over greater distances than natural lumber, and are also solid between the top and bottom.
Dimensional 2x3s and 2x4s are commonly used to manufacture floor trusses, although larger dimensional lumber is used in some situations. Barbed steel plates are used to fasten components together. Some trusses are a mix of horizontal and vertical wood components and diagonal metal struts. Steel floor joists can be formed from cold-formed channel members or heavier more solid components.
Wood joists are typically 2×6, 2×8, 2×10, or 2×12 #2-grade lumber from a variety of softwood species. The stronger the species, the further it can span based on different factors. I-joists use finger-jointed or solid 2x3s or 2x4s for the top and bottom flanges and OSB or plywood for the web.
LVL joists use thin layers of veneer from softwood to form varying thicknesses and depths of joists. Both I-joists and LVL rely on adhesives to bond layers or components together. Steel joists are fabricated from cold-formed heavy gauge steel.
Trusses are more complex than common wooden joists as they are designed and engineered to span specific distances and support calculated loads. The dimension of the chords and web components, cut angles, and spacing between chords and web parts are determined computationally for different distances and loads.
Joists are comparatively simple by comparison, even engineered LVL and I-joists, although more complex than a simple joist don’t have the complexity of trusses. Most blueprints include the Engineer’s specs for the type of floor joist or truss based on anticipated spans and loads. However, the most complex part for the builder or DIYer is the ordering and installation.
The load-bearing capacity depends on the span and spacing between joists or trusses. Loads can be calculated or set for dead loads of 5, 10, 15, or 20psf (pounds per square foot) and live loads of 20, 30, or 40psf, or more. Joists and floor trusses must be able to support the anticipated combined structural loads, plus all external forces that may apply.
The depth and width of the support member along with wood species and grade determine its strength and load-bearing capacity. Floor trusses have greater load-bearing capacity than LVL joists, I-joists, and solid wood joists. CFS floor joists have greater strength-to-weight than wooden joists but are not as strong as floor trusses.
The strength of floor trusses is determined by their depth, width, geometry, and spacing, plus the metal connector plates. The strength of dimensional lumber joists is based on width, depth, wood species, and grade. LVL and I-joists derive their strength from their width and depth, the adhesives used to bond components or layers together, and the type of material used for the flanges and web. Floor trusses are stronger than joists.
Floor trusses offer greater stability than joists. All types of floor joists and trusses, though, require blocking, cross-bracing, or strapping to provide lateral stability. For joists that stabilizing component may be every 8ft or midway between bearing points.
For trusses, the location for strapping is typically identified on the structural drawings. Stabilizing components are often checked by the building inspector before signing off on that stage of the build, so it’s important not to skip them.
Spacing of floor joists and floor trusses is center-to-center, or on-center (OC). Joists can be spaced at 12”, 16”, 19.2”, or 24” OC apart, with 16” OC being the most common spacing. The OC spacing for trusses is the same as that for joists, with 16” and 24” OC being more common.
The spacing of either type of floor support greatly depends on the subfloor thickness and planned floor finishes. The wider the spacing, the thicker the subfloor material. Spacing of 24” OC often requires 7/8” or 1” thick subflooring.
The span a floor joist or floor truss can bridge depends on various factors including depth, width, spacing, deflection, and whether sleeping or living area. For some joists, wood species and grade also need to be considered. There are also load factors that need to be taken into account too. There are numerous span tables and calculators available online to provide information and in the building code.
Depending on all applicable factors and different manufacturing techniques, the distance a joist or floor truss can span will also vary. A 2”x12” joist can span between 10’-1” and 25’-7” to comply with the 2021 IRC (International Residential Building Code). An 11-7/8” deep I-joist between 14’-4” and 21’-8” and an LVL joist 12’-6” to 26-7”.
A 12’ deep 2×4 floor truss can span between 20’ and 26’ under similar conditions. A cold-formed 12” deep steel joist can span 17’-6” to 27’-3” depending on all factors, including the thickness or gauge of steel used.
It should be noted, the thicker or deeper the dimension, the greater the span potential. Floor trusses and LVL joists can span up to 60 feet, while cold-formed steel joists span between 10 and 30 feet. For accurate distances, we recommend consulting a Structural Engineer or similar professional.
The amount of expertise to manufacture floor trusses or joists varies depending on mechanization, automation, and the magnitude of the operation. Cold-formed steel joists, floor trusses, LVL, and even I-joists require more expertise as they take raw materials and turn them into something else. Selecting the appropriate floor members requires more expertise than just looking at span tables.
Solid wood joists require the least expertise to select, as spans are easily determined based on wood species, grade, spacing, and loads. However, support structure location and materials, use of joist hangers, and beam location and dimensions also must be considered. So, consulting an experienced expert or Structural Engineer is recommended.
Installation depends on the type of structural members being used and their span. The further the span, the greater the weight, so some members may require more laborers or even machinery to place.
Placing each joist or truss in the proper location and fastening them into place typically is done in similar ways. However, some are delivered at the required lengths, and others may need to be notched or cut to length at the building site. Solid wood joists also need to be placed crown up too.
Floor trusses work well to provide large open areas and to reduce sag, bounce, and wobble. The open webbing makes it easy to run utilities, so drop ceilings and bulkheads aren’t necessary. They are also ideal for heated floors, supporting heavy tiles and other floor finishes, stone, quartz, or concrete countertops, kitchen islands, and high traffic.
LVL joists and I-joists are stronger than dimensional lumber and can span further to create larger spaces. Like floor trusses, they’re ideal for all floors and can also support heavier loads. The wider flange of the I-joist makes fastening subfloor sheathing and ceilings easier too.
Dimensional lumber joists are readily available, quick, easy to install, and comparatively inexpensive. Ideal for shorter spans but typically have mid-point support, so smaller open spaces. Solid wood is also slower to burn than trusses and has none of the off-gassings from resin adhesives used in LVL or I-joist construction.
Cold-formed steel floor joists provide slightly longer spans than dimensional lumber, are reasonably lightweight, and resist fire well. They won’t swell, warp, shrink, or rot due to moisture, are impervious to insects, rodents, mold, and mildew, and won’t sag. They also minimize noise transfer between floor levels.
The web can also be punched to make the placement of utilities easier too. They are ideal for damp locations or those susceptible to termites and other insect invasions. CFS joists don’t creak and can carry comparable loads to floor trusses and other joists.
The cost of floor trusses and joists depends on dimensions, design variables or requirements, labor costs, and ZIP or Postal Code. Floor trusses, depending on type, variables, and length, range from $7 to $18 a linear foot. Cold-formed steel joists range between $5 and $20 a linear foot depending on gauge, depth, width, and other variables.
An I-joist ranges from $5 to $10 or more a linear foot depending on the depth and other structural requirements. LVL floor joists, depending on all variables, range from $5 to $24 a linear foot. Dimensional lumber, depending on the wood species, grade, and depth, ranges from $1.40 to $3.25 per linear foot, making it the least expensive option.
What Are Standard Floor Truss Sizes?
Standard floor truss sizes depend on load variables and span requirements. They are commonly manufactured using 2×3 and 2×4 dimensional lumber and fastened together with barbed, heavy steel connector plates. Standard floor truss depths are 9”, 12”, 14”, 16”, 18”, 20”, 22”, and 24”. The length depends on the spacing between trusses, live and dead loads, and deflection, along with other factors.
Floor Joist Size in Residential Construction
Dimensional lumber floor joists can be 2×6, 2×8, 2×10, or 2×12, with the last three being the more common choices. I-joists typically are available in widths of 1-3/4” through 3-1/2”, and depths of 9-1/2”, 11-7/8”, 14”, and 16”. LVL joists can range from widths of 1-1/2” through 5-1/4”, and depths from 3-1/2” to 24”.
Cold-formed steel joists are made of 18-, 16-, 14-, and 12-gauge steel. They have flange widths of 1-5/8” through 3”, and common depths of 8”, 10”, 12”, and 14”, with greater depths also available. The joist size depends on the span desired, load variables, and deflection values, plus other factors.
Advantages and Disadvantages of Floor Trusses
Floor trusses are designed and engineered to span different distances depending on their depth, width, configuration, spacing, and deflection value, among other factors. The greater the distance to span, the more advantageous floor trusses are.
- Trusses are designed and engineered to meet codes and loads, limit deflection, and reduce vibration
- Factory-made to exact specifications so no waste, less shrinkage, twisting, warping, or creaking
- Reduced construction and labor time and costs, for a faster build and reduced overall costs
- No bulkheads or interior supports required, so larger open spaces and higher ceilings
- Provide greater support for heavier floor and countertop finishes, and kitchen islands
- Open webs make placement of electrical, plumbing, and HVAV easier and quicker
- Trusses can span greater distances than dimensional lumber or I-joists
- 2×3 and 2×4 flanges provide greater nailing surfaces for sheathing
- No support pads, posts or columns, or beams are required
- 24” OC spacing decreases installation time and costs
- Custom designed to meet build requirements
- Easier to cantilever or build in balconies
- Easier to install radiant floor heating
- Less build-site theft
- Very strong
- Heavy and difficult to place
- Expensive compared to dimensional lumber
- Edges of metal connector plates can damage wiring
- Allow fire to move quicker and structural components to fail faster
- Built offsite, so scheduling issues and little or no room for job-site error or trimming
Pros and Cons of Floor Joists
Floor joists refer to dimensional lumber or closed-sided I-joists, LVL joists, and cold-formed steel (CFS) joists. While each type has its own pros and cons, they function to transfer loads in a similar way.
- Dimensional lumber joists are readily available and the least expensive option
- Traditional design and layout
- LVL and I-joists are designed, engineered, and built offsite to structural specifications
- All types can be cut or trimmed at the building site
- Some types span greater distances
- Comparatively lighter in weight
- Some types offer wider nailing surfaces
- Solid and metal joists slow the spread of fire
- LVL joists have greater stiffness, bending, and shear strengths, and resist splitting, checking, warping, and shrinkage
- I-joists resist shrinkage and twisting and can span further than dimensional lumber
- CFS joists are strong and relatively lightweight
- Steel resists mold mildew, termites, other insects, and rodents
- CFS joists are dimensionally stable and have stamped openings for plumbing and wire conduits
- Dimensional lumber doesn’t span as far as other options
- Smaller open spaces underneath with some types
- Need to be drilled or cut for wires, plumbing, etc.
- Often require bulkheads for HVAC or plumbing
- LVL, I-joists, and steel joists are not readily available
- All types are more expensive than dimensional lumber
- CFS joists require special fasteners and installation skills
How far a structural floor member needs to span and the loads it must support typically determine which is better. Floor trusses are better for creating larger open spaces underneath and supporting greater loads. There are also different design configurations for various foundation styles and flooring options.
The open webbing makes the installation of plumbing, HVAC, and electrical easier and faster too. The higher cost per unit is usually offset by the reduced structural and labor costs, and savings due to faster build times.
Joists often require additional support points or beams but are better suited for shorter spans and smaller open spaces. Joists typically can be trimmed or cut at the building site for last-minute corrections and adjustments. Their solid web design means joists must be drilled or cut for plumbing and electrical passage.
It also commonly means HVAC passes underneath, creating the need for bulkheads and lower ceiling areas. While dimensional lumber is less expensive than floor trusses, LVL, I-joists, and CFS joists aren’t necessarily less expensive or lighter.
Floor trusses, LVL joists, I-joists, and CFS joists are designed, engineered, and manufactured offsite. Structural drawings commonly identify one or more floor support options based on load and structural requirements as identified by a Structural Engineer in consultation with the builder or homeowner. Hopefully, you have a better understanding of the differences between floor trusses and joists and are ready for your next build.