When building or remodeling a home, one important structural consideration is determining how many joists are needed to properly support a given floor span. Joists are horizontal supporting members that run from wall to wall, resting on the load-bearing walls and beams. They provide crucial support for the subfloor and finished floor above. For a typical single-story home with 16′ between load bearing walls, the number of joists required can vary based on several factors. In this article, we’ll discuss what joists are, how they work, and how to determine the correct quantity and spacing for a 16′ floor span. Properly sized and spaced joists are vital for safety, preventing sagging floors, minimizing bouncing and noise, and meeting code requirements.
What are Joists?
Joists are horizontal lumber beams, typically 2x8s, 2x10s, or 2x12s, that run parallel from one load bearing wall to another. They form the skeleton of the floor system. Plywood or OSB (oriented strand board) subfloor panels are then fastened atop the joists to provide a sturdy base for the finished flooring. The joists transfer the weight of the structure above downward into the supporting walls. This includes not just the dead load of the building materials themselves, but the live loads of people, furniture, appliances, and anything else placing weight on the floor.
Joists are typically made of dimensional lumber, such as 2x8s, 2x10s, 2x12s. The dimensions refer to the height and width in inches, so a 2×8 is actually 1.5″ x 7.25″. The depth provides stiffness and strength to prevent excess sagging under loads. The actual size lumber used depends primarily on the floor span length.
The ends of each joist rest on the wall plates of the load bearing walls, facilitating load transfer. Joists are spaced at regular intervals, typically 12″, 16″, or 24″ on center depending on the loads and span. This spacing must remain consistent to prevent floor bounce and sagging in some spots. Proper joist sizing, spacing, and fastening are critical to meet building codes and ensure proper structural integrity and performance.
How Joists Support Floors
When weight, also called a load, is applied to the floor above, the subfloor and finish floor materials transfer these loads downward into the joists. The joists then transfer the loads sideways to the load bearing walls. These walls support the total load through compression of the wall framing members down to the foundation.
To transfer loads effectively, joists act similar to a simple beam suspended on two ends. When a load is applied in the middle of the joist span, it causes bending stress along the length of the member. The top edges are compressed while the bottom edges are put into tension. The joist must be sized so it has sufficient stiffness to minimize deflection while carrying these stresses.
In addition to bending under loads, joists may also twist or oscillate up and down slightly. This can cause floor bounce, sagging, squeaking, and noise problems. Choosing the proper size joist and spacing helps control this movement.
The ends of joists rest on the wall plates or atop supporting beams. The connections must be properly designed and constructed to transfer forces into the supporting walls without excessive deflections. Proper joist hangers are typically used.
The preserve structural integrity and performance, building codes provide prescriptive rules for joist sizing and spacing based on the floor span length. However, joist layout can also be designed analytically using structural engineering principles.
Sizing Joists for a 16′ Span
For a 16′ span between load bearing walls, there are several joist size options that meet building code requirements, primarily based on the joist spacing. Wider spacing requires a larger joist to handle greater loads. Here are common prescriptive joist size options:
2×8 Joists:
– Spaced 12″ On Center: Does not meet code for 16′ (max span 13′)
– Spaced 16″ On Center: Does not meet code for 16′ (max span 14′)
– Spaced 24″ On Center: Does not meet code (max span 11′)
2×10 Joists:
– Spaced 12″ On Center: Meets code (max span 18′)
– Spaced 16″ On Center: Meets code (max span 16′)
– Spaced 24″ On Center: Does not meet code (max span 13′)
2×12 Joists:
– Spaced 12″ On Center: Meets code (max span 20′)
– Spaced 16″ On Center: Meets code (max span 18′)
– Spaced 24″ On Center: Meets code (max span 16′)
As you can see, 2x10s or 2x12s spaced at 12″, 16″, or 24″ on center will meet prescriptive building code requirements for a 16′ span. 2x8s are not thick enough at any spacing for this length of span.
How Many Joists for a 16′ Span
To determine the total number of joists needed, start by finding the joist spacing you plan to use. For example, let’s say we choose 2×10 joists spaced 16″ on center:
1. Our room length is 16′ , or 192″
2. We are spacing joists 16″ on center
3. 192″ (room length) / 16″ (joist spacing) = 12 joists
Therefore, using 2×10 joists at 16″ on center, we would need 12 full-length joists to span the 16′ room width.
The same calculations can be done for other spacings:
– 2x10s at 12″ on center would need: 192″ / 12″ = 16 joists
– 2x12s at 24″ on center would need: 192″ / 24″ = 8 joists
The fewer number of joists with wider spacing means each joist must support more load. This is why the larger 2x12s are required when going to 24″ spacing.
In addition to the full length joists, shorter joists are also needed along the perimeter walls to complete the floor system. But the above calculations give the minimum number of main, full-span joists required structurally.
Joist Layout Planning
Once you’ve determined the correct joist size and spacing for your span, the next step is mapping out the full joist layout. Here are some key factors to consider when planning the layout:
– Joist direction: Joists typically run perpendicular to the floor span direction, so would be placed front-to-back in a rectangular room.
– Bearing points: Joist ends should align above load bearing walls and beams below. Make sure the bearing points align vertically.
– Perimeter joists: Shorter joists are needed around the perimeter to tie into the main joists.
– Openings: Header joists may be needed to frame around any floor openings for stairs, chimneys, etc.
– Utilities: Factor in any special provisions needed for running plumbing, electrical, HVAC, etc.
– Alignment and spacing: Use layout lines to make sure joist spacing remains precise and consistent.
Create a detailed floor framing plan to map out the joists and call out all required components before construction begins. Proper planning and layout helps ensure the floor achieves the required structural performance.
Meeting Building Code Requirements
It is important that the joist layout meet all requirements set forth by the local building code. Key provisions may include:
– Joist size for span length
– Maximum allowable joist spacing
– Minimum number of full length joists
– Perimeter joist requirements
– Header sizing and framing at openings
– Bearing dimensions at supports
– Fastening type and schedule
– Notching and boring limitations
– Special girder/beam specifications if needed
The building code provides minimum acceptable standards for safety. Joist layout and sizing should be verified to comply with all applicable codes. An experienced contractor can help ensure your floor framing plan meets code. You can also consult your local building department early in the design process for guidance.
Using Engineered Joists
An alternative to solid sawn lumber joists are engineered wood I-joists. These manufactured products have an I-shaped cross section with thick top and bottom flanges and a vertical web in between. The flanges help resist bending stresses while the web provides shear resistance.
I-joists are often made with laminated veneer lumber (LVL) or oriented strand board (OSB) bonded together with adhesives. They can span longer distances than typical dimensional lumber with smaller member sizes. For example, an I-joist with 1.5″ wide flanges may be equivalent in strength to a 2×12.
For a 16′ floor span, I-joists spaced 24″ on center are commonly used. This allows an open web truss design that facilitates running ductwork or utilities through the joist cavities. An engineered joist option should be designed by the manufacturer per application to ensure proper structural performance.
Joist Installation Process
Once detailed plans are completed, the joists are ready for installation. Here is a general overview of the joist installation process:
1. Construct load bearing walls and support beams below. Verify all bearing points are level and positioned correctly.
2. Install perimeter joist tracks if needed. Often metal hat channels are used at the exterior walls.
3. Layout joist positions using chalk lines based on the floor framing plan spacing.
4. Install end blocks where joists bear on beams or interior walls. This elevates the joist ends to provide proper bearing clearance.
5. Set and fasten the first joist in position. Temporary braces may help hold it plumb until others are installed.
6. Install remaining full-length joists using the layout lines for spacing. Fasten to the perimeter tracks using joist hangers.
7. Install header joists and framing around any planned floor openings.
8. Add blocking and bridging between joists at intervals for bracing and load transfer.
9. Complete perimeter joists and final framing connections.
Proper safety precautions should be used during installation, including fall protection safeguards as required. The local building department may conduct inspections at various stages of the process.
Subflooring Installation
Once joists are installed, the subfloor sheathing can be put in place. Plywood and OSB panels are screwed down on top of the joists to create a smooth working surface. Panels are oriented perpendicular to the joists and installed in a staggered layout. Gaps are left between sheet edges for expansion. Proper nailing patterns and glue are used to bond the subfloor to the joists below.
The subfloor provides a stable deck for installing finish flooring materials like hardwood, laminate, carpet, tile, and vinyl. It also helps stiffen the floor structure by tying all the joists together into a diaphragm that further distributes lateral loads into the walls. A quality subfloor is critical for proper floor performance.
Considerations for Joist Installation
– Verify joist lengths, spacing, and orientation matches the approved plans.
– Joists should be straight, without excessive crowning or warping.
– Use proper joist hangers at all bearing ends and connections.
– End blocks create flush end bearing surfaces.
– Bridging and blocking helps brace the joists in place.
– Watch for plumbing, wiring, or other utilities interfering with joists.
– Subfloor helps stiffen the floor as a diaphragm when fastened properly.
– Inspectors will check for compliance with all codes and the approved plans.
Proper joist sizing, layout, fastening, and subfloor installation creates a code-compliant floor structure that will support all required loads without excessive bounce or sagging. Careful planning and execution of each construction step helps build a high quality, durable floor.
Joist Span Table Summary
Here is a summary reference table for allowable joist spans based on size and spacing:
| Joist Size | 12″ Spacing Span | 16″ Spacing Span | 24″ Spacing Span |
|---|---|---|---|
| 2×8 | 13′ | 14′ | 11′ |
| 2×10 | 18′ | 16′ | 13′ |
| 2×12 | 20′ | 18′ | 16′ |
Conclusion
Determining the proper joist sizing and layout is an important part of any floor framing project. For a typical 16′ span with standard lumber joists, 2x10s or 2x12s spaced at 12″, 16″, or 24″ on center will meet code minimums. The total number of full-length joists required depends on the on-center spacing used. Wider spacing reduces joist quantity but requires a larger joist size. Once size and spacing are established, a detailed framing plan should be developed accounting for all openings, perimeter blocking, utilities, and connections. With proper planning and installation, the joisted floor assembly can provide the required structural integrity, safety, and performance. Always consult local building codes for all applicable framing specifications and requirements.
