Floor Joist Spans for Home Building Projects

By: Danny Lipford

Floor joists on home addition.

There are a number of factors to consider when framing a floor for a home building project, including:

  • Type of wood used.
  • Grade of the lumber.
  • Width and thickness of the boards.
  • Spacing between joists.
  • Load placed on the floor.
  • Length the joists span.

Wood Species

Different types of wood vary in stiffness and resistance to bending, with some species being much stronger than others. Even within the same species there is considerable variation in strength depending on the growing conditions of the tree.

The lumber from slow growing trees, which have more end grain growth rings per inch, is much stronger and denser than faster growing trees of the same species.

The strength of common wood species used for framing includes:

  • High Bending Strength: Southern yellow pine and Douglas fir.
  • Medium Bending Strength: Hemlock, spruce, and redwood.
  • Low Bending Strength: Western red cedar, Eastern white pine, and ponderosa pine.

Lumber Grade

The grade of lumber used for joists is also a factor in determining the strength of the wood. Since knots and other defects weaken wood, higher grades of lumber (designated as clear, select, or #1) are considered stronger than lower grades. Higher grades of lumber, however, are also much more expensive.

Lumber for floor joists.Lumber graded as #2 is the most common choice for floor joists and other framing lumber. It has more knots and defects than higher grades, but usually not enough to cause significant loss of bending strength.

Lower grades of lumber (designated as #3, utility, and #4) often have too many large knots or other defects to be used for floor joists.

Regardless of the grade of lumber used, try to choose boards which have few knots or defects and that are straight, with little crook along the length of the edge.

When possible, position floor joists with any crook in the board facing up, so the weight on the joist will tend to straighten it out. Any large knots found near the edge of the board should face up as well, so the weak wood of the knot will be compressed rather than stressed.

Size of Lumber

The width of a board is an important factor in determining the distance a floor joist can span, playing a much larger role than the thickness.

For example, if you double the thickness or number of joists in a floor, the distance the boards can span will increase by approximately 25%. But if you double the width of the boards, the distance the joists can span increases between 80% to 100%, even though you’re using the same board feet of lumber.

Floor joists in room.

Load on Floor

The amount of weight on the floor is also an important factor in determining joist size and span length.

The weight of the building materials (joists, subfloor, flooring, and ceiling) is known as the dead load and is commonly rated at 10 pounds per square foot.

Bedroom with furniture.You can calculate the dead weight yourself by adding up the weight of the building materials, then dividing by the number of square feet the materials will cover.

The weight that will be placed on the floor is the live load and includes furnishing, people, and anything else you plan to store on the floor. Live loads range from 30 pounds per square foot and up, with 40 pounds the norm for most residential rooms and decks.

Obviously if you’re a piano collector or plan to crowd as many people as possible onto your deck, you should err on the side of caution and increase the size of the floor joists or reduce the span.

Joist Span

Putting all these factors together gives the minimum size joists needed for various types of wood, grades of lumber, sizes of boards, spacing of joists, and the load that will be placed on the floor.

You should also check your local building codes before starting construction and consult a structural engineer in unusual or extreme situations.

Below is a table giving the minimum size floor joists needed for 16″ and 24″ on center (o.c.) spacing when using #2 lumber with 10 pounds per square foot of dead load and 40 pounds of live load.

Yellow Pine
Douglas Fir
Redwood
Hemlock, Spruce
Western red cedar
Eastern white pine
Joist Size 16″ o.c. 24″ o.c. 16″ o.c. 24″ o.c. 16″ o.c. 24″ o.c.
2×6 9′ 9″ 8′ 3″ 8′ 8″ 7′ 6″ 7′ 6″ 6′ 3″
2×8 12′ 8″ 10′ 8″ 11′ 0″ 10′ 2″ 10′ 5″ 8′ 6″
2×10 16′ 0″ 13′ 0″ 14′ 6″ 12′ 4″ 12′ 9″ 10′ 5″
2×12 18′ 6″ 15′ 0″ 17′ 6″ 14′ 4″ 14′ 9″ 13′ 0″

You can always use bigger lumber, closer spacing, or a smaller span; just don’t go smaller, further apart, or a longer span. In other words err on the side of caution rather than pushing the envelope.

Further Information

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24 Comments on “Floor Joist Spans for Home Building Projects”

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  • Chris S. Says:
    September 21st, 2016 at 11:28 am

    I am replacing subfloor in a bathroom due to some water damage. I used a toe kick saw to remove the damaged subfloor. I know I need to add blocking around the edge to secure the new subfloor to. Does that blocking need to be the same dimensions as the floor joists (2×10) or can I get by using a 2×6?



  • Phil Says:
    June 5th, 2016 at 10:43 pm

    Putting in box headers to reinforce a host the needs to be notched. Which would be a better option – doubling up 2×4s or a single 4×4? Unfortunately I don’t have much more clearance on one of the sides for more than 4 inches.



  • John Says:
    February 9th, 2016 at 3:37 pm

    I’m constructing a second floor living room space, currently the living room is a two story foyer that i want to close in and make living space above. i need to keep it in the 2×8 range because then it will match existing ceiling height and floor. i am thinking of putting in two LVL headers at a depth equal to a 2×8 and then ill run my 2×8 perpendicular to the header and teco them in i need the headers to be flush. do you think this is sufficient enough?



  • Mick d Says:
    January 20th, 2016 at 12:25 pm

    I am putting on a flat roof span 3.5 mets what size joists will I req and what centres will they be set at
    Thanks



  • Matt Says:
    December 15th, 2015 at 2:06 am

    Hello, I bought my parents house, I was born here. It’s 100 years old this year. The builders of the house used no sub flooring. The tongue and groove wood flooring is nailed to the joists. Can I install sub flooring over it? I’m sistering joists from below to stop the bounce. The style of the house is a 2 1/2 story American Foursquare. Was it common to find these houses with no sub flooring ? Thank you



  • Greg Says:
    December 11th, 2015 at 12:13 pm

    On floor trusses with 40-20-5 loading, the 40 is live load, the 20 is dead load, what is the 5?



  • Dale Roppel Says:
    November 29th, 2015 at 11:31 am

    I’m building a loft above my garage for storage. loft will be 19’7-1/4″ wide by 8’deep. can i get floor joists that will span almost 20′ without using column support? if so, what spacing between floor joists do you recommend? what size floor joists should i use (2″x8″)?



  • chris Says:
    October 23rd, 2015 at 7:11 am

    my house is 100 years old. The joists are real 2 x 12 spaced at around 20 inches centered. The span is 16 feet from the centre beam. The problem is the noise when walking on the floor. Im thinking blocking will reduce the movement. What is the best way to reduce the noise/deflection. There are heating ducts and a couple of plumbing lines so 2 x 12 blocking cannot be continuious. What is the best solutions.



  • Deisko Duff Says:
    October 19th, 2015 at 11:31 am

    Hi, I’m building a 16’x16′ room off the kitchen. How far apart do I need to have the floor joists, and what size joists do I need for the floor?



  • bob Says:
    October 14th, 2015 at 8:18 pm

    Im building an island in the kitchen, 5 ft by 6 ft in the middle of the floor, the span is 14 ft, 2 by 10s (southern yellow pine #2), 16 in on center. The granite top weighs 540 lbs, is that enough support? Or do I need to double up my floor joists?



  • melissa Says:
    September 9th, 2015 at 10:10 am

    If I’m building a building that is 12′ by 24′ how many floor joist, ceiling joist and rafters for a gable roof would I need? Thank you



  • Oscar Says:
    September 3rd, 2015 at 5:23 pm

    Hello,

    A hypothetically question, in order to build a two story house with two balconies that are approximately 246 sq. ft., does that reflect with the size of joist that should be utilize and the spacing between the rafters o.c.? Also, if an A/C unit was to be installed, how much space between the first floor and second floor does it need? Furthermore, the entire sq ft of the house is approximately 2,520 sq. ft., including both floors.

    Please advise



  • Iris Says:
    September 1st, 2015 at 9:23 pm

    I need to know the size of header I need for a 5 foot opening on exterior wall with 2nd floor above



  • Bruce Williams Says:
    August 23rd, 2015 at 6:38 am

    I’m enclosing a small shop in my airplane hangar. The dimensions are 16×20. The ceiling will not have much load on it at all. I was planning on using 16′ 2×8 ceiling joists @ 16″ oc with plywood decking. The only thing up there will be a small air handler for the AC and dust collector. Your thoughts please?



  • scott Says:
    August 9th, 2015 at 11:42 pm

    hello, i am looking at building a second story room over my large dinning area, it is over 15 ft long and over 23 ft wide , if i run a support beam 23 ft can i do this without putting a pole in the middle of that room? or is there joist strong enough to run 16 ft to cover the 15 ft span?



  • Emil McBride Says:
    May 28th, 2015 at 4:46 pm

    I too want to build an Underground home with earthbags and a sloping flat roof. The snow load spec for the area calls for 50 lb load per square foot for starters and I want to put only 1 foot of soil on top. How close must the header boards to support a 100Lbs per square foot load assuming you think this is correct at 100lbs psf



  • Todd Weeks Says:
    May 15th, 2015 at 5:58 pm

    I have a 32’X40′ pole barn that I built to store vintage cars.
    I plan on placing the floor on 6″X6″ pillars spaced 8′ apart so I will be building 20 8’X8′ sections. Each 8’X8′ section will need to support 1/2 a vehicle. What spacing and what size boards will I need?



  • don pascucci Says:
    April 29th, 2015 at 6:43 pm

    I want to build a underground house. the span will be 25 feet, with 3 feet of dirt on top. What kind of joist should i use.

    thank you

    don



  • Steve Says:
    March 28th, 2015 at 6:56 am

    I wish to put a floor in an outhouse for storage. The size will be 6 m long and 8 feet wide. It has been suggested I use timber 4×9 and 6 m long. Can you advise as to whether this would be the right size, whether spruce or pine would be ok. The floor is to store various household items. We need to be able to walk on it to get access to the items stored.
    Thanks



  • chris chapman Says:
    February 11th, 2015 at 4:59 pm

    I’m building a small cabin. 12 ft wide and 16 ft long what is your recommendation for floor joists. Also will have a second floor.


  • Official Comment:


    Ben Erickson Says:
    January 31st, 2015 at 7:27 am

    Hi Konrad,
    Honduran pine is a hard pine, so it should have a high bending strength comparable to Southern yellow pine, but it doesn’t hurt up make your joist wider than the table suggests (other than the additional cost of the wood). I don’t know what the span of your joists will be, but for 10′ between posts you would need at least 3 2″x12″ together on edge (or a 6″x12″ beam on edge) for the sill beams to support a 20′ joist span.



  • Konrad Gola Says:
    January 29th, 2015 at 3:15 pm

    I’m going to build a house in Honduras on concrete posts, 8×8 and spaced at 10’x10′.I’ll be using honduran pine. I’m still trying to figure out the spacing for the floor joists and what size to use. I’d appreciate it very much if you could help me out.
    Konrad



  • Shelia Says:
    January 8th, 2015 at 11:36 am

    If your deck is sitting on a solid foundation like concrete is the only way I would consider using 2×6 floor joist,and 2×8 only if the you have plenty of support blocks under stress points. I personally would not use anything less than 2×10 on 16″ oc



  • Rick Brosge Says:
    April 29th, 2014 at 6:23 pm

    I want to build a 10’x12′ deck I have most of it calculated but could you help me decide on floor joist and the outside framing. Can I use 2″x6″ all the way or do I need heavier 2″x8″? This deck will hold a grill and table and up to 8 people at most at one time. I am very cost conscious but I want a long lasting safe deck.


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