How to Build a Floor for a House




Framing A Floor Using "I" Joists

This article teaches you how to build a floor for a house on a
concrete foundation. This framing system can also be used for framing storage shed floors.This floor framing system is typical over basements and crawl spaces.

Joist spans and spacing are determined by engineering that is published by the manufacturer.

  • Every floor joist manufacturer has product literature and

telephone support with engineered span and spacing tables specifying typical joist spans and floor loads to help design a custom floor system.

  • Lumber yards that sell floor joists will have the literature and skill necessary to help you order the proper materials to build a floor for a house. This article will teach you how to build a floor for a house.

Brief steps to framing a floor for your storage shed or house.

  1. Square The Sill Plate Layout On The Foundation
  2. Cut, Drill And Attach The Sill Plates

  3. Build Basement Bearing Walls, Where Necessary

  4. Layout The Joist Spacing On The Sill Plates

  5. Attach The Rim Joists

  6. Cut And Install The Floor Joists Between The Rim Joists

  7. Stairway and Balcony Openings Are Framed Using Bearing Walls Or Beams Below

  8. Beams In The Floor Require Joist Hangers

  9. Beams Under The Floor Joists Allow The Floor Joists To Go Over

  10. Cantilevers Allow The Floor To Be Framed Out Past The Foundation

  11. Sheeting The Floor With OSB Or Plywood

Tools and materials

  • Floor joists
  • Rim Joist
  • Joist Hangers
  • Fasteners, (nails)
  • Construction Adhesive
  • 3/4" O.S.B. (Oriented Strand Board)
  • Construction Pencil
  • Chalk Line
  • Tape Measure

Step 1: Square the Sill Plate Layout on the Foundation

This image shows the chalk lines measured in 3 1/2" for the plate and 2" for brick.

The first step to building a floor for a house is to attach the sill plates to the foundation of the house. The foundation will have bolts set into the concrete along the perimeter. You will use these bolts to attach the sill plate to the foundation. The sill plate must be square, even if the foundation was poured out of square. This will make your floor system square and hopefully keep the rest of the house framing closer to square as you go up to the roof.
Squaring a floor is done by making a "345 triangle" on the top of the foundation. The numbers stand for the lenght in feet, 3' 4' 5'. The larger the triangle the more square your floor will be. Simply double the 345 by making the triangle legs 6'-8'-10' or larger. These are the steps to square up the sill plates on a house without brick, if your house has brick you will need to add the brick thickness to the 3 1/2" measurement in from the outside of the foundation:


Find the longest continuous foundation wall, measure and mark 3 1/2" in from the outside of the foundation at each end.

  • Put a chalk line mark between these pencil marks. (down the entire length of the wall).

MARK THE CORNER: Find the second longest wall that touches the first wall and measure in 3 1/2" from the outside of the foundation wall and put a mark on the chalk line. (Call this the "Corner Mark". This marks the sill plate inside corner.)

MARK THE 3' LEG: From the Corner Mark measure down the chalk line 3' and make a mark on the chalk line. (Call this the "3 Foot Mark").

MARK THE 4' LEG WITH AN ARC: Have someone hold the end of the tape measure on the Corner Mark and measure down the wall without a chalk line on it 4' and draw an arc with the pencil. An arc is used because we do not know where the point on the arc to make the square wall is yet. Somewhere on the arc will be the intersection that makes the two lines square.

MARK THE 5' LEG WITH AN ARC: Have someone hold the end of the tape measure on the 3 Foot Mark and make an arc through the 4' leg arc. The intersection of the two arcs is the point you will pull the chalk line through to mark the sill plate location on the second shorter wall.

DRAW THE SECOND LINE: Have someone hold the chalk line firmly on the Corner Mark and another person pull the other end of the line to the other end of the second wall, (the one without a line on it yet). Have a third person stand over the intersection of the two arcs and then "pull through" the arc intersection, make sure you go all the way down the second wall to fully mark the sill plate location.

After these two lines are on the foundation you will be able to easily
pull measurements across the foundation to the other foundation walls and mark parallel lines for the other sill plate locations. Try to get them 3 1/2" in from the outside of the foundation. These measurements are rarely the exact same as the plans because the concrete foundations are not always square.

Step 2: Cut, Drill and Attach the Sill Plates

These are the steps to mark and drill the sill plates:

  • Place the treated sill plates directly over the chalkline location they will be installed on, rest it on the bolts and then hit the plate with a hammer into the bolts so that the anchor bolts make an indent in the sill plate.
  • Roll the plate over and use a 3/4" drill bit to drill a hole where the mark from the bolt is.
  • After all the holes are drilled place the sill plate on the bolts and check to make sure it is lined up with the chalk line. Adjust with the drill as necessary until it is perfect.

These sill plates are held back from the edge of the foundation so bricks can be installed.

Step 3: Build Basement Bearing Walls - Where Necessary

This is a image of a basement stairwell wall and main bearing wall.
Notice the wood blocks next to each wall stud. Notice how the plate is 1/2" in from the outside of the foundation at the corner but farther away it is flush with the outside of the foundation. This foundation is out of square.

After you have the sill plate installed you will be able to measure the
exact height of the basement bearing walls or the beams used for bearing in crawl spaces. Either way the process to find the height of the bearing plate on the interior lower walls is about the same. Measuring from the basement floor to the top of the sill plates better insures a flat floor because basement concrete slabs are never perfectly level but your wood floor can be by following these steps:

  1. Find the location where the bearing wall meets the perimeter walls and mark the exact location on the sill plates that you have just bolted down.
  2. Place a nail in the sill plate in approximately the center of the basement wall location. Repeat this for the other end of the wall as well.
  3. Pull a nylon string line between the nails, it will be directly over the new basement wall. Make the string line very, very tight. There can be absolutely no sag in it.
  4. Layout the wall location on the basement floor and mark it with a chalk line.
  5. Stack three 2x4 blocks on the concrete floor at each stud location and measure the stud length for each stud along the wall by measuring from the string line down to the top of the blocks, (1 block for the sill plate and 2 blocks for the top plate)
  6. Cut the studs to length making sure to keep them in their proper order.
  7. Frame the basement bearing wall.

Step 4: Layout the Joist Spacing on the Sill Plates

Floor joists are typically spaced 16" o.c. or 12" o.c. This article will assume 16" o.c. Start at one corner of the foundation and layout the joists 16" o.c. If you hook your tape measure on the sill plate that runs parallel to the joist layout then you will find 16" on the tape measure and subtract half the thickness of the joist to mark the edge of the floor joist. So for a 2" wide flange on a joist you will find 16" on the tape measure and go back to 15" to make the mark. Continue this on all the marks along the sill plate.

Mark the opposite sill plate from the same end so all the floor joists are square with the foundation. Use a ladder to mark the top of the bearing wall with the same joist layout from the same side of the foundation as the two exterior walls.

Step 5: Attach the Rim Joists

The rim joist is attached at the ends of the joists to keep the joists
stable in the upright position and to carry the weight of the walls and roof above. The rim joist will typically sit on the edge of the sill plate.

It is common to notch the rim joist over the anchor bolts.

Step 6: Cut and Install the Floor Joists Between the Rim Joists

Now you are ready to cut and install the floor joists.

  • Measure the distance between the rim joists. (the lengths should be the same because you took the time to make the sill plates square and parallel to each other)
  • Cut the floor joists.
  • Put the floor joists on the layout marks on the sill plate.
  • Nail each end of each floor joists to the sill plates according to the manufactures requirements (usually 3 -10d nails)

Floor joists attached to rim joist.

Floor joists laid out between the rim joists.

Step 7: Stairway and Balcony Openings Are Framed Using Bearing Walls or Beams Below

If there is a stairway opening you will frame the walls for it just like
you did the main bearing wall. The floor joists will run to the inside of the stairway and be headed off with a rim joist on the inside side of the stairway.

Stairway opening in floor joists. Notice how the rim joist sits on the
inside of the stairwell wall so the floor joists can bear on the wall. The two joists at the top of the stairs provide additional support to hold the stair stringers.

Step 8: Beams in the Floor Require Joist Hangers

Sometimes you may want to have a beam holding the floor but you don't
want the beam to be below the floor joists. This problem can be solved by installing a beam in the floor joist system and hanging the floor joists on the beam. Make sure to use construction adhesive in the seat of the hanger to prevent floor squeaks.

The beam is typically sized to be the same height as the floor joists.

Step 9: Beams Under the Floor Joists Allow the Floor Joists to Go Over

Other times you can simply put the beam under the floor joist. This beam
will get trimmers under each end to hold it up before the framing is finished.

Step 10: Cantilevers Allow the Floor to Be Framed Out Past the Foundation

Cantilevers are used to extend the floor beyond the foundation. This is
popular with fireplaces, breakfast nooks and bay windows. Follow the engineering specifications when determining how far the joists need to extend under the floor to allow your desired cantiliever.

Step 11: Sheeting the Floor With OSB or Plywood

The final step in building a floor for a house is to sheet it with OSB. The OSB or plywood must be staggered like stacking bricks to give it strength. OSB floor sheeting is typically 3/4" thick and comes with a tounge and goove to give the spans between the floor joists strength.

Pick a wall to start installing the floor sheeting from, typically you will start at the same end that you laid out the floor joists from so you can start with a full or half sheet.

This is the floor sheeting around the stairwell. Notice how the rim board and the sheeting and the wall below line up perfectly. Also notice how we left the sheeting over hanging the first step so there will be a stair nosing coming off the floor. Wait until the stairs are in to cut out the sheeting over the bottom part of the stairs to make sure there is the code required 6'-8" of headroom.

Now that you have built the floor for the house you are ready to frame the exterior walls!

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    20 Discussions


    Reply 2 years ago

    In fact, when I submitter the building plan to the building inspecter. He told me that they don't build houses like that anymore. Maybe I'm old school,but when I put something in, it's in to stay!!


    2 years ago

    not a fan of I beam, they typically do not last that long before you have structural problems( glue separation, warping etc), , but then again modern house standard for north America is only life span of 25 years. but wondering why only a 2X4 plate and not a 2X6 or 2x8 for better weight distribution on the Ibeam . and considering only osb for the end plate . if that house was built in a cold or damp climate that floor would be failing in 10 years of less, as moisture would of deteriorated the resin to the point you could poke your finger through it. and the outside walls would begin to sag..

    9 replies

    Reply 2 years ago

    In a country where you have lots of tornados I don't understand why you keep building fragile wood houses.

    Iron and concrete is far more secure, will last a lifetime, and you will survive to bad weather !


    Reply 2 years ago

    Here in "Tornado Alley" you see newer houses demolished by tornadoes while the houses that were built fifty plus years ago survive at a much higher rate. The older houses were built to higher standards. You see a lot of them that were built with 2x6 roof and ceiling trusses and 2x6 wall studs instead of the cheap white pine 2x4's used today. Pine was seldom used in the construction of these older homes.


    Reply 2 years ago

    yeah older house survive better, because how they are built I built my house 20 years ago. diagonal shiplap for the exterior wall board for the roof and tongue and groove for the floor.- they do not build them like that any more because it takes a lot of time ,a week to put down a floor and roof and a day or two to sheet a wall. but look at my roof compared to plywood all the plywood one sag between each rafter, mine is straight as the day I put it on also older house where not the cheap mc mansion of built of OSB, and particles board. usually bungalow and 1200 square feet and under built from soild wood material ( giving them a fire safety rating of 20 minutes. compare to the 5 minute and less for a modern house)


    Reply 2 years ago

    I have been in two tornadoes here that have sucked up roads. After one, I was wondering what all of the holes in the ground were then I realized that they were where the fence and power poles had been. It sucked some poles right out of the ground and others were snapped between ground level and six feet. Another tornado hit a farm implement store and was whirling multi-ton tractors in a circle with over a quarter mile radius even damaging railroad track. I saw concrete structures obliterated. However, weak wood houses that had some give between the gusts were better able to handle the gusts. Lately though, the big threat is the following rain. You can't survive an EF 5 unless you are in a hole. The topside goes bye-bye regardless of construction. With global warming, the tornadoes are now more rain wrapped and you need to get out of the hole before your drown. For lesser tornadoes, the method of attaching shingles to roof and anchoring roof is the biggest factor in the house surviving.


    Reply 2 years ago

    Have to agree.

    Having seen my timber-framed two storey house survive the only recent hurricane in the UK (1987), I would subscribe to the virtues of having some structural movement in the construction. Traditional, weak block and brick houses were demolished but not one of the twenty timber-framed buildings in my development sustained more than superficial damage like like lost tiles.

    The pictures from Haiti are devestating - where do you begin in this already poor country to put things right? The dwellings are no better than tents. Future builds wherever need to be bombproof


    Reply 2 years ago

    a well built wood frame house should survive a tornado and hurricanes . problem being well built houses are not being built. considering US construction standard allows for cardboard wall sheathing. instead of a minimum osb or plywood sheathing that only allowable in Canada . and now standard if osb outside sheatheing internal plywood sheathing under the drywall


    Reply 2 years ago

    Unless the wood is very thick I also doubt it would survive a tornado or hurricane. Nothing better than reinforced steel and concrete plus industrial brick in my opinion.

    ST NZmarvelmx

    Reply 2 years ago

    Except for our Wellington super rugby team (called the hurricanes)! :D

    ST NZmarvelmx

    Reply 2 years ago

    We don't have hurricanes here, so I'm all good.


    2 years ago

    Don't you get a moisture trap between the wood and the concrete? I think you should put something in between those two. Something like lead or insulation.


    2 years ago


    I cringed at the usage of OSB as support beams :o


    2 years ago

    "tounge and goove" should read 'Tongue and groove' and is now commonly called center-matched lumber.

    1 reply

    Reply 2 years ago

    Lounge and groove sounds cool though. Perfect for building the entertainment portion of house


    2 years ago

    Nice little indestructible.

    An observation, not a comment about the OP. I thought UK houses were built as cheaply as possible and pretty poor today, but wow! No wonder we often see swathes of houses mown flat by hurricanes in the US, and sash99 states a 25 year lifespan! (I have window frames that are nearly 90 years old!) Surely such poor standards must verge on the criminal? No wonder we see your president telling people to flee for their lives in the face of a blow!

    BTW, how long do chipboard staircase stringers last? I see no bracing in the studwork, and I suspect the I-beams could use some herringboning too.

    Buy some bricks or quarry some stone guys! Didn't you hear about the wolf that huffed and puffed?


    2 years ago

    I found this interesting because in my country we dont have basements (although I'd like to) nor do we use wood for construction except for molding up concrete.


    2 years ago

    I feel bad for the family that gets this house! So many weak construction practices in this, from OSB to 2x4 perimeter wall studs ( limits the insulation ).

    This is cheaply built in a hurry to make a buck rather then workmanship and quality.


    2 years ago

    Total garbage is the way houses are built today. Use the cheapest materials to save a few bucks, charge more for the home, and pass the added maintenance on to the unsuspecting buyer.

    Nice job on the instructable - I'm sure it's not your fault home construction has reached this sad state.


    2 years ago

    I would like to elaborate a little on the 3,4,5 rule, or Pythagoras theorem so you can use it for any sizes.

    It works like this: 3² = 9, 4² = 16. 9 + 16 = 25. √25 = 5. Now you can just plug in you own measurements.

    eg: 52² = 2704. 65² = 4225. 2704 + 4225 = 6929. √6929 = 83.24.

    So if the width is 52' and the length is 65', the diagonal should measure 83.24' and you would have a perfect square.