How to Build a House: Installation of the Floor System

The installation of your floor system will depend on the design of your house. The guidelines we will use are 24ft wide x 48ft long with a cantilevered bay window. The floor joist will be 2inch x 10 inch we will space at 16inch centers. The girder and sill plates you learned how to install in part 1.The first thing that we are going to want to do is square up our floor or foundation. This is a repeat process and the reason for this is you will want to constantly check for squareness.

Take your tape measure and measuring corner to corner or from the right corner to the back left corner and from the left front corner to the back right corner. These two measurements should be the same,if not adjust and make them the same so you have a square starting point. Now that you have a square foundation with your seal plate down and the girder in place, We will need a string line. We will want to take the string line and run it from front to back checking for high spots and/or low spots. If you have any high or low spots you will want to shim or notch accordingly. You should not have no more than a 1/8 inch variance.

The next step we will need to follow up with is sorting our lumber in our scenario we will separate all of our 2 inch x 10 inch x 12 foot floor joist. We will check for crowns,twist,bows and warps. The importance of separating your lumber is so you will not install a floor joist that is going to cause you problems especially a bad crown. This will also allow you to get the bad lumber replaced so you are not at a stand still in the middle of framing your floor.

The next step we will do is to lay out for our floor joist, you will want to mark where the bay window is going first. In doing this you should eliminate double marks, lets start at the back right corner. Hook your tape measure on the outside edge of the sill plate, you will want to pull 15 1/4 inch,31 1/4 inch and so on. The reason for this is so that when we start to lay our plywood the seams will break in the center of the floor joist. You will want to continue subtracting 3/4 of an inch until you complete the layout at the other end of the foundation. You will continue this process on the front also remember to start on the same side of the house as you did on the back. For any off sets you might have you can mark your girder and then pull your layout. Use your string line to find your mark on the wall. Remember that your layout on the front wall will be 15 1/4 set back with your joist, on the back wall it is 15 1/4 set ahead with your joist.

The floor joist that you set aside earlier will now be used for the floor. We will want to make sure that the crown on all the floor joist is pointing up. This will ensure that the floor is built at maximum strength and will eliminate dips. Making sure that the floor joist have been squared up on one end, this end will go towards the outside of the house now you can start installing your floor joist. You will want to nail the joist from the side into the sill plate 2 – 16D nails on each side. along the girder you will want to use the same nailing pattern and also nail the floor joist from the back to the joist from the front.

The bay window will require what we call a cantilever this means that for every foot you hang out over the foundation you will need to double that going back into the foundation. This is simple in our case as we are using 12 foot material, so at our bay window we will upgrade to 14 foot material letting the extra 2 foot hang over. If you can not use longer material then you will add the floor joist for the cantilever to the side of the existing floor joist. Allowing the joist to hang over the desired length therefore creating your cantilever for your bay window.

The next thing that we will do is to install the band board around the outside of your floor joist. Making sure that the band board is crowned up the same as the floor joist. If you are going to install a deck you will want to use pressure treated lumber for your band board at this spot, let the pressure treated band board go past the area for the deck at least 16 inches on each side. Down the center of the girder you will want to install fire blocking. This will come from any extra material from your floor joist,scraps,twisted lumber etc…You will measure in between each floor joist along the girder and cut a block to fit, do this also on the ends. That will finally complete the installation of the floor system and part 2 of Building a house.

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