Installing Floor Joists

For installing floor joists, your architect should have calculated your joist and girder sizes for you and included them on your house plans. If not, you’ll need to calculate them yourself. This is somewhat of a trial-and-error calculation, where you make some assumptions, calculate the loads, and see if it will work.

Framing your home consists of three basic steps:

  • Frame a floor
  • Frame the walls
  • Frame the roof

If your home is more than one story tall, the first two steps are repeated as many times as necessary, to account for as many stories as you are going to have. Also, if you are building on a slab, without a basement, you start by framing the walls, and then frame the upper floor. Framing the walls is the easy part, framing a floor on the other hand, is much more complicated.

Before You Begin:

Since your house is probably bigger than 20’ x 20’ you’re probably not going to be able to just run floor joists from one side of your foundation to the other. In these cases, what is done is to run a beam or girder the length of the house, dividing the space to allow for shorter floor joists. Most of the time, if the shorter dimension of your home is greater than 30’ an additional girder will be run; splitting the distance that the joists have to run into thirds. To prevent the girder, which is usually a steel I-beam, from having to carry the weight over an excessive span, one or more posts may be run in your basement.

Your architect should have calculated your joist and girder sizes for you and included them on your house plans. If not, you’ll need to calculate them yourself. This is somewhat of a trial-and-error calculation, where you make some assumptions, calculate the loads, and see if it will work. If it doesn’t work, or you’ve overshot your needs, adjustments are made and new calculations are done.

The type of material you use for your girders and joists is extremely important, as it will affect how much weight the floor will hold. Typically, tables are used to calculate these. While this is fairly simple for joists, it is rather complicated fohe girders. If your architect has not calculated this for you, I’d recommend going back to have him do it.

How to Build it:

The girder will be supported by notches in your foundation wall and steel posts every so many feet throughout the span. Your foundation plan will have shown you where to place footers for these posts. The posts and girders are bolted together and the posts are bolted to the floor. Before bolting, adjust the height of the post to insure that the girder is level. They generally have a threaded section at the top end for this purpose.

Your joists will need to span from the foundation wall to this girder. There are two different ways of having them come together at the girder; overlapping and butting. If you chose to overlap them (most common) you can simply nail them together and attach them to the girder. If you choose to butt them together, you must attach them together by nailing through special perforated steel plates made for the purpose.

At the foundation wall end of your joists, you will need to attach a sill plate and header for the joists to attach to.

Any place where you have a wall, you’ll need to have a floor joist under it. So, if your wall runs parallel to the floor joists, you simply add an extra joist. If your wall runs perpendicular to the joists, you add blocking, the same size as the joists, nailing it between the joists, where the wall will go.

Likewise, it is advisable to install an extra joist under your bathtub, refrigerator, and if you are planning on using a waterbed, under the waterbed’s location.

In the case of your second floor, your joists will sit on the load bearing wall and the exterior wall of your home. A header (usually of the same size dimensional lumber) is placed all around the perimeter and across the load-bearing inner wall to attach the joists to. If there isn’t enough lip for the joist to sit securely, or in the case where there is no lip at all, joist hangers are used to secure the joists to this header.

Many builders “block” between adjacent floor joists for extra rigidity. This can either be done with a solid piece of the same sized dimensional lumber that is used for the joists, or with smaller dimensional lumber that is crossed in an “X” and nailed to the floor joists. Please note that the building code does not require this “blocking” although your local building inspectors may ask about it.

Framing Openings into Floors:

There may be times where you need to frame an opening into the floor, for the purpose of piercing the floor, such as with a staircase or fireplace base. In those cases, you should put double joists on either side of the opening, with headers at each end of the opening, for the joists to attach to. Since there will be no support under the joists, you will need to use joist hangers to connect them.

What You Need to Know About the Building Code:

The building code is very specific about minimum material requirements and spacing for floor joists. Basically, the ground floor must be built for a live load of 40 PSF (pounds per square foot) and the upper floors must be built for a live load of 30 PSF.

The other thing that the building inspector is going to look for is how you connect your floor joists together and to the header. You must be sure to either have them resting on a girder or foundation wall, or if they are butting into a header, use joist hangers.

Tricks & Tips:

You can totally eliminate the problem of installing the girder by installing a framed, load bearing wall in its place, in your basement. The wall will need to be finished to pass inspection. It will also need a footer underneath it. Of course, if you are building on a slab, this isn’t an issue at all. Check with your building inspection department before doing this, to verify that they will accept it.

Money Savers:

There are engineered joists available today which are both cheaper and less expensive than using dimensional lumber. Your local building materials center should be able to supply you with information about these engineered joists, specifically their acceptable spans. We cannot provide that information here, as it is different for every manufacturer.

If your local building inspector doesn’t require you to put blocking between the floor joists, don’t bother. The subfloor will provide the necessary horizontal rigidity to prevent your joists from wobbling.