Details for Framing a Floor System with Wood

Learn the details for framing a floor system with wood with this simple DIY guide. It includes a diagram, step by step layout instructions, and mistakes to avoid.

The floor is the part of the framing under virtually constant contact from the occupants. The framing members of the sub-floor literally hold you up every step you take. The workmanship is noticed more often. Due to the constant interaction between you and the floor, there are many areas  which can actually speak to you. They may tell you how there are gaps which were not attended to properly and when your weight centers over that spot, the floor will cry out in its own language “something is wrong!”. To our ears it sounds like squeaks and other unwanted noises. These are by far the most common mistake made when framing a floor system (even by some professionals).

Cross Section of the Floor Framing Detail

Floor Framing Detail Cross Section

Floor Framing Details

Use a contractor grade sub-floor glue. It is well worth the extra expense. Apply it to every joist with a 3/8″ bead before you drop the sub-flooring in place. Do not use it in the rain or very wet conditions. Do not apply it until you are ready for it. If left in a bead to long, then it will cure into the shape of the bead, and the sub-floor will ride on top of it. Use 16d coated sinker nails for all of the framing members. We recommend 8d coated sinker nails for the sub-floor as well. For nail guns, the 16d are best substituted with 12d. Do not be shy with the nails, but be careful. Most squeaks are caused from bad nailing. Truss specification documentation will accompany them upon delivery. It will include exact details on nailing. Joist hangers require a special nail. Fill every hole when using hangers.

Be sure to use the “tacks” (the little stamped out pointed things). Build the girders first. Start with the longest one. Make sure its placement puts it where it needs to be. Most girders are beneath load bearing walls. Double check to make sure there are no errors with the blueprints. It is not uncommon for the plans not to be entirely accurate! Use strings and braces to make sure the girders are as straight as possible. Lay the girders and plates out for doubles. Doubles are two joist sandwiched together and placed below walls that run parallel with the joist. Make sure the crowns are up for all girders and joists. Lay the girders and plates out for the joists. A fail safe method is to mark the first center minus 3/4″ on the end where it is best to start the plywood from. For example, mark 15 1/4″ for a 16″ oc layout. From that layout, mark every 16″ integer on your tape (they are highlighted in red). Be sure to put an “X” on the side of the layout mark away from the direction you are pulling from. See our truss layout page more details and a large diagram. Cut the joist to length and nail them in. Be sure to inspect each board for quality. Install any ledgers with three nails under each joist and doubles. Fill every hole with joist hanger nails if you are using hangers. Nail up a 2 x 4 under the joists in the center of each span. This is for lateral support and to straighten the joists. For second story applications use metal bridging. An easy way to install it is to first nail the tops. Then run a temporary 2 x 4 on top to straighten the joists. Finally, nail the bottoms of the metal bridging. We do not recommend using wood bridging block. It is just too easy to get squeaks. Pop a line in 48″ on the side you want to start the plywood on. Keep the female edge directly on this line. Make certain the plywood breaks very close to the center on each joist. Nail it as it is laid, There are dots indicating the nailing pattern. Use a 2 x 4 and a sledge hammer to seat the tongue into the groove on all of the rest of the floor. Check over the floor for missed nails and listen closely for squeaks; now is the best time to correct them. We hope you found some useful information with out floor framing details guide.

Common Floor Framing Mistakes

A properly framed floor gives attention to detail and will forever will be silent and rigid. Here are some suggestions for good workmanship: Use only new kiln dried No. 2 Pine. or engineered trusses. Name brand AdvanTech® tongue and groove sub-flooring outperforms every other product, and it is guaranteed for 50 years! We do not recommend rough sawed lumber even though it is our material of choice for many other projects. Do not over span any joist or beam. Over spanning causes springy floors, and leads to sagging. Span charts stretch this to the absolute limit. Most of them are beyond our standards by at least 10%. A good rule of thumb is not to buy any 2 x 8′s longer than 10′, 2 x 10′s longer than 14′, or any 2 x 12′ longer than 18′. If you don’t have any available, then you can’t use them! If you need longer joist, then use trusses.