CST-111 Floor System Installation Assess the Foundation: The foundation comprises the first system of the house. In many ways the floor system depends on the foundation. In many cases, the floor system can make up for small imperfections in the foundation, so the floor system provides an optimal base for the wall and roof systems which follow. Also, if there are issues with foundation that don’t directly impact the floor system, they are still easier to correct before the floor system is fixed on top of the foundation. For example, if the house will have a sealed crawlspace, the building’s envelope will include the foundation. Proper installation of plastic liners and insulation with spacing to allow for termite inspections should all be accomplished prior to floor system installation. But the main things the floor system is looking for from the foundation are accuracy of dimensions, levelness, and squareness of corners. Check for these and address any issues. Levelness can be adjusted by grinding high spots or padding out a mortar bed in low spots. Minor issues in squareness can be addressed by having the floor system sill plate overhang at out of square corners. Also, expect to do some simple cleanup of the foundation before installation of the floor system. Use a wire brush to scrape loose debris from the top of the foundation, and any mortar which may be clogging the threads of the anchor bolts. Install the Sill Plate with Sill Gasket: There are a few ways that the floor system can be attached to the foundation. We use anchor bolts which have been embedded into concrete in the foundation, to attach the sill plate to the foundation. The sill plate is made up of 2x6 pressure treated boards such that the boards. If the masons followed the foundation plan, the anchor bolts should allow for attachment of the sill plate, can be cut and installed with a maximum of 12” from the end of each board and a maximum of 6’ between anchor bolts. Snap a chalk line to mark the internal edge of the sill plate along the foundation. Again, it is best for there to be a slight overhang of the outside edge of the foundation if it is required to make the floor system square. Lay down the sill seal. The sill seal provides airtightness for a sealed crawlspace or a basement. It also provides a capillary barrier between the sill plate and the foundation. Select and cut the sill plate boards. When prepping for installation, arrange any wanes toward the inside and crown toward the outside of the house. This is to ensure the rim band will be fully supported by the sill plate. Scribe the anchor bolt placement on the sill plate boards. Lay a sill plate board along side the anchor bolts that will attach it and use a speed square to mark the outside edges of the bolts. Measure the distance from the chalk line to the center of each anchor bolt and mark this distance on the sill plate board. Drill holes that are ¼ to ½ inch larger than the diameter of the anchor bolt, to allow for slight adjustments as the sill plate is installed. Put the board into place over the anchor bolts and verify that its inside edge can align properly with the chalk line. Install the washers and nuts to attach the sill plate board. Tighten the nuts so the board is secure, but not too tight just yet. Wait for verification of the squareness of the entire sill plate for final tightening. Repeat this process for the rest of the boards making up the sill plate. Double check the sill plate for squareness. Use the wiggle room of the holes around the anchor bolts to make small adjustments. If bigger adjustments are necessary, drill larger holes where necessary. When everything is looking good, give the nuts a final tightening on the anchor bolts. Install the Rim Joists: A few notes about the installation of the rim joists: We use conventional lumber 2x10s for the rim and floor joists. The nails which attach the rim joists to the sill plate must be galvanized because the sill plate boards are pressure treated. To ensure that the floor system is airtight, construction adhesive is used at all seams. This includes the entire length between the rim joists and the sill plate. It also includes junctions between rim joist boards, at the corners of the structure and at the ends of the boards where multiple boards must be used to run the length of the sill plate. Rim joist boards must be installed crown up. This provides the best support for the wall system which depends on the floor system. Attach the rim joists to the sill plate by toe nailing galvanized nails. NC building code specifies using 3” x .131 galvanized nails fastened every 6” o.c. or 3” x .120 fastened every 4” o.c. Perform a final check for squareness. While not easy, individual rim joist boards may be removed and reinstalled at this point. Beyond this point, once the floor joists are in place, any fixes to squareness become much harder. Install the Floor Joists: Refer to the building plans for the floor joist layout. Expect 12”, 16” or 24” on center. Each of these layouts allows for the edges of 4x8 sheets of subfloor to have joists align with the edge of each sheet. Use a tape measure and a speed square to mark the layout on the rim joists on both sides of the house. Remember to offset the first joist by ¾” to ensure that the edges 4x8 subfloor sheets to align with the center of a floor joist. One trick for doing this is to nail a temporary ¾” block on the outside of the rim joist and hooking your tape measure over that block. Using the layout marks as a guide, install the floor joists. Use 3” nails to attach the floor joists to both the sill plate and to the rim joist. The nails attaching to the sill plate must be galvanized, because the sill plate is pressure treated. The nails attaching to the rim joist can galvanized or not. You can save a little in materials if you use non-galvanized nails for attaching to the rim joist, but it may cost more for labor given the need for more nail guns and more time ensuring which nails are used where. At each end of the floor joist, use four nails through the rim joist into the end of the floor joist and 4 nails, two per side, toe nailed attaching the floor joist to the sill plate. Install the Subfloor Install 4x8 sheets of subfloor over the rim and floor joists. We use Advantek, t & g subfloor. The sheets have tongue and groove edges. Install the first sheet with the tongue edge along the rim joist. Successive sheets in the row will have their tongue edge secured into the previous sheet’s groove. Before placing each sheet, apply subfloor glue to the top of the joists. We use Huber purple subfloor glue. Use 2 ⅜” nails 6” OC along the edges and 12” OC in the field to attach each sheet to the floor joists. Between each row of sheeting, a ⅛” spacing between sheets is required. An easy way to ensure this spacing is to temporarily secure 8d nails into a couple floor joists directly next to the sheet of the previous row. The diameter of this nail will insure the 1/8” gap. Air Seal Outside Joints The final step to finish the floor system is to use sealing tape along all exterior joints of the floor system. Often this step is delayed until the wall frame has been installed, prior to the exterior wall sheathing going up. This is because the joint between the bottom plate of the wall system and the subfloor of the floor system must also be taped*. Delaying the taping of the exterior floor system joints until the wall framing is erected allows the taping to be done all at once. * This seems to show a discrepancy between the airtight rim video and our construction plan. We plan on building the walls, framing and sheathing, including the 10” overhang of the sheathing to cover the floor system bands, before erecting the walls. This plan will not allow for taping the joint between the bottom plate and the subfloor. One potential change in plan might be to not include the 10” overhang to cover the banding, but instead sheath two inches shy of the bottom of the bottom plate. Then when the walls are erected, the joint between the bottom plate and the floor system are still exposed and can be taped. Then we can rip 12” strips of sheathing to install over the rims and the bottom plate. Another potential compensation would be to stick with the plan, and run a generous and continuous bead of high grade caulking material at or just below the joint of the bottom plate and subfloor. This caulking would provide a seal between that joint and the sheathing, keeping the inside of the joint within the building’s envelope. Turns out the second is what we did, and thinking about it, it is common practice. It seems the air gap would always be taken care of by this, and taping between the bottom plate and the floor platform is redundant.
