In this floor framing walkthrough, AwesomeFramers takes viewers through the anatomy of a floor system built over a crawl space, covering PWT-treated LVL mudsills, Simpson Strong-Tie Titan HD screw anchors, I-joist layout, LVL beams, squash blocks, web stiffeners, floor diaphragm fastening schedules, and the trade-offs involved in specifying joist depth for floor stiffness. The video also addresses the common concern about I-joist fire safety head-on. For more, visit AwesomeFramers YouTube channel.

VIDEO TRANSCRIPT:

Seems like it should all be flush, right? The whole point of engineered products is that they’re supposed to be perfect. It turns out the rim sits about a 16th of an inch deeper than the I-joist, and there’s a good reason for it. The rim takes the gravity loads from above. The floor joist takes only the floor load. It doesn’t take both.

PWT-treated LVL mudsill

We are all framed, so let’s go through the anatomy of floor framing over a crawl space. The connection from house to concrete starts with the mudsill. In this case, we’re using PWT-treated LVL, and we staple foam to the back side before setting it so that it separates the concrete from the wood. Code doesn’t require it, but we’re going for a net-zero energy-ready standard, so it’s part of the program.

If you’re not familiar with LVL, it’s a series of laminations laid up and glued together to make structural beams. With PWT treatment, the treatment goes into the glue, so the material is treated from the inside out. Most pressure treatment in the Pacific Northwest doesn’t actually penetrate to the center. It’s forced in through incising marks. PWT-treated LVL carries a 25-year warranty.

What’s especially useful is that it comes in 60-foot lengths, which means we can order a single piece for every wall on the job. The longest on this one was 44 feet. The advantage is laser-straight material with no warp, checking, or splits. Anyone who has worked with conventional pressure-treated lumber knows what that means.

Code requires an anchor bolt within 12 inches of the end of a mudsill run. When you’re butting standard 16-footers together, you end up with extra bolts at every joint to meet that requirement. With a continuous LVL mudsill, those extra bolts go away.

Simpson Strong-Tie Titan HD screw anchors

To anchor the building to the concrete, code requires J-bolts, cast-in-place anchor bolts, typically half-inch at six feet on center. We stopped using cast-in-place anchors about five years ago. We got our engineer to approve Simpson Strong-Tie Titan HD heavy-duty screw anchors, and we haven’t looked back.

No special inspection is required. The length is stamped on the head, which makes the inspector’s job straightforward. Drill the same diameter as the shank: a 5/8-inch bit for a 5/8-inch anchor. Too big and you can pull them out. Too small and they won’t go in. Drill about an inch extra so the dust settles before driving.

In our seismic design category, we use 3x3 bearing plates, and code specifies a minimum thickness. We’ve done videos in the past showing import bearing plates that don’t meet that spec. Worth paying attention to.

These cost more per piece than J-bolts, but the total installed cost is competitive. We set our mudsill dead on the line, do all the layout, then install the anchors at 48 inches on center, which is every other stud on a 24-inch layout. Because we’re using PWT-treated LVL, we never need one within 12 inches of the end, which saves a bolt on most runs.

The common question is whether the hooked J-bolt is stronger. Surprisingly, no. There’s a good engineer video on YouTube that covers this, and I’ll link to it. J-bolts handle base shear, keeping the house from sliding off the foundation when the ground moves. The Titan HD is rated for both shear and uplift. We’re not giving anything up by using a post-installed anchor.

I-joist layout and LVL beams

The joist layout for both floors is stapled to the side of the joist for reference on site. All the details you need are printed right on the plans. The first-floor sheet tells you every joist, including how many of each length. I-joists are manufactured in 60-foot lengths and cut from there, which is how you can get 44-footers or 10-footers out of the same stock. No crown, no camber.

Wherever bearing walls land, the plans show either doubled joists or, in this case, LVL. Here’s a cutoff from a 2.1E LVL. Insanely strong compared to sawn lumber. They glue up the laminations and rip them to size, so they come out of the plant perfectly sized in both depth and width.

Store them correctly, and they stay that way. A lot of contractors have seen a bowed LVL on site and concluded the product is inconsistent. Usually it wasn’t stored right. I visited the manufacturing plant. When these come out, they’re perfect. Strong and a very good use of wood fiber.

Pony wall detail and floor diaphragm

Instead of a pony wall plus a separate rim, we framed the pony wall straight to the top of the floor system. That eliminates a hinge joint. Does it matter structurally? Probably not. What it does do is give the subfloor a 5.5-inch bearing surface instead of 1.25 inches. We have the full 2x6 to glue and nail the subfloor to, and the wall above sits right on top of it.

The floor diaphragm fastening schedule runs 6 inches on center at the mudsill, with the rim attached to the I-joist on a specified fastening schedule. The subfloor goes on at 12 inches in the field and 6 inches on the edges. All of that works together to transfer loads from the top of the assembly down through the foundation. Think of the floor system as a beam on its side.

Squash blocks and web stiffeners

The I-joist is strong and stiff, stiffer than an equivalent 2x10, with long available lengths that translate to flatter, more comfortable floors. What it’s not designed to do is handle point loads from above on its own. That’s why the details include squash blocks and web stiffeners.

The web of a tall I-joist under a concentrated point load would want to fold. Reinforcing those locations prevents that. Squash blocks are cut a 16th of an inch taller than the joist, so the block carries the load from above and transfers it straight down, not through the joist.

The window header location here shows three studs. The rim and blocking carry the load, not the joist. It’s almost as if the whole thing was designed to work together as a system.

Fastening at the rim is one nail through the top flange, one through the bottom, and a couple on either side at the base. We never think about single fasteners in isolation. We think about systems. The subfloor over the top, fastened at 12 and 6, ties the whole diaphragm together.

Deeper is cheaper for floor stiffness

The floor manufacturer runs proprietary software that models the complete assembly. We’re framing at 24 on center. If we need a stiffer floor, we could tighten spacing to 16 or 12 on center, but the more cost-effective move is to go deeper. These are 9.5-inch joists. If we step up to 11-7/8, identical in every other way, just taller, we get a stiffer joist, and the framer does no additional labor. The material is just taller. Deeper is cheaper.

I-joists and fire safety

The common and reasonable concern about I-joists and fire is worth addressing directly. A 9.5-inch I-joist will burn more quickly than a 2x10. Does that make this house less safe than the neighbor’s built with sawn lumber floor joists? The answer is no.

A house is a system of systems. Each system is designed to meet minimum code. The fire rating is determined by the assembly, and there are UL fire-rated assemblies that incorporate wood framing, insulation, drywall, and various siding types, all tested and documented. Google “UL fire-rated assemblies” and you’ll find a lot of detail. None of this is new. This assembly meets code. So does the neighbor’s house framed with 2x10s. We just get there through slightly different paths. I’ll link to an APA article covering several of these assemblies for anyone who wants to go deeper.