I-joists are awkward to cut because the top and bottom chords are wider than the web. To overcome this difficulty, make a simple jig with 3/4-in.-thick plywood. Cut a rectangular piece of plywood to fit between the chords and serve as the base of the jig. Screw a longer piece to the first piece, positioning it to guide a 90-degree cut. The edge of the top piece guides the base of the circular saw, as shown in the photo at right. Lay the guide on the I-joist, set the saw on it, and make a square cut. It’s that simple.

Cut I-joists with a guide. Scrap sheathing that is nailed or screwed together creates an effective guide for cutting I-joists. [Photo by Roe A. Osborn, courtesy Fine Homebuilding magazine © The Taunton Press, Inc.]

swell, shrink, crack, or warp the way solid lumber does. They are much lighter and easier to carry than 2x joists. And they’re uniform in size. In a load of 2x joists, you might find up to 3/8 in. of variation in joist width. I-joists don’t vary; once installed, they create a dead – level floor. Nails driven through the sheathing into the top chord are less likely to come loose and create a squeaky floor, especially when the sheathing is applied with adhesive. In terms of price, they are competitive with standard-

dimension lumber. Installation details for I-joists are slightly different than those for 2x joists. I’ll cover those differences just ahead.

Nail rim joists first

Rim joists form the exterior of the building and are the first joists to be installed. The layout of other joist locations are marked on the top edges of the rim joists. Cut the rim joists to length and toenail each one flush with the outside of the sill. I drive one 16d nail every 16 in. around the perimeter (see the photo at left). Don’t forget that nails going into PT wood should be hot-dipped galvanized. In earthquake and high-wind areas, code may require that the rim also be secured to the sill with framing anchors, so check with your local building inspector. If there are no vents in the foundation, they can be cut into the rim joists. A standard screened vent fits in a 4//2-in. by 14//2-in. opening.

If you’re framing a floor with I-joists, you’ll probably use the specially made OSB rim joists supplied with your I-joist order. Install rim joists along only one side of the house. Then lay the I-joists flat across the sills, butting the end of each joist fast against the installed rim joist. The opposite ends of the joists will extend over the sill at the other side of the house. You can now

snap a line across the ends to establish where the I-joists need to be cut. A simple jig, explained in the sidebar on the facing page, makes it easy to cut the joists smoothly and accurately. After cutting the I-joists to length, complete the rim joist installation.

Joist layout goes quickly

When a single joist spans a house from edge to edge, the layout is identical on parallel rims. Just hook a long tape on the end of the rim joist and make a mark on top every 16 in. (32 in., 48 in., etc.) down the entire length. Put an “X” next to each mark to indicate which side of the line the joist goes on.

When the joists lap over a central girder or wall, the layout on the opposing rim joists must be staggered. On one rim joist, mark the 16-in. o. c. locations with an “X” to the right; on the opposite side, lay out the joists with an “X” to the left. This allows the joists to lap and nail over a girder or crib wall, where they will be stabilized with blocks (see the illustration on p. 68).

Your joist layout may include openings (called headouts) for a stairway or to provide clearance for plumbing or vents. Your plans should show these openings, but it’s always a good idea (and it could save a lot of time and effort) to check with the plumber. A common mistake is leaving insufficient room between

Roll and nail the joists

Once the joists are cut to length and in position, carpenters say that it’s time to “roll” them. This just means setting the joists on edge, aligning them with their layout, and nailing them in place. If you are working with 2x joists, it’s im­portant to sight down each joist to see whether there’s a bow or a crown, and then set the joist with the crown facing up.

Drive two 16d nails through the rim joist directly into the end of the joist—one nail near the top and one near the bottom (see the photo below). Most codes also require that joists be toenailed (one 16d on each side) to the sill plates and supporting girders. To nail off an I-joist, drive a 16d nail through the rim joist and into each chord, then nail the chord to the sill on both sides of the web.

Make sure that all the joists are nailed securely. This is important for safety reasons, for quality workmanship, and for meeting code requirements. Once all the joists are nailed upright, stop and check for symmetry—make sure the line of one joist is parallel with another.

the joists for the tub’s trap and the toilet’s drain. You may need to frame a headout to make room for plumbing. For headout framing details, see the sidebar on p. 67. When framing with I-joists, remember that, like any other type of engineered joist, they cannot be notched or cut midspan with­out destroying their structural integrity.

Cut the joists in place

If you trust your eye, try cutting 2x joists in place rather than measuring each one indi­vidually. As you become comfortable using a circular saw, you’ll be able make a square cut without using a square (see the sidebar on the facing page). This technique is definitely worth learning. Over the course of framing a house, it will save a significant amount of time.

Driving a couple of nails through the side of one 2x into the edge or face of another creates a strong connection. This is a good way to join two 2xs at a right angle. But sometimes this isn’t possible, either because the board is too thick or because its face is not exposed. That’s when you resort to driving a nail at an angle, or toenailing.

To toenail two boards together, hold the nail at a 60-degree angle and start it about 1 in. from the end of the board. If the nail angle is not correct, the connection between the two pieces of wood will not be as strong. Back up the wood with your foot to hold the board in place as you toenail. With practice, you’ll soon gain skill, speed, and confidence.

attaches to the pier post and holds it in position. Secure a TT-in.-thick pressure-treated pad to the top of the pier if the posts will be cut from untreated lumber. You can measure for the post’s length just as you measured for the crib wall’s studs, stretching a line above a piece of girder stock placed on the pier. Cut your posts to length, then secure them directly to the piers or toenail them to the blocks attached to the piers. Make sure the posts are plumb and paral­lel to one another.

Girders must butt together over a post. When the end of a girder fits into a pocket in the foun­dation, you’ll have to shim up the girder to get the top surface level with the top of the sill. The shims used beneath girders will bear the full weight of the floor, so they must be cut carefully from pressure-treated stock. Cut uniformly thick shims instead of tapered ones and make them large enough to fit in the bottom of the founda­tion pocket. Rather than foundation pockets, we inserted a metal 4x post base in the footing next to the stem wall to hold the end of the girder.

Solid lumber versus engineered I-joists

Up until 20 years ago, most of the floors in this country were built with standard 2x joists. These days, more floors are being built with engineered I-joists. So named because of their “I” profile, I-joists have plywood top and bottom chords connected by an OSB web (see the top left photo on p. 62). They offer several advan­tages over solid lumber. Being an engineered product, they are knot-free and can span long distances without interior support. I-joists don’t

PNEUMATIC NAILERS

Volunteers who help build Habitat houses use only hammers to drive nails. But these days, buildings are nailed together with all kinds of pneumatic nailers. These are good, reliable tools, available for framing, finish work, siding, and shingling. However, there are basic safety considerations to keep in mind.

■ Treat a pneumatic nailer with respect. Be mindful of what you are doing. Never point a nailer at yourself or at others.

■ Read and follow the instruction manual regarding its maintenance and use.

■ Don’t walk around with your finger on the trigger. You could acci­dentally fire a nail.

■ Adjust the air pressure as needed. Larger nails require more pressure.

■ Wear safety glasses or goggles.

■ Disconnect the nailer from the air compressor before clearing a jammed nail.

■ When nailing on a sidewall, don’t hold the nailer in front of your face. Hitting a metal strap or other hardware beneath the surface could cause the nailer to recoil into your face with considerable force.

■ Drain moisture from the compressor tank after using it. A rusty, compromised tank can explode under pressure.

■ No one under 18 years of age should use a pneumatic nailer.

■ Pneumatic nailers should only be used by a trained professional or an experienced volunteer under supervision.

Unless you are building on a slab, you’ll prob­ably need to provide midspan support for the floor joists. Without additional support some­where between the sills, the joists can be over­spanned, resulting in a finished floor that sags or feels bouncy. A friend once took me through

his 18th-century home in rural Connecticut. The beautiful, hand-hewn floor joists in the basement were fascinating—dry, free from rot and termites—but far overspanned. Upstairs, it felt like walking on ocean waves. Clearly, what was needed was some support to keep the joists from sagging and bouncing in the middle.

Codes require a minimum of 18 in. between the earth and the joists in a crawl space. These days, two systems are commonly used to provide midspan support for the joists: crib walls (also called pony walls) and post-and-girder systems. A crib wall is just a shortened version of a regu­lar stud wall and is supported along a continu­ous concrete footing. With a post-and-girder system, a solid or built-up girder (also called a beam) is supported by posts every 6 ft. or so, depending on code. The girder usually fits into a recess or pocket where it meets the foundation wall. Joined to the girder by metal connectors or plywood gussets, the posts bear on concrete piers (see the illustration on p. 48).

In both these systems, the joists rest on and are nailed to the top support member.

The width of the joists and the length of the span determine how much support is needed. With 2×6 joists, for example, posts and girders are often placed every 6 ft. With 2 x12s or

Build a crib wall. It’s best to use PT wood in underfloor areas. Crib walls (also referred to as pony walls) are short, stud-framed walls and can provide midspan support for joists that extend over a crawl space. After bolting the wall’s bottom plate to the footing, stretch a line over the sills to measure the length of the crib wall’s studs (see the photo above). Place two pieces of top plate stock on the wall’s bottom plate and measure up to the line. Toenail the studs to the bottom plate, spacing them 16 in. o. c. or 24 in. o. c. depending on your code requirements (see the photo below). [Photos © Memo Jasso]

Wait to carpet over concrete. Make sure you let a con­crete slab dry out well (for several months) before laying carpet on it. If you don’t, the carpet adhesive may not hold properly and your carpet could rot, possibly posing a health hazard.

Stretch a string tightly from one sill to another. For the post length, measure from a piece of girder stock to the taut string. To find the stud lengths for a pony wall, measure from the top of a stack of three wall plates to the string.

engineered I-joists, only one support in the center may be needed.

Crib-wall construction

Just in case a moisture or termite problem develops in the future, I like to build crib walls with pressure-treated wood. To determine the length of the crib wall’s studs, first bolt the wall’s bottom plate to the footing. Then tightly stretch a string above the crib wall’s bottom plate and across the mudsills installed on the stem walls. Set two pieces of top plate stock on the bottom plate. The distance between the top plate stock and the string is the length of the crib wall’s studs (see the top right photo on the facing page).

Toenail the studs to the bottom plate, spac­ing them 24 in. o. c. but leaving a 30-in.-wide opening somewhere in the wall so that plumbers, electricians, and others can get from one side of the crawl space to the other. When toenailing the studs to the crib wall’s bottom plate, you can drive either four 8d toenails or three 16d nails per stud. If you’re using a pneumatic nailer,
make sure you follow the safety guidelines explained in the sidebar on p. 65.

Once the crib wall’s studs are in place, nail on the two top plates. Drive a pair of 16d nails through the first plate into the top of each stud. Secure the double top plate with a single 16d nail at each stud location, and toenail the plate’s ends to the sill on the foundation wall at each end of the house. I like to sheathe sections of a crib wall with pressure-treated plywood (my first choice) or OSB to provide good lateral bracing (see the bottom left photo on the facing page). Be sure not to sheathe over the crawl-through opening you framed in the wall. As an alternative to sheathing a crib wall, you can stiffen it with 2x diagonal braces nailed to the top and bottom plates and across at least one stud.

Build a post-and-girder system

Posts used to construct a post-and-girder system can be anchored directly to a metal post base that is set in the concrete footing. You can also use a precast pier with a metal strap that