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Building a Dream



Sills, joists, sub-floors and walls
Your home’s taking shape
Including the halls

Hammers and nails
Rough stairs and rails

Now will begin
All those carpentry tales

As the previous chapter points out, a solid foundation is the best investment you can make in your new structure. It's the hardest thing to fix if things go awry, and it's one of the cheapest things to make right with a little foresight and planning. A sound home, however, doesn't stop at the foundation. On top of the foundation is yet another foundation. In a certain sense, a typical residential structure is a series of platform foundations stacked vertically, each designed to support the load above it. In a wood-frame home, the mud sill provides the transition from the concrete foundation to the floor and wall framing. Typically, the mud sill circumnavigates the exterior perimeter of the house. However, with increasing use of interior concrete stem walls (as opposed to isolated posts and girders), mud sills will also be found on all internal stem walls as well.

Since the mud sill is in direct contact with concrete, it’s prone to moisture, termite attack, and rot. For these reasons, building codes dictate that mud sills be foundation-grade redwood (which has a natural decay resistance), or preservative-treated lumber. The latter is more common due to its widespread availability and proven resistance to moisture-related decay. Since preservative treating involves highly toxic chemicals, some consider it a potential health and environmental liability, despite its proven resistance to deterioration. Typical chemicals used in preservative-treated lumber includes Ammoniacal Copper Zinc Arsenate (ACZA), Chromated Copper Arsenate (CCA) or Ammoniacal Copper Quatenary (ACQ). A good reference on the subject can be found online at the Sustainable Building Sourcebook website.

Recent advances in boron-impregnated lumber show promise, yet this technology is still in its infancy and availability is limited. Since boron is water soluable, it can leach out of the wood if exposed to moisture, rendering it less effective. Generally, wood used in humid parts of the world must be pressure treated exclusively. The bitter taste repels termites. A lumber yard I saw on the island of Kauai sold exclusively pressure treated lumber. Anything else would have trouble enduring the torture of wet tropical weather and ravenous termites.

Minimum mud sills are typically 2x6 pressure-treated Douglas Fir, commonly referred to as PTDF. If you're willing to spring a few extra bucks and want a more substantial mudsill, consider using 3x6 or even 3x8 mudsills such as those used on better residential and commercial jobs. The three-by material is a full inch thicker (2-1/2" vs. 1-1/2") and makes an excellent nailing base for the floor joists and plywood shear walls. The extra thickness also allows for anchor bolts to be recessed below the top surface of the mudsill, eliminating interference between floor joists and protruding anchor bolts.

Forty years ago when a 2x6 was really a full two inches thick, using three-by mud sills might have been overkill. But as dimensional lumber continues to shrink in actual cross-section, two-by lumber looks pretty flimsy in some applications. Since 3x lumber is heavier, some framers find it more troublesome to work with. But if they know they're working with 3x material, they'll take extra pride in your job because it's the savvy client who demands beefier lumber. A carpenter's job is largely based on the gratification of building something good. A foundation with 3x mud sills provides a strong and durable attachment transition. Even your neighbors will comment that your house looks "really strong." Our neighbors thought we were building a parking garage!

While some will be justifiably critical of "overbuilding," I am seeing increasing use of heavier mudsills (and wall studs where adjacent sheathing panels adjoin), particularly in areas prone to natural disasters such as tornadoes, hurricanes, and earthquakes. The most recent model building codes specify 3x material where excessive sheer forces can be expected to occur. I recently toured a new development in hills overlooking San Francisco Bay and noticed that 3x8 mudsills were used throughout, with 5/8" anchor bolts spaced on 16" centers. 

What’s more, every three years the building codes get tougher and tougher. That's because with each natural disaster we learn more about the deficiencies in existing designs. To correct these deficiencies, building experts enact new legislation to tighten standards. Despite its increasing scarcity, skimping on lumber in a structural situation is something you should never do in your home. If you're going to build something, build it right. Build it so you never have to worry about it again. Build it so you don't leave a hidden time-bomb behind.

Sizing Floor Joists

In this light, it's important to remember that building codes are designed to ensure minimum safety. The codes themselves don't guarantee a quality end product. For example, the Uniform Building Code allows a 2x10 #2 Douglas Fir floor joist to span up to 16 feet when spaced on 16" centers. This number is based on a maximum allowable deflection of 1/360 of the joist length in inches. That means a 16-foot joist could deflect up to 1/2" under fully loaded conditions and still meet code. While this meets requirements for structural safety, a floor this springy would feel like a trampoline under your feet. I hate floors that flex and creek when I walk. Remember that the building code doesn't attempt to specify requirements for perceived quality. Rather, the codes set forth minimum standards to ensure general occupant safety.  And rarely do codes become more lenient.

Once the mudsills are in place, the floor joists are laid crown up. Depending on the spans and whether the structure uses intermediate supports, such as girders, the sub-floor joists range anywhere from 2x6 to 2x12 dimensional lumber. On very long spans, this may be increased to double 2x12 or 3x12. However, the growing acceptance of engineered floor joists, such as the Trus Joist MacMillan Silent Floor® system (www.trusjoist.com), is replacing traditional dimensional lumber in many floor joist applications.

Engineered joists are made of a plywood sheathing or oriented strand board (OSB) turned on its side, with top and bottom nailers made of Fir, Pine, or laminated veneer lumber. A cross-sectional view looks like a thin "I" beam, which is why some call them "I-joists." Even though they appear less substantial than solid lumber, the advantage of engineered floor joists include lower installed cost and greater strength for a given span compared to traditional lumber.  In addition, the flexibility of engineered lumber allows longer clear spans that would be prohibitively expensive with traditional lumber.  Typical depths range from 9" to 24" and lengths up to 52' or greater.

The efficient use of materials also saves on natural resources -- as much as 50% per joist. Another advantage is their improved "squeak" performance. There’s nothing worse than building a new home, only to hear the sub-floor squeak as you walk overhead. Since engineered joists are made from cured lumber, there is no shrinkage that can lead to later nail pops. The ultimate strength is achieved when the sub-floor is glued to the joists, and fastened with ring-shank nails or, better, quality sub-floor screws.


If you're planning to add tile or marble anywhere, a stiff floor is essential to prevent cracks from appearing in the grout lines or through tiles. Many new homes today use 3/4" tongue and groove plywood or oriented strand board sub-floor, although recently I've seen some sneaking by with slightly thinner material. Since a major portion of our home has heavy quarry tile floors, we specified 1-1/8" tongue and groove APA-rated Sturd-i-Floor as our sub-floor. We then used a 1/2" layer of Hardibacker, securely mortared and screwed to the sub-floor.  Sturd-i-Floor is a combination sub-floor and underlayment that makes an excellent substrate for carpeting, hardwood floor, and tile. Standard 1-1/8" plywood has voids under the laminations that make it less suitable as an underlayment for carpeting. The concentrated weight of a high heel shoe, for example, is more than enough to puncture right through thin plywood veneer hiding internal voids. Insist on Sturd-i-Floor. Although the beefier sub-floor wasn't required for structural safety reasons, the floor feels very stiff and makes an excellent base for the tile installation.  Combined with the the tile backer board or traditional mud base, the stiff floor helps to prevent flexing and cracking in the tile and/or grout.  Indeed, our tile floor has shown no signs of any hairline cracking over the years.

To minimize nail pops and squeaks, always run a generous bead of heavy-duty construction adhesive along the top of the floor joists prior to laying the sub-floor. It's a good idea to "fit" the sub-floor first just to make sure there's nothing in the way of a clear fit, such as a damaged tongue or groove. Once the sub-floor is in place and squared up with a mallet, securely fasten it to the floor joists with ring-shank nails, or better, high-quality screws. Traditional smooth nails can lead to squeaks down the road, particularly if the floor joists shrink as they lose moisture. As mentioned earlier, the use of engineered wood joists helps to reduce this concern.

If you have a crawl space, planning for one or more inside access points is important. Consider using a closet or some other inconspicuous area to cut an access hole -- about 30"x24" or so. Be sure to block the opening properly since it's likely you'll have to cut one of the floor joists to fit this clear opening.


In a traditional wood-framed house, the walls are either 2x4 or 2x6 studs and plates. Unless your home is in a very temperate climate, use 2x6 framing on all exterior walls. In very cold climates where temperatures frequently drop well below zero, even thicker walls, such as 2x8, may pay dividends over the long haul. The thicker walls not only allow ample space for heavier insulation, which provides better interior comfort and energy savings -- but they improve sound proofing and strength as well. Another benefit of thicker walls is having sufficient a cavity for plumbing waste and vent stacks.  Since some of these pipes can be as big as 3" in diameter or more, using 2x6 walls helps to maintain the structural integrity of the top and bottom plates. In any cases, it's always a good idea to nail a metal strap across any plates which have been compromised by large drilled openings, such as is common for plumbing or HVAC ducts.

Where openings exist in the framing, such as for doors and windows, a header is installed over the opening span. While the size of the header depends on the length of the span and the weight above, typical headers are 4x12 or 6x12. Since 4x12's are generally strong enough for most typical openings, these work well with 2x6 framing, providing a 2" pocket on the inside for added insulation -- such as 2" rigid expanded polystyrene (EPS). Increasingly, the use of LVL (laminated veneer lumber) headers and beams is replacing traditional solid-wood headers. LVL's offer significant strength and stability with minimal shrinkage or cupping, typical of sawn lumber.

Generally, the exterior of the house is covered with 3/8" or 1/2" structural plywood or OSB to provide shear strength and structural stability. However, unless you're building this house for yourself, don't be surprised if your developer cuts corners by eliminating this external sheathing from many walls. Over the past few years, I've seen million-dollar homes in new, prestigious Silicon Valley developments that use just the bare minimum of external sheathing.  This practice amazes me given the area's predisposition to major earthquakes. The builders install the bare minimum of plywood to meet shear strength requirements of the building code, then staple stucco paper right over the open studs, supported by nothing more than thin wire. Down the road, this has the potential to lead to water or moisture infiltration. For the price of these homes, unsuspecting buyers are none the wiser.

More About Wood and the Manufacturing Process

To learn more about plywood, oriented strand board (OSB), I-joists, and glulam beams, visit the APA website.

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