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
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
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