What are Floor Joists?
How to Construct a Raised Floor with Floor
Joists and Joist Headers
By Mark J. Donovan
Floor joists are used on raised
floors, or floors that are not built on a slab. Floor joists are structural
members that support much of the weight of a home. They are horizontal framing
members that span the gap between foundation walls and support beams and which
the subfloor sheathing is attached. Floor joists tie into joist headers that run
perpendicular to them. In addition, the joist headers act to cap off and band
the floor joists together.
If your home includes an unfinished basement you can observe the floor joists
from the basement.
Again, they are the long horizontal
2x10s, or 2x12s pieces of lumber that span the basement ceiling and rest on the
foundation sill and center support beams.
Floor joists are typically spaced 16 inches on center, however in some cases
they may be spaced at 12 or 24 inch centers, depending upon the structural
design of the home and building code requirements. Floor joist sizing, in terms
of span length and lumber dimensions, is determined by building code and the
specific blueprint requirements of the construction project.
Using undersized floor joists, at a
minimum, can lead to excessive deflection in the raised floor, so it is
imperative to use the right size lumber for floor joists.
Floor joists never rest directly on the concrete foundation. Instead a layer of
pressure treated lumber followed by a non-pressure treated layer of lumber is
fastened to the concrete foundation sill. Typically there is an insulation
blanket that is sandwiched in between the concrete foundation and the pressure
treated lumber. The floor joists are then stood on edge and rested on the wood
sill. They are then nailed to the joist headers. In many cases joist hangers are
used to more securely fasten the floor joists to the joist headers.
The non-sill end of the floor joists
typically rest on a center beam, where they are often spliced with other floor
joists coming from the opposite side of the foundation wall. Spliced floor
joists should over lap each other even when they rest on a center beam. In some
splicing cases, if there is no center beam, the floor joists are butted
end-to-end and spliced with plywood gusset plates. Where openings in the raised
floor are necessary, e.g. for a stairway opening, floor joists are doubled up at
the edges of the opening and perpendicular joist headers are used to cap them.
Once the network of floor joists and joist headers is constructed, subfloor
sheathing is then attached to the floor joists to finish the raised floor.
The sheets of plywood are placed over the floor joists perpendicular to
them. Each course of plywood deck sheathing is staggered from one row to the
next to create a strong raised floor.
Fixing an Existing Raised Floor
If an existing raised floor has excessive deflection, the floor can often be
stiffened up by sistering additional lumber to the existing floor joists.
Photos by Mark Donovan
In some cases lumber of the same size as the existing floor joists are used and
are simply fastened to the existing floor joists. In other cases, smaller lumber
than the existing floor joists are used. Alternatively, additional lumber can be
placed on either side of each existing floor joist, effectively sandwiching each
one, to make the floor stronger.
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