Raised Floor Framing
The Basic Steps and Components in a Framed
Raised Floor System
By Mark J. Donovan
Raised floor framing consists of a
system of sill plates, floor joists, joist headers, sheathing and blocking. A
raised floor system offers numerous advantages over a concrete slab floor. First
and foremost a raised floor gets the home off the ground, thus helping to
protect a home from water and insect damage. A raised floor frame also enables
an area underneath the home for installing electrical and mechanical equipment.
In some cases the area underneath the house could be literally a basement or a
Finally, a home that includes a
raised floor frame simply offers more curb appeal by raising the house up off
the ground a foot or two and making the home look more stately.
Raised floor framing begins with installing pressure treated lumber on top of
the concrete foundation sill. Placed underneath the pressure treated lumber is a
thin layer of moisture resistant blanket insulation to provide a tight seal
between the concrete and the pressure treated lumber.
Resting on top of the pressure
treated lumber is a second layer of non-pressure treated lumber to complete the
sill plate. The sill plate lumber is anchored to the concrete foundation sill
via bolts or metal straps that protrude upward from the concrete foundation
Any required center beams are then installed in between the foundation walls of
the home and temporarily held up with lumber posts. Eventually the lumber posts
are replaced with Lally columns (concrete filled metal tubes).
Once the sill plate has been attached
to the concrete foundation sills and the center beam(s) is in position, floor
joists can then be installed to span the distance between the sill and center
beam(s). Typically floor joists are installed on 16 inch centers. The floor
joists make up the heart of the raised floor framing. They normally are 2x10s or
2x12 boards that are placed on edge and run perpendicular to the foundation sill
and center beam(s). They are capped off on the ends with joist headers. The
floor joists are either nailed directly to the joist headers and sill plates, or
joist hangers are used to tie them together.
Floor joist sizing is dependent upon building codes and the shape and style
of the home. It is important that the proper floor joists are used to
provide sufficient structural integrity of the home. The raised floor
framing supports much of the weight of the home. Spliced floor joists should
sufficiently overlap each other over the center beam and should be fastened
together for maximum strength and rigidity. In splicing cases where there is
no center beam, floor joists should be butted end-to-end and tied together
with gusset plates.
Photos by Mark Donovan
Where there are openings in the raised floor for stairways, floor joists should
be doubled up around the perimeter of the opening and perpendicular joist
headers installed to cap them off.
The next step in raised floor framing is to install the plywood sheathing on top
of the floor joists. Typically ¾ inch tongue and groove plywood or OSB sheathing
is used. The plywood or OSB sheathing boards are placed perpendicular over the
floor joists. The sheathing is staggered from one row to the next to ensure a
strong raised floor. An adhesive should be installed on top of the floor joists
just prior to actually placing the sheets of plywood on them. The adhesive helps
to ensure a better bond to the floor joists and prevent squeaks.
Finally, bracing is installed in between each floor joist bay to provide added
rigidity to the raised floor system. If metal bracing is used, the metal bracing
brackets should be installed prior to installing the raised floor sheathing.
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