I have cut a window opening for installing a new window in an exterior
concrete block wall.
The window brochure says this is a "H33" window, the size is 26 1/2" W x 38
3/8" H and it called for a rough opening to be 1/2" larger in either
dimensions. This means the rough opening need to be 27" W x 38 7/8" H which
is what I cut the concrete opening to be.
However, when I looked at other window brochures, I see more terms - CALL
size, BUCK size, FLANGE size and they are all a bit different...
Also, I talked to one window contractor and he told me he has to fill in the
concrete block voids with solid concrete then screw the window onto the
concrete with Tapcon. Another window contractor told me he would install an
interior frame of PT lumber around the concrete opening, attach the wood
frame to concrete and then attach the window to the wood frame. The first
contractor says this does not work as the window would be blown away during
Now I am wondering, what does the "rough in" mean, it means the concrete
opening or the opening after installing the wood frame? Do I really need a
Thanks in advance,
Many windows are installed with nailing flanges, but others may be
installed directly into a bare masonry opening, for example, a
basement window. Find out what window design you have to make a
decision about monting method. IMO, the treated wood frame sounds like
a better way. Tapcons are good, but the window design must have the
frame strength to use them properly. HTH
RO refers to the size of the opening the window will install into. If you
want wood frame around the window the opening is going to need to be large
enough to hold the frame AND have the proper size opening for the window.
Did you cut the block out all the way to the top course? Or are there
blocks over the top of the window?
Not if there are no blocks over the top of the opening, you fill all voids
and use a replacement type window. Personally I would prefer to use the wood
When you say "new window " what do you mean? Is it a window for new
construction (probably has a nailing flange) or a replacement window
(no flange since it's attached through the frame into the wood around
the rough opening.) In other words, windows with a flange have the
fasteners pointed into the house while replacement windows will have
the fasteners installed along the same plane as the walls.
Typically the installation instructions for a window will call for a
rough opening slightly larger than the window itself to allow for
squaring and insulation. Once the window is square and insulated, it
is attached either through the external flange or though the frame.
If it's a replacement window and the RO is only 1/2" larger than the
window, I don't see how a PT frame could be installed inside the RO.
If it's a new window with a flange, how is "an interior frame of PT
lumber around the concrete opening" going to allow the exterior flange
to be used?
I can see TapCons being used with a flange, but I wouldn't take a
chance on using TapCons through the frame. It's not uncommon for a
TapCon to fail during installation, and you really only have one
chance to get it right. If the installation fails when going through
the flange, you could just pick a new spot. If it fails while going
through the frame, things could get ugly.
It is common in Florida to install PT bucks on masonry to receive
windows when the ouside walls are to be stuccoed and trim around the
aluminum/vinyl windows is to be drywall. Windows properly installed
on them absolutely won't be blown away.
(The reference to flange size is for windows to be installed in wood
construction...the flange overlaps the wood structure and is nailed to
The window should be on a pre-cast sash which slopes downward from the
window and extends beyond the block wall. It has a raised water
stop/lip on the top upon which the window sits thus retaining the
window. There is also usually a recessed drip line on the underside
outboard of the block wall line. There is a lintel above the window
opening that spans it and extends onto the blocks at either side. My
lintels are precast concrete. Some builders use steel.
The bucks are a diagonal half of a piece roughly 3/4 x 3/4. One right
angle edge is against the concrete block, the other faces out. The
diagonal edge faces inside sloping from window frame to block. There
is/should be another piece of PT which extends 1/4" beyond the block
on the inside edge of the block along the vertical edges of the
To install, the window is lifted over the sash lip then screwed to the
bucks at sides and top.
The rocker shoves a piece of 1/2" drywall in against the sloping
buck...the slope pushes it toward the window frame...the rock is
nailed to the other piece of PT...the rock lays off the block because
the space is 3/4" and the rock is 1/2".
The inside surface of the concrete sash can be covered with whatever.
Builders often use polyester "marble" because buyers think it is real
marble and therefore luxurious. One can trim the other edges of the
window with materials other than drywall as well. All mine (except
one room) are Saltillo tile. The other room has butternut.
As far as rough openings goes, keep in mind the fact that all block
homes are limited in dimensions to a minimum 8" unit - a half block.
Openings in new block walls are going to be divisible by 8 and
manufacturers of stuff to go in the hole know that. Best suggestion I
can make is to call the manufacturer/distributor/retailer of the
window and ask. Best time to do that is before you chop holes in your
What I described above is normal for new construction. In new
construction, every opening is bounded by a vertical stack of blocks
with steel tied into the foundation; the top course of horizontal
blocks are "U" shaped; that "U" shaped lintel has steel put in the "U"
and tied into the steel from the posts; both the "U" and the posts are
then filled with concrete. The result is a very strong post and
lintel structure that is tied into the foundation; all the other
blocks are just "filler".
However, yours isn't new and you've cut through blocks exposing hollow
cells. Apparently, there is no lintel either. If that is the case
then you have compromised the structural integrity of your house and
I'd suggest you get someone out there that knows WTF they are doing.
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