need to make hole in structural cmu wall larger, then add reinforcement
Existing 18 wide x 12″ height hole near floor, thru cmu structural wall. Wall supports roof rafters. Need to enlarge hole to 18 x 18″.
1-Hole was cut thru wall by previous owner or their contractor. I don’t know if they put a header in that hole.
2-It looks like one of the rafters is laterally very close to one side edge of the hole. Not sure if it is above the hole or just beside it. See diagram.
I won’t know either #1 or #2 until I pull the old air handler.
Im getting quotes on new hvac system. So far quotes say new air handler requires 18 x 18″ return.
Im thinking of adding 2x4s, straps, and cross member. Also a header above the hole.
Points on diagram. Sorry it’s pretty bad, but kinda scale.
Red is 2x4s (can I liquid nail to walls opposed to screwing?)
Blue is ends of rafters sitting on top of 2×8/10 on top of CMUs. Ill push the red 2x4s tight below those and strap them together.
Brown is what Im calling a cross member. Maybe it should be higher? If I dont need to stagger the right red 2×4 should I still use that cross member?
Cutting the cutouts shown at top sides of 18×18 hole for header will be tricky.
Your thoughts?
Thanks,
P
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Replies
Personally, I do not think 6 inches will make much of a difference in this case. I suspect the hole was cut without any additional support above.
If you want a relevant opinion, file for a mechanical permit and have the zoning office/inspector make the call. (you likely need a permit for the hvac install as well, even if the contractor may say otherwise)
Increasing the HEIGHT of the opening in a CMU wall, near the floor, will not adversely affect the load path from what previously existed.
Are you cutting the CMU at the bottom of the existing opening or at the top?
Top
Top. I dont suspect an issue, but since im there & after the air handler is installed i cant get there, it would be foolish not to add supports. Wood & metal are cheap
CMU is typically 16" (15-5/8) long.
If during the course of removing the CMU above the opening it results in 16" block above the new opening having no support from the remaining CMU on either side of the opening, that one block may require modest support.
As far as the loads applied to the top of the CMU, increasing the height of this opening should have no adverse effect on the wall as a whole.
Good point. Ill ck before i cut.
One thing to consider is getting a two foot long steel angle iron and cutting along the mortar joint just above your opening.
(Using an angle grinder with a masonry cutter will give a clean cut for the opening, and let you slice along the mortar joint. It will make a tremendous amount of dust, even if you pre-wet and run a vacuum. )
Make sure to wear good breathing protection. and eye protection.
I was thinking of that, doing both sides of wall. However I was going to cut into block on both ends of upper edge of enlarged hole. That would most likely be cut into actual block opposed to mortar. Which would be stronger?
I think it a wash, but suspect it might be easier to cut into mortar, with less chance of generating a crack in the cmu to complicate your task.
You could cover the inside wall (where the air return is from) with some plywood or drywall and get a nice vent or grille instead of the filter.
I have not done this, but you might want to start with drilling a couple holes at the ends of your opening and any slots you cut.
"you might want to start with drilling a couple holes at the ends of your opening and any slots you cut"
Why?
Otherwise it is difficult to stop at the end of the cut.
You can just over-cut and patch it after.
If you think you want to be picky, get a CMU and practice making a cut in the middle of a face.
They do make long sawzall blades for cutting CMU and other masonry.
Choice of tools depends on what is in your toolbox already.
You can also just drill holes close together and finish with a hammer and chisel.
You could also do the whole cut with a hammer and a couple chisels. (narrow one for the mortar cutout for the angle irons.)
A nice frame could cover a lot of rough edges
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