3 1/2" Hole In Cmu - Masonry - Contractor Talk

3 1/2" Hole In Cmu

 
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Old 03-10-2015, 10:20 PM   #1
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3 1/2" Hole In Cmu


I may need to run a 3" pipe through 12" cmu (hollow). A friend advised to drill a series of small holes in a 3.5" circle then chip the center away. Is that a viable option, considering I may hit the web?

I have a big sds plus hammer, but not a 3.5" core bit. I could probably rent one for the day, but his method piqued my interest. I don't want a nasty looking opening. I know the core bit will be smooth, but is the chip out method capable of acceptable results?
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Old 03-10-2015, 10:22 PM   #2
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Re: 3 1/2" Hole In Cmu


Quote:
Originally Posted by philcav7 View Post
but is the chip out method capable of acceptable results?
Yes

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Old 03-10-2015, 10:23 PM   #3
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Re: 3 1/2" Hole In Cmu


Difficult to obtain if you want a clean tight hole.

Use the core bit if appearance is important.
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Old 03-10-2015, 10:25 PM   #4
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Re: 3 1/2" Hole In Cmu


Quote:
Originally Posted by philcav7 View Post
I may need to run a 3" pipe through 12" cmu (hollow). A friend advised to drill a series of small holes in a 3.5" circle then chip the center away. Is that a viable option, considering I may hit the web?

I have a big sds plus hammer, but not a 3.5" core bit. I could probably rent one for the day, but his method piqued my interest. I don't want a nasty looking opening. I know the core bit will be smooth, but is the chip out method capable of acceptable results?
Yes
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Old 03-11-2015, 12:35 AM   #5
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Re: 3 1/2" Hole In Cmu


Can't you move the hole a couple inches to miss the web?
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Old 03-11-2015, 06:01 AM   #6
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Re: 3 1/2" Hole In Cmu


Can always parge around the pipe or put an escutcheon on.
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Old 03-11-2015, 06:29 AM   #7
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Re: 3 1/2" Hole In Cmu


Yeah, if you don't have a core bit, a series of smaller holes will certainly work.

I've knocked a many of holes with a hammer. Knock a hole right in the center, then chip out the circle, and mud right around the pipe. I do all our thimble holes that way.
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Old 03-11-2015, 06:51 AM   #8
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Re: 3 1/2" Hole In Cmu


Quote:
Originally Posted by philcav7 View Post
I have a big sds plus hammer, but not a 3.5" core bit.
Same here. I've done the small-holes method many times in block and brick with decent (though certainly not perfect) results. But then one day a countertop guy gave me a 1.25" core bit he considered worn out.

With a little finesse that does a near-perfect job without the need to invest in a larger bit.
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Old 03-11-2015, 08:27 AM   #9
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Re: 3 1/2" Hole In Cmu


pretty sure home crapo rents the bits and drill
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Old 03-11-2015, 04:13 PM   #10
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Re: 3 1/2" Hole In Cmu


Drill a small pilot hole if ye hit the web move over an inch or so, drilling several holes will work great, Home Depot rent the large bits to, $40 or do for 24 hours in my neck of the woods.
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Old 03-11-2015, 07:30 PM   #11
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Re: 3 1/2" Hole In Cmu


Thanks guys. It worked out awesome. My helper drilled the holes and broke it out. I didn't get to see the process, but it turned out well.

Turned out to be a 4" hole, fit right between the webs.
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Old 03-11-2015, 07:32 PM   #12
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Re: 3 1/2" Hole In Cmu


As long as its not near a corner, if you hit it with a sledge hammer right on you will have a 3" hole.

I should put that on a fortune cookie.
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Old 03-11-2015, 09:34 PM   #13
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Re: 3 1/2" Hole In Cmu


Just a little background and big picture for some contractors--

It is always better to get an idea of the core configuration of the block you are going through. - Look at the top of the wall for a clue. It could be 2 core or 3 core (too common in many eastern and NE areas of the U.S.). In some areas, more modern shapes with one core, open ends are used commonly depending on the age of the building.

For a 12" block, the face shell must be 1-1/4" to 1-1/2" minimum, but the shape of the cores (square or pear-shaped) can affect the and the top face shell may be thicker, but the core arrangement is somewhat consistent if the number of cores are known.

The specs for CMUs are not prescriptive(exact), but are minimums for face shell thicknesses or are based on the minimum web thickness per foot to accommodate the older style units between 2 core and 3 core units. Some block have only 1 centered core with open ends or even a single half-high web. - The latter ones would be rare, especially on older buildings.

Every area has different habits and demands for block configurations to suit the local contractors demands.

Going in with a sledge 4" from a corner may work in some areas, but that is also a prime spot to fill the cores solid with grout and rebar.
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Old 03-12-2015, 12:56 AM   #14
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Re: 3 1/2" Hole In Cmu


Scoring the block (stop sign octagon) with a 4"-5" diamond bladed(11.00$or less) grinder would reduce the chances of a cracked up block. you can bore a 4"+ hole with a 4" blade, it will take a few dusty minutes.

Do it right, put the next size of pipe in as a sleeve, and mud it in.
Install the 3" nominal pipe and some foam rope and caulk, drive on.

For 95% of block in the USA, measure over 5" from a head joint to center of your new hole, that is half way between the webs mostly.

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Last edited by Fouthgeneration; 03-12-2015 at 12:59 AM.
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