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How to drill through fiber cement wall from inside?

andrewc, zone 5A's picture

I would like to add an external exhaust to my over-the-range microwave.  Duct work will go up through top of cabinet, then turn horizontal (with slight downslope) and go through another framed cabinet and then out the exterior wall that has fiber cement cladding.  Standard 2x wal construction.

Assuming that I get that far, the plan was to drill a small hole out through the siding from the inside where I want to center my 6" duct, go outside, find the hole, and then cut a (6") hole in the fiber cement.

Question: if I use a standard drill bit of sufficient length, can I just go slowly, or am I likely to crack the fiber cement?  Re-phrasing, what's the best way to drill this hole?

Follow-up question: best method to cut a +6" hole in fiber cement siding when it's already installed on the house?

Obviously, I haven't done this before as a homeowner.


Thanks for your thoughts,

Hi there, Yes you can do what (post #215000, reply #1 of 8)

Hi there, Yes you can do what you described, but you should think about what might be in the wall before you start drilling.  Obviously you want to miss plumbing and electrical lines, but you also want to miss structural members.  With some simple math you should be able to miss the wall studs.  If you can't adjust the location for the ductwork you will be either removing cabinets and wall finishes or siding and sheathing to head off an opening for the duct to pass through.  Assuming you have established a clear path, you can drill through from inside, slowing down as you come through the back of the siding.  You will want a flat surface to which you attach the wall cap for the ductwork.  For example, f you have fiber cement lap siding you need to cut in a flat block of trim to create that flat surface.  So you won't be cutting a round hole through the siding, you will be cutting a square hole through the siding.  With fiber cement you have a few options for cutting it in place.  Best option is a multitool with a carbide tile blade, but a grinder or circular saw can be used to.  The trim block (we usually use PVC) should be flashed at the top and rabbetted at the bottom so the siding below can tuck into it.  Hope that makes sense.  Good luck.   

reply to FineFinish r.e. fiber cement-mounted wall cap (post #215000, reply #3 of 8)

Thanks for the many suggestions.

Do you have a pic or link to a PVC trim block that you use?  I was thinking that I'd just get a wall cap / exhaust hood with an attached section of round duct and then just push that back through the wall to connect with interior duct work.  Apparently there are better ways...


Making the connection between (post #215000, reply #4 of 8)

Making the connection between duct and wall cap often requires a touch of cleverness.  Sometimes you can get pieces of rigid duct to fit together nicely and tightly, but often this doesn't work at all or doesn't give you much confidence that it's airtight.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The trim block I use of PVC (post #215000, reply #7 of 8)

The trim block I use of PVC or a nice wood like red cedar is not a manufactured piece.  It is simply a square block cut from scarp material.  

There is apt to be some (post #215000, reply #2 of 8)

There is apt to be some flaking of the far surface as the drill penetrates.  This should not extend more than about a half-inch from the hole, however, if you aren't pressing too hard.  So if you drill your hole more or less in the center of the to-be-cut-out space you should be OK.

However, it's unclear whether you have sheet cladding or plank siding.  If plank siding, and your hole will have to cross the edge of plank, there is a bit of a danger that the edge will split off if you drill too near to the edge, so you'd want to plan to avoid that.

For cutting the hole you use a sabre saw.  Just be sure that you have clearance behind (and a short enough blade) so that the saw blade doesn't bump into stuff.

For a circular hole one MIGHT consider using a fly cutter, but then you have concerns about splintering.  Or there are hole saws available in some large sizes.

(But you've probably already spent too much time worrying about this.  Just make sure you don't hit any pipes or wires.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Locate a clear section from (post #215000, reply #5 of 8)

Locate a clear section from the inside first. In other words make sure there are no studs, pipes or wires in the way.  I do this witha drywall stab saw. I stb the saw in to the center of where I want the hole. If it's to be a 6" hole I saw slowly left 3 inches then right 3 inches. If I hit something I go back and saw the other way until I have the 6 inch gap I need. Once you know where that is you can cut out the hole on the inside and easily mark the outside. Don't worry about breaking the Hardi, you're going to be cutting a 6 inch hole around the center anyway. I use a jigsaw on the Hardi with a blade with as few teeth as possible. You're going to trash the blade anyway.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thank you for all the (post #215000, reply #6 of 8)

Thank you for all the suggestions.  Appreciate it, should help me avoid the most egregious errors.

vent hole (post #215000, reply #8 of 8)



If you have already cut the hole, keep this method for future reference.

From the inside, determine where you will be drilling. I assume you will purchase a

 6 1/2"  or 7"  hole saw to drill the hole for the 6" duct to pass through. Get a 12" long

drill bit that is the same diameter as the pilot bit on the hole saw, typically 1/4" or 3/16"

drill the pilot hole from inside to outside as level as possible. Then go outside and

use that hole as the pilot for the hole saw and drill from the outside in. You will have less chance of

any splintering or damage to the Hardie siding drilling from this direction. Now get a piece

of 5/4" stock about 12"x12". Mark the block  w/ a pencil line from corner to corner in both directions,

basically an "X" on the block. Drill a 1/4" hole, same as the wall pilot hole, in the center of the block.

Using the hole saw again, drill through the wood block. Now set this block on the wall and align it with

the hole in the wall and mark with a pencil the outline of the square block onto the siding. Now you can use

what ever saw you have available to cut out just the siding, leaving the sheathing, now you can install

the block to the sheathing/wall with a screw in each corner, no more than 4 are needed! Now attach your

duct (flex duct?) coming from the wall to the exterior damper, then push it back into the wall and secure

with a screw in each of the 4 corners, many come with  pre-drilled holes for the fasteners in each corner.

Hope this is of help. Be patient and you will good results. Be aware of the potential for "kick-back" when 

you are using a larger hole saw and always be " ready" by having a good firm grip on the drill AND a solid

footing on what ever you are standing on, especially a ladder!!

Good Luck