Recipricating saw or angle grinder to cut cement board?

[MiamiCuse]

I am remodeling this 35 year old house and throughout the house the walls are made of a double layer of gypsum board as the layer underneath and a layer of plaster / cement material on top. Total thickness is about 3/4".

This presents a problem in matching new wall with existing. If I use the standard 3/8" or 1/2" wall board it will not match. I could use them in areas where I torn down the entire wall from end to end, but in other areas I have to match the existing thickness. I guess I have three options:

(1) Use one layer of 3/4" sheetrock and try to match it, or (2) Use two layers of 3/8" sheetrock, or (3) Furr out wall a bit to use one layer 3/8" wall board

Not sure which one is the most convenient and economical.

Also, electricians, plumbers, window installers have worked on the house and there are numerous places I need to make a patch but the patching is impossible since the cuts are done in an irregular fashion. See pic:

I try to cut them on a straight line and it's hopeless. Can't use a recipricating saw because of the concrete layer underneath. Tried a angle grinder and there were so much dust I could not stand it. Any advise?

Thanks,

Sum

[dadiOH]

Plaster?

[Mikepier]

There is no 3/4" sheetrock, only 5/8". I would think using 2 3/8" sheetrock would be easiest.

Use a circulating saw and adjust the thickness to 3/4". There will be dust but it will get the job done quickly. Try wetting down the area a bit or have a buddy hold a shop vac close to the cutting to minimize dust.

[RicodJour]

Yep. Don't let your tradesmen cut nasty holes for you to patch. You should cut the openings or have the tradesmen cut straight lines.

A reciprocating saw will work just fine, you'll just go through a lot of blades. Most reciprocating saws accept the blade in a reversed position (teeth are facing up instead of down when the saw is held horizontally). Reversing the blade allows you to have a much lower angle of attack which is necessary since you don't have blade clearance behind the drywall. Tape the vacuum hose to the end of the saw so it will pick up most of the finer dust as you cut.

As an alternative, it's easier to cut complicated outlines in the new drywall than trying to cut into the existing wall. You'll have to clean up the edges of the cutouts, but you can use scribe cutting techniques to cut any sort of outline to follow the contours of the cutout. You mark the scribe line on the new drywall patch with a compass and cut to the line with a drywall keyhole saw. It goes fairly quickly and will cut way down on the amount of dust you'd be blowing around cutting the drywall with a power tool.

I attach scrap plywood blocking behind the drywall so it's fastened on both sides of the cutout every foot or so (the blocking ends up looking like a ladder running up inside a long cutout). Then you can attach your drywall, or cardboard/plywood shims, to the blocking. I prefer to have the cutout repair drywall sit just a bit lower than the original wall surface as it makes taping easier. You'll end up bridging over the entire cutout and it's better to have one smooth, gradual bump, than to have bumps on either side of the cutout from the tape.

I would not use a single layer of 3/8" drywall. 5/8" should work nicely. If the existing wall surface is flat and true, you could shim that out a bit with some cardboard to you won't need as much joint compound. Standard fiberglass mesh tape would be a poor choice for your repair. It's noticeably thicker than paper joint tape and the loose ends end up requiring more compound to cover them (and you can't sand them), so the bump ends up being bigger. Either use paper tape, or something like Tuffglass tape (TKO Coatings) - that's a flat woven tape that is much thinner than standard fiberglass tape.

R

[Joe]

Buy the cheapest saber saw (or jig saw) that Harbor Freight has. Break off or grind off a blade to the exact length that will cut the thickness you want and have at it. Pros use this same trick for removing just the layers of old underlayment, flooring and what have you without tearing up the subfloor. HTH

Joe

[MiamiCuse]

I might either use 2 3/8" or may be fur out the wall a bit more with 3/8" furring strips. May be furring strips will work out better.

Thanks, It's hard using circular saw moving up and down with that weight. I wish there is a laser beam cutting tool.

[MiamiCuse]

I tried to tell them to cut straight lines but they never listen to me. They told me this is not regular sheetrock so they had to pound it out.

I am getting a lot of kicks when the saw blade hits the concrete wall right behind it. It's not working this way I have to think about a different approach.

RicodJour thanks I think I may try to fur out the wall to flush with existing surface instead of using thicker or double sheets. This will make future repair easier, I think...it's good to know about Tuffglass tape I will need it where I am matching new to old walls.

[spammer]

I use a utility knife to score the cement board and then snap it. Works great and no flying dust.

[MiamiCuse]

Thank you, I might try that. It will be tricky to break off a blade. I wonder if I can somehow set a depth, is there any attachment to a jigsaw that will allow you to reduce the depth?

[MiamiCuse]

It is not cement board it is 3/*" thick plaster on top of sheet rock, around corners and edges it has this metal wire mesh too.

It is hard to snap when it's already mounted on the wall with screws every 8 inches or so.

[SteveB]

FWIW, I had to cut some stucco, which is similar to cement board. A SawZall made terrible jagged cuts, and pulled on the stucco making fractures that looked like a giant cobweb. I got a SkilSaw with a metal cutting blade, set the depth properly, and breezed right through it. I did have a helper with a vacuum to help keep the dust down, but there was still a lot.

Steve

[RicodJour]

If you're hitting the concrete block the angle isn't low enough. If there's furring attached to the concrete block, you will be able to use a longer blade installed backwards in a sawzall. That will give you the clearance you need.

If there isn't any furring between the concrete block and the wall covering you could just infill the whole thing with plaster. Clean up the loose edges and loose paper on the drywall and start plastering.

R

[Jeff]

This looks like it would be perfect for your requirement.

However a brief, very brief, net search didn't find a U.S. counterpart. The customer reviews also indicate it isn't very powerful so might not cut through your walls anyway. You may have better luck finding an alternate brand with more power locally.

[a]

Ooooh, that's nice - I'll pop over to Can Tire and buy one for the op and ship it to them for my cost only...

a