Cutting cement board?

[Doug71]

I am building a fire surround for a customer, it will be mainly MDF with some Pine mouldings but the sides of it facing the wood burner will need to be non combustible so for those parts I'm using cement board.

I have not had much to do with Cement Board/ Hardiebacker type board so am wondering the best way to cut it to get as clean edge as possible? I see people just scoring it and snapping it? Will this work with 12mm, can I take some sandpaper to it to get a nice finished edge as it will be painted?

I see some saw blades available for cutting it, could I stick one of these in my TS55, would that give a cleaner edge than snapping or grinding?

I seem to remember (hopefully) at least one side of cement board is smooth enough for painting?

Final problem is I need 2 lengths at about 1500mm long, it seems to mainly come in 1200x800. Do you think I will be able to butt 2 pieces together, fill it and get a near invisible joint or will I have to source an 8x4 sheet? I only need 3 strips about 175mm wide so a small sheet would be more than enough if I can getaway with joining.

Any advice appreciated.

Thanks, Doug

 

[petermillard]

I’ve done it with an old 48t stock blade in the TS55 - great finish! Not sure about butting two pieces together though - I’d try for a bigger sheet, personally. HTH P

 

[AJB Temple]

By far the best way in my experience is a track saw or a hand held circular saw on a sacrificial board. You can also use a multitool with suitable blade but it is slow that way.

Edges are always a bit chippy. I would get a single sheet.

 

[okeydokey]

If it will be seen then single sheet if not then you do it with a joint but the joint will be a weak point as regards non combustibility

 

[Woody2Shoes]

I have cut it quite cleanly with a disposable hardpoint handsaw in the past. Joints are best taped with fibreglass scrim. I'd be tempted to skim the whole surface with a plastering sand and cement render (the boards, and especially their edges) aren't really designed to be seen as a final surface.

 

[eribaMotters]

You will need an 8 x 4 sheet as if you butt two smaller sheets together the joint will always show.
I have tried a score and snap approach, jigsaw and finally diamond dust disc in my 115mm disc cutter. The latter won hands down.

Colin

 

[Spectric]

The last time I used it for a shower I just scored it and used a length of square edged timber to rest the score on and then snaped it.



Use a grinder and you will vanish in the dust,

 

[Cabinetman]

Watched that very vid and tried that method with a special carbide tipped cutting tool and despite gouging deeply from both sides it still doesn’t leave a clean edge - at all. Ok for behind tiles but not for a visible edge. Ian

 

[Rustic Mike]

The last time I used it for a shower I just scored it and used a length of square edged timber to rest the score on and then snaped it.



Use a grinder and you will vanish in the dust,

What did you score it with

 

[Ollie78]

Diamond blade in a grinder with extraction of course. An old blade in a track saw is good as well.
I have never had a decent result with the scoring method. Certainly not on the 12mm stuff.
I think it often comes in smaller sheets because it's so heavy, try tile suppliers for different options the stuff I used for my fireplace was not the James hardie stuff but I forget the brand ( could have been knauf maybe) but it had a very smooth finish one side and was 12mm.

Ollie

 

[MikeJhn]

Don't know if its the same as cutting glass, but deep scoring does not work well, one score and then snap gets a much better consistent result.

 

[JBaz]

I used a diamond disc in a 115mm grinder with a Herzo dust control shroud and shop vac.

It cut OK and left a reasonable edge, though I wouldn't want to do it indoors.

I think you will struggle to get a joint that won't be seen under paint. You might consider tiling on top of the board, which I did in the alcove surrounding our wood burner.

As to a 2400 x 1200 sheet 12mm thick, you will need help to lift it!

 

[Spectric]

What did you score it with

https://www.toppstiles.co.uk/fixing...hardiebacker-board/hardiebacker-scoring-knife
There are several brands out there but much the same. Go for several lighter scores than a single heavy one.

 

[Cabinetman]

Don't know if its the same as cutting glass, but deep scoring does not work well, one score and then snap gets a much better consistent result.

Yes that’s how I started to do it, and it’s why I moved onto deep score both sides.
If I was to do it again I would definitely go back to my original method and use an angle grinder with all the suitable PPE. Ian

 

[Spectric]

In theory a cheap jigsaw and

https://www.toolden.co.uk/p/Irwin-irw10505815-abrasive-materials-jigsaw-blade-t141hm/
or a reciprocating saw which I have used on masonary so cement board a breeze,

https://www.screwfix.com/p/bosch-expert-s1543hm-carbide-reciprocating-saw-blade-240mm/302ky
https://www.screwfix.com/p/bosch-expert-carbide-reciprocating-saw-blade-150mm/889pf
If @JobandKnock is around he may well have some helpful input to this topic.

 

[Cabinetman]

I like the idea of using a reciprocating saw, I used one for the first time in anger recently to cut up a fibreglass shower cubicle here in the states, saved an awful lot of work.
They call them sawsall's here as in, they saw all things. Ian