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A previous owner of our property fixed a fibreboard plinth to the concrete floor under our kitchen worktop to raise the height of the dishwasher there; I'd like to remove that plinth to fit a taller dishwasher, but I'm struggling to find a way to actually get it up.

The plinth appears to be made out of some sort of fibreboard with a solid, matt veneer, and it appears to be very well affixed (possibly using cement?) to the concrete floor. It's about 2–3cm thick, and about 45cm square.

I've attempted to take a crowbar to it, but I've not managed to get the end of the crowbar sufficiently under the plinth to get any useful leverage. I was able to get a chisel into the fibreboard, but it was very hard work, and attempting to lift the entire thing up like that seems like it'd be many hours of miserable work.

This is going to be out of sight, and the new dishwasher has adjustable feet, so it doesn't need to be particularly pretty or perfectly level. How can I get rid of this plinth?

  • It does sound like it was glued down. 1) are there any signs of mechanical fasteners like nails or screws holding down the fiberboard material? 2) How much clearance is there underneath the new dishwasher? i.e. if you could get the feet to the original floor, would the rest of it clear the plinth? – FreeMan Nov 19 '18 at 13:52
  • @FreeMan no sign of any nails or screws or anything like that. And I'd need to remove a fair chunk from the top to be able to fit the new dishwasher; it wouldn't have to be all of it, but it would need to be ~50% of the depth at least. – me_and Nov 19 '18 at 16:24
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Since you say that there is no sign of mechanical fastener, it does sound like the fiberboard was glued to the subfloor.

I see two options for you:

1)
If the new dishwasher has leveling feet, and they're the lowest point by a fair margin, you might be able to use a circular saw to make a couple of saw kerfs from the front to the back of the fiberboard. Have these kerfs line up on either side of the feet with enough clearance to give you some wiggle room. Using your crow-bar, hammer, chisel, some elbow grease and sweat, remove the fiberboard from between the saw marks to make two dados (or tracks) for the feet to slide back through. This will minimize the amount you have to remove.

2)
If you don't have the clearance to use the above method, you'll have to remove the whole piece of fiberboard. Using your circular saw, make multiple cuts through the fiberboard in a variety of directions. Once you've cut up the surface, use the tools + elbow grease + sweat method to get it up off the floor. The cuts should make it easier to remove chunks at a time, since any place that doesn't have glue directly under it should be able to pop off once it's been released from a nearby glued section.

A couple of notes:

  • You will, of course, have to ensure that your depth of cut is just deep enough to get through the fiberboard without actually hitting the concrete. You'd probably want to leave is a scant mm above the concrete to avoid damaging your blade.
    • If your blade has brazed on carbide teeth, it's possible (though I don't know the likelihood) that hitting the concrete could cause a tooth to shatter/pop off the blade and it would become a dangerous missile. Use appropriate caution! (Even if this is an extremely low likelihood, I feel better leaving a caution than not mentioning it.)
    • You may consider using a diamond-edged concrete cutting blade so it doesn't matter if you hit the concrete. They're not terribly expensive where I live. It's an option.
  • You may find that only the edges of the fiberboard were glued down in which case your job just got a whole lot easier!. Break up the edges and the middle will lift right up.

The flush cutting saw recommended by Kris would go a long way toward replacing the hammer & chisel/pry bar, however, I think you'd still need to cut the fiberboard from the top in order to be able to get that flush cutter to a newly glued down area. You can do it without investing in new toys tools if necessary, but, of course, no one can deny the joy of a new toy tool purchase. ;)

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I would use a flush cutting end saw like this one
enter image description here

Ease the wood cutting blade in between the plinth and the concrete. Break away the pieces as they are separated from floor with chisel or flat pry bar. Maybe you will get lucky and only perimeter is glued or there are just a few beads of adhesive instead of full glue down.

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