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We are replacing the carpet around our Heart Shaped tub with tile. For better waterproofing we are using cement board on the top surface. The tub is sunk in the concrete floor with a 7" step up to the edge of the tub on one side. The other side of the rise is to the wall, finished off with a tile backsplash.

The problem is that the step follows the curved edge of the tub about 10" out, at a radius of about 10 feet. We can tile by using 1" squares or other small configurations, but how do we curve the backer-board to go behind the tile on the stair riser?

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If I ever had to tile a heart shaped tub (and God willing someday I will) I would be looking at the Schluter Kerdi system. They have a great website with videos, pdf's of installation procedures, and videos demonstrating techniques. And their products are easier to work with than traditional cemetitious backers. They even have a board specifically made for curves. enter image description here

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I have used HardieBacker, Wonderboard and Durock and I'd use 1/4 Wonderboard in this application. IIRC, it is the most ductile (bends before it breaks) of the three. On a stair subject to being kicked, you may want to use two sheets of quarter inch (the same trick for bending drywall) with troweled mortar between them. Basically, backerboard easily lets you fake a "dry pack" mortar bed:

Traditionally, for wet applications, the arguably proper method is a "dry pack" mortar bed sloped to a tile drain followed by a hot-mop asphalt application, followed by floated cement followed by tile set in thin-set mortar. It is a method that is quite labor intensive involving specific steps. This is still a common method for commercial and residential applications, but becoming less so with the advent of modern alternative materials and techniques. – Jimmy Fix-it

The 'tar' waterproofing is accomplished by the application of an anti-water membrane over your backerboard; embedding fiber tape along any seams or cracks.

Even if one or both of the sandwiched backerboards does crack as you install it; the mortar will fill and set it back into a solid piece... hopefully. If the framing is sound, using screwed-down mortared-in-place backerboard and a liquid applied membrane will go a long way towards achieving a professional tile job.

This is all assuming you have a nice, gently curved plywood substrate that you can screw to every 6~8" and not just 16"oc studs.


Ditra mat is possibly what you could use but I never have, and therefore cannot attest to it's functionality nor whether it is applicable, vertically: (emctiles.co.uk)

enter image description here

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A jigsaw fitted with a carbide or metal cutting blade will cut through the backer board without a problem. Sounds like a pretty long cut, so you may go through a few blades. But those are cheap.

However, the dust created is not healthy. At a minimum, do all the cutting outside and if it were me, I'd use a respirator.

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If you want to cut Hardiboard on a curve just use an angle grinder with an old diamond blade on it. Will cut it like butter. You will need goggles and a very good mask though - and do it outside.

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    I don't think cutting it is the problem (they do sell nipping screw-gun attachments for cutting backerboard). I picture the stair riser curved, abutting the heart shape. – Mazura Nov 8 '14 at 0:00
  • @Mazura - you are right, read it wrong. I wouldn't use any backer board for that. – DMoore Nov 8 '14 at 6:17

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