Framing for an under mount tub installation
One question that gets asked a lot online is how to "Under Mount" a tub. It's a really good question because there is no real guidelines to follow, at least not from a tile installation point of view. I have been helping out this guy in Ontario (will call him Steve B) with some design ideas for his new bathroom renovation. Yesterday he sent me some pictures of his work.
I'd like to share them (with his prior permission of course) since I think he is doing a really good job. The framing looks strong enough to support a Space Shuttle and the framing designed to allow for a mortar bed both under the tub but also under the top lip of the tub.
Take a look at these framing pictures;
If I had to guess I would assume that the tub framing is 100% level. Perfectly square. A little excessive in stud spacing but it's his place and so what if he spend an extra $11.00 on lumber. With the tub dry fit on this framing the bottom sits about 3/4" off the ground. A couple bags of mortar mix will make up this space and provide support along the bottom. That's the easy part.
Supporting the 4"-6" tub top is the tricky business. Since the tub will have granite slab we wanted to add in extra support to this rather weak section of a typical tub.
On a recent job out in Coquitlam we used Ardex 8+9 and a little silicone to install a Schluter Profile to a tub deck. We did not fully under mount the tub but rather framed out the tub deck smaller and tiled. The Ardex 8+9 did a great job bonding the Schluter profile to the tub deck.
Here is a peak at that.
More considerations for a under mount tub installation
Some fine work here from Steve B in Ontario. The Ardex 8+9 coating is to improve the bond strength of the setting material (for stone) to bit to the tub deck. The underside of this lip (top) is getting filled prior to the tub install with a little Laticrete 3701 fortified mortar bed. This will fill the void and add strength to the install.
Once again I'll remind everyone that this is not an endorsed method from either the TTMAC, TCNA, Laticrete or Ardex. It is how I would do it and the plan my buddy Steve is following in Ontario.
Locking the tub into a mortar bed and tub deck
We used a couple bags of mortar mix to support the weight of the tub.
Look how high I piled it in. At least twice the height I needed...
This picture taken from the bathroom looking under the tub face (long side).
Here you can see some poly we used as a bond breaker for the bottom side of the tub. In a renovation down the road it will be easier to remove the tub because of this poly layer.
On Steve's job he will be thin setting the tub top lip to the tub deck with some Laticrete 254. This will give the tub a solid feel around the perimeter and fully support the stone that will be later installed over top.