Thoughts on building a platform for a shower pan vs busting concrete?

I built a platform for a basement suite last year becuase the slab was five inches thick and full of rebar. I used 2x8 pressure-treated lumber for the framing and topped it with 3/4-inch Advantech subflooring. To keep that waffle-grid Kohler pan from flexing, I didn't trust spray foam. Over time, foam compresses and you'll start hearing that annoying "thunk" every time you shift your weight during a shower.

Instead, I mixed up a bag of Mapei 4-to-1 mud bed mix and spread it about an inch thick over the plywood subfloor. I laid a piece of 6-mil poly film over the wet mortar before setting the pan down. This creates a custom-molded base that perfectly matches the grid pattern but doesn't bond permanently to the plastic. If you ever mess up the drain alignment, you can still lift the pan back up. The step-up height does feel a bit high—nearly eight inches once you add the pan lip—but if you trim the side of the platform with the same tile you’re using on the floor, it looks intentional. Just make sure your 2-inch PVC trap fits comfortably in that 7.25-inch joist space so you aren't notched too deeply into your supports.

Honestly, skip the platform entirely. I did the platform route for a basement guest bath once and regretted it within six months. The eight-inch step-up makes the shower feel cramped and awkward, expecially if you have standard eight-foot ceilings in a basement. You lose that overhead clearance fast, and it always feels like a DIY workaround rather than a finished room.

Renting a jackhammer from a local shop for sixty bucks is a much better use of your time than building a wooden stage. It took me maybe two hours to bust thru the slab for a standard 2-inch PVC trap. Once you are through, you just scoop out the fill, set your trap properly, and patch the hole with a bag of Quikrete. It is messy for an afternoon, but the result is permanent.

For the pan support, sitting it directly on a mortar bed on the actual slab is the only way to get that dead-solid feel. Those waffle-bottom pans, like the Sterling Accord series, relly need that full contact to prevent flexing. Even with high-end subflooring, wood eventually moves or shrinks, and you will end up with a squeak that drives you nuts every morning.

Seen plenty of guys try to cheat the slab, but if you're dead set on the platform, don't use that spray foam garbage. Back in the day, we’d pack stiff deck mud under those pans and it never moved. If you go the wood route, make sure you glue and screw everythign like your life depends on it. A squeaky shower is the mark of a hack. Personally, I'd still grab a 20lb sledge or a rental chipping hammer and just bite the bullet. It’s a two-hour headache now versus forty years of stepping up onto a throne just to get clean. Have you checked exactly where your main stack is? You might not have the pitch you think you do.

I went the platform route and it’s still solid. Instead of standard mortar, I used Structo-Lite basecoat plaster under the pan. It’s lighter than a mud bed but sets up just as hard. I spread it thick, dropped the pan, and stood in it to bed it down. The trick to making it not feel like a cheap fix is the framing. I spaced my joists 8 inches on center under the pan and glued every contact point with PL Premium. Zero squeaks. If the height bothers you, build a wider 4-inch step in front of the shower so it feels like a design choice rather than a mistake.