Toilets, Sinks, Showers, Tubs and Disposals - Where surround meets the tub question

Alright, we have an older tub that doesn't have that flashing lip that runs around the outside like modern tubs. I don't quite know how to go about waterproofing that seam. Yeah, you can go and caulk it but Is there any better method? Can you flash it before I put in the surround? If so, what should I use for flashing? and what's the best waterproof caulk/adhesive?

You're right. That could possibly be problematic.

I have had to deal with this situation already, with old cast tubs designed simply to be tub and not a combo tub-shower.

Silicone caulk alone, has worked and held up for me.

The flashing idea would give yourself more surface area of which to sandwhich caulk under the bottom of the flashing.

Whatever material is used, the caulk would have to be able to stick to the flashing material. For example, if you were to take something supple and bend it, say like rubber roofing material, to sort of glue it to the top edge of the tub, you'd have to find out what can stick to both it and the tub, permanently.

You would not want to use some metal whose sharp edge might protrude beyond the tub.

Or else try to use some plastic 'inside corner' angle.

Another similar approach would be in not relying on beneath- the- surface flashing, but to say glue on external inside corner trim. This too would add surface area for the caulk to stick to.

One landlord I work for has applyed that around the top of the tub, at the surround bottom, to also help ensure the surround does not come unstuck down at that edge where you caulk.

Similarly, recently, I simply 'glued' on a white foam trim inside corner to the vertical corner of a tub surround. It stuck, with no nails or temporary tape needed!