Problem- contractor failed to support an acrylic/fiberglas tub installation with any underneath- the- tub mortar slurry (it has small 4 position bump out tiny legs at bottom but maker "recommends" leveling with mortar). Now when tub is filled with water (50 gallons = maybe 400 pounds or so) the tub depresses downwards and the the lip is pulled down and away from the caulked tile to tub flange intersection leaving a good sized and unsightly gap all along the top tub perimiter. But, then when the water is drained out the caulk seems to go back into place and assume a normal appearance. Floor was solid enough to begin with and a layer of dens-shield was added and tub went in seemingly level on top of that backerboard.
Nevertheless, I need a fix for this reappearing gap that could allow water into the wall over time. I could open up a section of floor underneath the tub from the crawl space and try to inject mortar (or expanding foam maybe-risky) under the tub base but that seems like overkill and fairly inaccessible with under layment reconstruction effort a difficult task and a chance the mortar doesn't even setlle in to the right spots anyway because of the reduced accessibility --or surrounds the drain assembly which I don't want to be impacted. So any ideas on how to keep this gap from forming every time the tub is used? I used a polyseamseal white caulk twice now with poor (unstabilizing) results but I don't feel that any other caulk would fair any better with this expansion/contraction issue. Incidentally, I've seen other acrylic tubs before and they seem much beefier all around than this Maax product and am wondering if these manufacturers are also creating part of the flexing problem by cutting down on materials used in order to get larger internal surface areas at the expense of a stable/strong product. The Maax firm has had some review issues that I've seen blogged about on line.