We have a tile shower with glass walls and a marble ledge that the glass sits on. It's a corner shower with the 2 back walls tiled from floor to ceiling. There's water seepage somewhere that's causing the fibreboard under the marble ledge to soak up water. I've put silicon on all the joints and sealed the grout and marble with tile sealer. This hasn't made any improvement. There's no obvious signs of water anywhere except the fibreboard which is damp. The base has a rubber boot which runs up the knee wall & wraps over the 2x4. The fibreboard sits on top of the rubber boot then the marble on top. Any ideas where the water might be getting in? I don't want to start pulling tiles off the wall if i don't have to.

Maybe the rubber boot was nailed atop the 2 X 4 ? It should wrap across the top and be nailed on the outside.

The rubber boot wraps over the 2x4 & is stapled to the frame. There's no sign of water on the boot (we cut out a piece of drywall on the outside of the knee wall.)