Conservatory window ledge

[chelsway]

Hi to everyone. Im pricing for removing the timber ledge all around a conservatory and tiling it. Its a low brickwall ,rendered inside and out.the wood has got wet only by lack of Silicon and condensation.now. How do you tile and grout if there is a cavity??? I dont want to fix on plasterboard etc because i dont want to raise the height of the front,ideas please!! Thanks

 

[White Room]

Try 6mm hardie but the trouble with ledges people tend to try and sit on them