problem with concrete subfloor

[mariaarthur]

hi all. first post here & i never thought i would need to after reading all the good advice on this site.

i am tiling my kitchen/conservatory area (about 26m2) the floor is concrete with approx 12mm bitumen topping & i will be tiling it with 450x450mm ceramic tiles.
all good so far i think.

running through the centre of the floor there is an 8" wide channel cut through the bitumen & below that a 4" wide channel cut into the concrete about 3" deep. it looks like the channel was chased out after the slab was poured, this being to allow the services to be run (cabling, gas & central heating pipes)

the original builder has put 12mm shuttering ply into the channel between the bitumen to make a level floor again but this is giving an amount of movement that is unnaceptable.

is there a product that is 12mm thick that i can use in place of the shuttering ply that will be suitable?

i reallydon't want to concrete in the services as this does not seem to be the right way to go.

any help/advice greatfully appreciated.

many thanks in advance

 

[gcol]

Screw down 12mm WBP (exterior) ply every 200mm and it should be ok. If you still see movement, then you'll need to add support to the ply. Use a flexible adhesive.

 

[mariaarthur]

someone has suggested today to fill the lower channel with sand & compact it down, then to cut to size some marble or similar natural stone tiles to fit in the channel between the bitumen & use tile adhesive to fix them into place.

this strikes me as a clever idea as the natural stone tile has no movement at all, it is dimensionally very stable. my only worry is if it is strong enough to bridge the 4" channel below it without cracking once the ceramic tiles have been laid over it?

thanks gcol for your input. if i get no good answers on the idea above i will go with your suggestion.

replies anyone?

 

[manchestertiling]

if not ply then the only other thing I would suggest is the hardibacker 500 cement board, at 12mm thick & can cut to size to fit your channel easy.

 

[mariaarthur]

thanks for the reply paul.

if you understand correctly the ply/hardibacker/natural stone tile will have a 4" wide void underneath it & will be supported either side of this void a further 2". ie an 8" wide strip.

whichever method i use i guess i am going to have to use screws & adhesive to fix in place?

is this hardibacker stuff strong enough to stand on (in your opinion) with a lack of support in the middle 4"? bear in mind also that i will be fitting 450mm floor tiles over the top so this will in effect make it stronger.

sorry to ask so many questions, but i have never come across this problem before & want to make sure i get it right first time.

as always many thanks

 

[gcol]

whichever method i use i guess i am going to have to use screws & adhesive to fix in place?

Screws only.

is this hardibacker stuff strong enough to stand on (in your opinion) with a lack of support in the middle 4"? bear in mind also that i will be fitting 450mm floor tiles over the top so this will in effect make it stronger.

So long as the tile edges don't finish on the strip you should have no problems with either WBP or backer board. As you suggest, when fixed, the substrate, adhesive and tile will make quite a thick floor that should be ok over such a small area compaired to the tile size.