Help!: tiling on heating mat on plywood floor

[luke8250]

Hi there
i'm sure this has been answered a hundred times but cannot find a link with prompts to search previous posts. I've never liked tiling on wooden floors but i understand todays flexible adhesives and grouts have improved somewhat in the last few years. Now everyone wants to add the further complication of a heating mat.
My situation is i have 18mm ply screwed down at 6" centres, Its a small (2 sq.mtrs) area. Do i lay the mat out on a bed of flexi adhesive and then butter up the back of the fairly large tiles i'm laying or do i glue the mat down with something else.
Oh, one other thing, the ply is wpb, should i prime it or something,
thanks for your help

[deanotile]

slc on top of the ufh then an uncoupling membrane on top of that would be the way I'd go.

[luke8250]

SLC, uncoupling membrane, sorry Deano your talking to an old bloke here could you elaborate por favor, what are they.

[Mike]

i'd recomend 6mm hardi boards or similar laid with adhesive and screws between the ply and ufh/slc. this will help deflect the heat up rather than into the ply and a better substrate for the slc to stick to. but it depends on if you can afford the extra 8mm or so in height.

[luke8250]

the 8mm is not a problem, but isnt that just adding to overall movement

[Mike]

slc is self levelling concrete. this will cover the heating cables so you don't get any hotspots and stop the cables getting damaged prior to or while tiles are getting laid. i've never needed to use the ditra matting but im sure deano or someone will help you here.

[Mike]

if it's screwed down properly on bed of adhesive it will make the floor more rigid.

[luke8250]

ok, with you now, sounds like i,m off to Wickes, another half day wasted doh !

[Mike]

not wasted as it's important you do it right. i did one last week that had previously failed due to the other tiler just tiling straight onto ply that wasnt stable enough. cost the customer a lot of extra money in the end including a new ufh cable.

[luke8250]

Thanks mike, just warming up the tranny, off to get the cement board, but slc, its 25 quid a bag and i need cupfull, cant i just lay the mat on adhesive and butter up the tiles ( so no nicking with trowel)

[Mike]

no you must ensure the cables are fully encapsulated in adhesive or self levelling. you could search for cheaper self levelling by going to the builders merchants not the diy shops. or you could lay the adhesive over the cables carefully with a plastic notched trowel.

[luke8250]

oh dear, wickes board is 12.5 mm thick, that'll be 14mm overall, havent got that ?

[Mike]

like i said , go to the builders merchants. they should have it

[luke8250]

found it, 6mm, stutt, i 'ate topps tiles. Dont shout but if the mat is bedded on adhesive and then the tile comes down (buttered up) that will encapsulate
the mat but i'll do what you say ok. Just looked at the mat, not much
more than 1mm but the temp.probe is somewhat thicker

[luke8250]

Just unrolled it, i see now, the actual heating element thicker also, bit like me. Got to cut it up to fit floor, oops !

[Mike]

if you just back butter the tile you are working blind, you dont know for defo the cables are fully covered. you are better using slc, you won't need much as the cable sounds thin. as for the probe you can always sink it a bit in the cement boards or run it through a gap in the join of the boards. i've got to go now but i'm sure others on here will help if you need any more advice. good luck

[luke8250]

thanks MIke, appreciate your help.

[Scott]

Chase a small channel in the floor for the probe and the larger end of the mat. This helps with the height issue.

[luke8250]

thanks for that,will do.