Tiling on asphalt screed?

[H.I.S]

Hi All,
I am new on here, and have just discovered this great forum with lots of good advice.
I have been tiling for a number of years now, not as a sole trade, i fit kitchens, bathrooms and am also a plumber and do other general building work.
i am after some good advice,
i have a floor to tile soon and its a substrate i have never tiled on before,
the property is a 1950s build and the floor has (i think) an asphalt screed this is about 20mm+ thick a brownish colour and solid, smooth to the touch. this floor is to have insulation board electric ufh and then porc tiles, i am worried about bonding to this substrate and would like to know what surface preparation and primer you all recommend for this substrate?
Thanks in advance for any advice.

[Ajax123]

oh oh oooh! Are you sure it is asphalt. It sounds typical of an magnesite screed. If you have a moisture meter (electronic type) bung it on the screed and see what reading you get. If it is positive it is very likely to be magnesite and as such you will struggle like heck to get anything to stick to it. I usually advise digging it out and putting something else in instead. Alternatively I guess you could drill and fix hardibacker or something like that to it.

[H.I.S]

I dont have one of these devices unfortunatly and i am not sure it is asphalt, i just came to the conclusion it was by serching the net, the description for mastic asphalt seemed similar to this substrate.
I have broken a small piece out from the floor and it smells similar to bitumin.
Is there any other way i can tell if it is asphalt or magnesite?

[Ajax123]

Not easily and safely. If it smells of bitumen it could well be that. If it were me I would SAFELY use a heat gun to see if it melts. This is Hazardous though as it will potentially catch fire if it is Bitumen based. If you do this be flipin careful. Safer way might be to pop to B&Q and buy a moisture meter. They are not too expensive. You might be able to "borrow" one from who ever you are buying the tiles from. Post some pics as well that might help to id the stuff.

[Dave]

Yep, Pics would be good..

[H.I.S]

Asphalt Screed 004.jpg I have just tryed to post pics as requested i think the files were to big, it has only let me add this one.
This is the small piece that i broke out from the floor, what you can see is its cross section (depth about 20mm) i have taken your advice and heated a small amount with my heatgun on a brick and it becomes quite soft when heated, then when cooled it is hard again, it also smells like bitumin.
Do you think this is conclusive that it is definately asphalt screed?

[Dave]

It doesn't look like any Asphalt i have come across, and my own floors are Asphalt pours..and are very black in colour.

[H.I.S]

Oh thats worrying thought i had sussed it, it certainly smells like bitumin when heated and goes soft with heat also. it looks as if it has been poured down. i think it is what was done as a dpm back in the 50s.
i cant think what else it could be, is mastic asphalt the same thing? could it be that? what primer did you use on asphalt you have come accross before?

[Dave]

I always slurry coat them.. creates a stronger bong.. but i am not saying that yours isn't Asphalt.. just not that type i have come across..

[H.I.S]

With it behaving the way it does ie smell and melting when hot i cant think what else it could be, would you key it first or just go straight over it as is? what is the process for slurry coating i have never needed to do this before, also do you think this floor will be ok with ufh due to it reacting with heat?

[whitebeam]

I'd be using sbr with a cement slurry for a key.

[Dave]

I would use insulation boards if using ufh..

A slurry coat is SBR mixed 50/50 with cement or rapidset.. and the floor needs to be as clean as possible, so a wash with sugar soap will suffice before slurrying.

[H.I.S]

I have got 6mm insulation boards already for the job as i cant go any higher, i was going to try and key/scarify the surface what do you think? so i can use cement or rapidset mix with sbr as slurry coat, do i wait till slurry is set before fixing the insulation boards down with spf? i had got some instarmac ultra prime it fp to use as an acrylic primer thinking it would do the job, glad i asked the question on here first.

[deanotile]

I'd get rid of it

[Ajax123]

melting and smelling of bitumen is pretty conclusive I would say. Magnesite would just get hot and with a blow torch would give of a bright yellow flame. As previously said though it don't look like asphalt. My opinion use epoxy to seal whatever it is then use uncoupling membrane.