Hi builder guys and renewables guys. Anyone able to tell me the best sand to use for screeding a floor over underfloor heating pipes? I've read sharp sand. But I don't know the name of that in Spanish. Don't know what I've got at the moment but I think its a softer form which I'm using for mortar.
I want to ask for the right stuff at the builders merchants. Tried a small section with what I had but it's cracking. I know I didn't make it deep enough but wondered if the sand was wrong too.
Hope you can help guys!:-)

Yes, what we call sharp (coarse) sand is the best for screeding. In Spain it's called "arena gruesa" but you will usually find that every builders merchants has their own name for it eg arena del rio etc. If you tell your supplier what you're using it for and go and have a feel of their sands you should end up with the right stuff!
However, the cracking in your floor screed was most likely due to lack of cover over the pipes (or if it dried out too quickly, but unlikely in this weather lol). A reinforced screed will have less tendency to crack.

Hi, fraid I´m more of a bodger than a builder but here´s what I´ve been getting down the local builders merchants;

Arena Normal - gritty sand - bricklaying
Arena Chinchilla - finer than Normal - laying narrow(5cm) "ladrillos" bricks and rendering.
Planche - premixed agregrate and rough sand.

How thick is the scree you plan to lay ? For a shallow scree (up to a couple of 2cm´s), we´ve been using using arena normal, for a thick scree (over 5 cm´s) concrete (6 parts planche mixed with 1 part cement). No problems to date with cracking but I try and avoid working when it´s hot and add a bit of diluted latex or "hydrofugante" (an additive which make the mix more resistant to water). This just my experience working on my own place with both Spanish and English builders.

I´d also be interested in what the builders have say.

Thanks very much for the quick reply Hillbilly. I'm running out of time for this job now as I have family coming at The beginning of june and loads more jobs depend on this being done first. The weather, and the lack of windows or a lucenario , i.e. a gaping hole in the roof had made it impossible so far. Now we're watertight :-) What do I reinforce it with. Metal? Fibres...Yak hair??:

Yeah. I wondered about the aggregate but thought it might be a bit rough on the pipes if there is any movement in future. Hillbilly seems to know what he's talking about. They must laugh at us bodgers sometimes.
I think I need around 6 cm to take account of the flexibility of the expanded polystyrene and the pipes on top also thin out the surface. So that would be around 4 cm over the pipes. The bit I did too thin was an absolutely necessary walkway and had to be done quickly. I feel a fool for not asking here first!! But we can pick out the cracked bits and overlay it again.

For re-enforcing, ask for "una malla" - steel reinforcing mesh of about 10cm squares. Again this may differ depending on where you are, but the local builders merchants sell it by the meter. You could find this difficult to use in a shallow scree, since I think it´s really intended for thick concrete floors (ie. min 5cm).

The min. recommended depth of floor screed over underfloor heating is 75mm I believe (but you need to check your manufacturer's spec). If you are laying a screed any less than about 60mm then you should really reinforce it.
As littleweed says, you may not have sufficient depth for regular reinforcing mesh. You could use chicken wire (not strictly recommended, but sometimes needs must!) or fibre (fibra del polipropileno, if you can get it. Never tried to source it here, so don't even know if it's available except to the trade). You could use glass fibre reinforcing mesh (malla de fibra de vidrio) which I do know you can get here.
On the other hand, does it matter if the screed has cracked? You could always seal any cracks that do appear. Or lay a latex self-levelling thin screed on top. If you're going to tile the floor anyway, just use a flexible adhesive to lay your tiles when the time comes.
BTW I'm a she, not a he
Edit: just re-read your post and you say you're only putting in 40mm over the pipes which isn't really enough. Can you not increase the depth?
Edit 2: here are some products http://www.collvila.com/refuerzos-para-morteros/

Many more thanks Hillbilly and Littleweed. I'm happy to put whatever depth is required! We got the architect to design high ceilings into the spec. So we'll get the mesh and put the extra depth in. Cheers again

Oh and sorry for getting the sex wrong. Really should think about that being a she myself and getting grit under my own fingernails;-)
respec!

We did our floors with a dry/damp screed mix well hammered down3-4 inches thick and reinforced with chicken mesh, then covered with tiles. So far so good.

That is how it should be done.

Thank you. I was told how to do it by a friend who is a structural engineer, so I listened very carefully when he was telling me what to do.

I found the square xsect tubes great for giving levels.

A semi-dry mix is great for small areas and DIY...but the problem when screeding over underfloor heating pipes if you use a semi-dry mix is ensuring that all the air has been expelled. If you have any trapped air pockets this will reduce the efficiency of the underfloor heating. It can be done but you have to take great care over compaction. With a "liquid" screed you can be sure that all the pipes are completely enclosed.

Either you've been going to night-classes Hillybilly or else you been watching too much Tommy Walsh....

LOL. It's my job (I don't mean watching Tommy Walsh ) for my sins!