Question
I am about to lay turf, and had planned to use sand as the final levelling medium, but my local building supplier does not have horticultural sand - he only has soft washed building sand, which he says is not recommended for lawns, but he couldn't say why not.  Since I don't have time now to buy horticultural sand before the turf is delivered, can I safely use the building sand?  I would only be putting a thin layer (of, say, 15mm uncompressed, 3 - 4mm compressed) below the turf - I cannot see how this could be harmful to the turf, but I would appreciate your advice.

Answer
If I'm understanding correctly, you have soil, on top of which you plan to spread sand to level things, on top of which you plan to lay sod?

If this is correct, then it is a bad plan no matter which type of sand you use. Having different layers of soil types that change sharply from one to the next creates a series of problems with drainage and rooting. Sand on top of soil is exactly this sort of layering problem that you should be trying to avoid.

I'd recommend using soil that is very similar to the existing soil in the yard.

Good luck.

-C.J. Brown