No, it often is dredged from the sea and is too salty. Use horticultural sand.

The amount of salt in sand is not really going to be an issue. The reason for not using builders sand is that it is soft and clumps together which is what it is supposed to do when being used for path laying or mortar. To alter the condition of a clay soil you needd to add sharp sand. This is also available at Builder's Merchants. It is easy to tell the difference, if it makes good sand castles then it is no good for drainage in clay soils.

I guess the clue is in the name!!

I use horticultural grit in clay soil, find its more effective than even sharp sand.

That would be my recommendation too, grit.

I too would use grit, actually any irregular shaped material will do, even the size is not that important. Have used the 7mm limestone road grit before now, to good efffect. The question was about Builders sand, so I answered that.
Also will add that Gypsum is as good as anything to help with a clay soil. Best source........crushed up old plaster board (without the cardboard covering of course).

Owdboggy, I know about gypsum, but also that you can't use it where there's existing planting - do you know how long it has to be left after using in unplanted ground before planting can take place?

New one on me, when I used it on our clay soil years ago, I did it on the newly dug vegetable patch and planted stuff almost immediately, they grew ok.
I would imagine that it would be safe to plant once there had been enough rain for the chemical reaction between it and the clay to have taken place. So, I suppose it would depend on how much rain fell and when.

I know that if you apply it in autumn and leave over winter, then it's fine, but I can't remember what the shortest time is, (oh the joys of getting older) but you're probably right, its dependent on rainfall.

One thing that has always puzzled me about "builder's sand": why would it be salty when any builder who knows what he is doing knows that any salt in the sand will keep the mortar or concrete he mixes it with from setting properly?!

Exactly!

I was always told you shouldn't use it because you didn't know what would be in it - lime in particular might be an issue with builder's sand, never mind salts. Any builder worth his salt uses washed builders sand anyway, specially for brickwork...

Ah! So washed builder's sand could be okay, though maybe more expensive than all-purpose or sharp sand.

Same problem with lime in sand as far as cement or mortar making is concerned. Its presence would/could spoil the mix.

It's irrelevant anyway Tugbrethil, I'd never put builder's sand on clay soil to help the soil, not much point for all the difference it makes.

Nope, you don't put it on, you mix it in. Eight cm of sand, dug in to a spade's depth, then turned over four times. Of course, now I'm talking about all-purpose sand, not builder's.

My experience with gypsum is that it is nearly inert. In fact there are some islands in the Mediterranean where the bedrock and soil are nearly pure gypsum, and support a wide variety of plants--all loving fast drainage, of course! That concern of leaving it over the winter sounds like using slaked lime, rather than gypsum, to me. One reason not to use gypsum in an established bed is that it works best when dug in. Just laying it on top might open up the top few cm of soil, but it is very slow to work itself deeper.

On or in, I still wouldn't, I'd go for grit every time, lol!

True, grit is better. Unfortunately scarce and expensive here in Arizona--for no reason that I can find!

Builders sand in the soil? Well, I had 4 bags left over last year. Was opening up a new area under lifted lawn. Needed to improve really heavy clay soil.
I moved aside the top 6 or 7 inches of soil, removed some of the rubble, mixed in the builders sand with the heavy clay stuff underneath, then replaced the moved aside soil, but added a bag of peat free compost to that bit. Have had a brilliant floral show this year. Dahlias and malva sylvestris in particular. (Just made sure I didn't plant any acid lovers in that bed just yet.)
I did the sand infill last winter, so it may be that weather has washed any salt or lime away. So, my feeling is you can use builders sand if you need to, just keep it deep.