Hmmm..around 5-6 inches if you are using grade 1 sand. If using purely grade 3 I think your sand bed needs to be alot deeper. Not too sure if grade 3 sand should be used to make dsb (it can be done, but would look like an eyesore). Just stick to grade 1 and all should be fine. I personally find grade 0 to be too fine. Just a personal opinion.