whats the best way to mend a leaking concrete shed roof?

I have a large shed built around 1940 and it leaks really badly from flat concrete roof mainly from joins in concrete slabs, I have tried cementing large cracks and then covered whole shed in thompsons roof seal it is a bit better but still leaking, can anyone sugest best way of fixing before I spend loads of money on different products that dont work, I was thinking of using a true bitumen product next, any advice gratefully received