Hi Steve.
A good old 4:1
4 parts sand and 1 part cement. It may also be a good idea to add some waterproofer to the mix, you can get this at most diy stires and builder stores.
Joe Dybowski
November 2010
I can answer this question...
the plaster /cement between my roof tiles and batons has dried and rotted w?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix roofing.