replacement sash window sill / Membrane between sill and brick?

Hi good folks of the DIYnot community.

I'm replacing the rotten sill of a sash window.

Having removed the rotten sill, i'm presented with row of bricks.

I'm going to lay a new bed of mortar for the new replacement sill to sit on.

Do I need to put some kind of semi permeable membrane in?

and if so, between which materials do I place it.

If I do have to place a membrane or some sort, any recommendations.

I hope i've explained myself clearly,

Pls have a look at the attached photo, I think it will be self explanatory what I'm attempting to do.

Any tips or advise will be most welcome.

Thank you for reading and thank you for any comments.

 

No, you don't need to put a membrane under the cill, but do make sure the new cill has a good drip bead under the outer edge. If you put a waterproof layer under the cill you actually risk creating a water trap should any drips get under the cill

As a matter of interest, are you taking the opportunity to install draught strips? you can get parting & staff bead with seals. e.g. https://www.reddiseals.com/product-category/sash-window/beading/