replacement sash window sill / Membrane between sill and brick?

[james19000]

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.

 

[mrrusty]

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/