Waterproofing window sill with too little incline

[Bruce]

Our BCO is not happy with the incline of our window sill and say it might lead to water ingress. They alluded that the sill should not need redoing but they were not allowed to tell us how. So I'm after any ideas that we can use/submit on how to retrofit a simple solution to this problem. I'm thinking maybe suggesting something like the below pictured diverter but wanted to see if someone else has done something clever before me? We have had this window like this since August, without any selaent under it (life got in the way) and it has never leaked.

[MikeGrahamT21]

Something like this would be ideal, cut to the relevant dimensions and put under the existing sill

 

https://ewistore.co.uk/shop/external-wall-insulation/over-sill-2-5m-100mm-depth/

[Bruce]

14 hours ago, MikeGrahamT21 said:

Something like this would be ideal, cut to the relevant dimensions and put under the existing sill

 

https://ewistore.co.uk/shop/external-wall-insulation/over-sill-2-5m-100mm-depth/

I like the look of that but not sure if it helps with the incline of the sill if just mounted on top of it?

[Mr Punter]

Is it the tiles or the plastic cill that are not right?  If it is the tiles, @MikeGrahamT21's overcill looks good.  You could put some packers along the back to give it some fall.

[Bruce]

On 29/12/2025 at 13:02, Mr Punter said:

Is it the tiles or the plastic cill that are not right?  If it is the tiles, @MikeGrahamT21's overcill looks good.  You could put some packers along the back to give it some fall.

It's the tiles that they complain about.

[Redbeard]

Can you show us a pic with a little 'boat' spirit level on the cill so we can see the nature of the fall (or not)? I *seem* to see a fatter mortar bed at 

the back than at the front, which suggests a fall, but an accurate spirit level will be a better judge than my old eyes. I have used the EWIstore overcills with success.

[Bruce]

On 31/12/2025 at 18:34, Redbeard said:

Can you show us a pic with a little 'boat' spirit level on the cill so we can see the nature of the fall (or not)? I *seem* to see a fatter mortar bed at 

the back than at the front, which suggests a fall, but an accurate spirit level will be a better judge than my old eyes. I have used the EWIstore overcills with success.

Thank, I will take a picture with my smallest level and post it. Not to be lasy but rather asking for experience, do you have a link  to the EWIstore that you used?

 

TIA

Edited Tuesday at 17:12 by Bruce

[Redbeard]

Here's a link to one size of overcill:

 

https://ewistore.co.uk/shop/external-wall-insulation/over-sill-2-5m-110mm-depth/?Size=110mm&mh_matchtype=&mh_keyword=&mh_adgroupid=&mh_network=x&gad_source=1&gad_campaignid=22700066520&gbraid=0AAAAADhexNg4SlS0vhlOGHW8qltiU573b&gclid=Cj0KCQiAgvPKBhCxARIsAOlK_Er-hrpGbM79I9UzdR6ULRJrRkfMPUlaJsyl8HDjKihsiIaeqvNunQEaAlohEALw_wcB