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 January 6 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

[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.

I forgot to take a picture but I measured the slope to about 3°, so definitely on " too flat" side.

[Nickfromwales]

1 hour ago, Bruce said:

I forgot to take a picture but I measured the slope to about 3°, so definitely on " too flat" side.

Is there mastic sealant under the cill? As it appears to be a unit that weeps out of that small shadow gap, onto the top of the cill and away, then I’d say you’ll be better off pumping a load of clear sealant under the cill to appease the BCO.

 

They often don't / won’t comment on solutions or methodology as they get into trouble or can become liable if their suggestion fails. 
 

Some BCO’s are helpful and pragmatic, others are clueless, obtuse dickheads.

 

As your builder if the weep / drip from the frame of the unit only comes out above the cill. If so, pump it full of clear frame sealant and you’ll be fine.

 

Out of curiosity, has it been in for long and any signs of ingress to date?

 

FYI, this is your builders problem to solve, not yours, so get them to take over solving the problem but tell them you want to discuss it before they “do something” and you don’t know what it was, plus your BCO will need to say they’re happy with it before it’s done; you could end up having to remove the sealant if they don’t like / approve it.