Are stub sills or knockons required on concrete sills with upvc windows?

Hi Tess , Not necessarily , it all depends on the type of drainage the window has.if the window has ' face drainage ' with little cover caps then a add on may be required to allow room for water to drain away. If the frame has ' concealed drainage ' then a stubb sill will be required as the profile of the sill section allows the water to escape.if none of these are present then water ingress can build up within the profile and overtime cause failure to locking mechanisms and sealed units ,corrosion to any metal reinforcing within the frame and possible damp problems to the building itself.

No As above face drainage is enough as it sits arounds 15 mm up from the bottom rail allowing it to drain away. Add ons to the bottom are usually to let the frame clear the internal sill thus raising the handle height. Lee

Stub sills cost a little extra, but you do get a far better external finish to the installed frame. Depending on the depth / site line of the internal window board would be the main factor to consider if a stub sill or knock on/extension or type of drainage required during the manufacturing process. All good qualified surveyor's would be able to advise accordingly.