A window that doesn't sit straight on the sill may not open and close correctly. For the best protection from the elements and insects, make sure your window sill is level and secure before installing your new window.

Should window sill be slanted?

The sill on a rough opening should be sloped so it can direct water that has penetrated the building envelope back to the outdoors. ... Don't forget to add to the height of the rough opening to accommodate the thickness of a sill-wedge or sloped pan.

What is sill angle?

The sill slope of a window is the angle of the sill (angle of the bottom of the window unit). The sills of most Andersen® products are sloped towards the exterior of the home.