We are going to replace old windows with new Pella Pro Line double hung. These are made for new construction, not replacement. The windows are the width of the wall. The window comes with a narrow (1.5 inches-2 inches) sill type ledge. We would like to install a solid surface sill (I have heard some call it a stool) on the inside that is about 4-5 inches. My question is do we make the sill as wide as the wall + the 4-5 inches we want it to stick out and have the window sit on top of the sill? or do we silicone a 4-5" sill to the narrow factory sill and finish around the edges with other framing? Is it even possible to put a sill on a window that is as wide as the wall?

 

Hi,

Here's a complete installation guide for Pella (new construction window in existing opening) The part about sill is near the bottom of the article. Hope this is helpful:
http://www.extremehowto.com/xh/artic...ticle_id=60193

 

Thanks that was very helpful. From reading it I guess that I need to use the correct terminology. We would like an interior window stool. The article said that it could be done, but not how. The article explained well how to do an exterior sill. We have installed an interior stool for a sliding window before, but not for a double hung that is the same width as the wall.

 

Assuming that the windows are the correct wall thickness for your wall, I would suggest that you install them in a manner consistant with "new construction window in existing opening".

When it comes time to add the stool, I'd suggest that you add one onto the interior of the window jamb by using a little wood glue and a few pocket screws underneath to attach the stool to the bottom jamb. If you are unfamiliar with pocket screws, just Google "Kreg pocket hole jig". The stool itself will not be the shape of the traditional stool, which fits like a cap over the sloped bottom sill of a window. Instead, it will be more like a wide jamb extension. I usually want my stools to protrude at least as far from the casing as the casing is thick. So if the casing is 3/4" thick, then the stool should protrude at least 1 1/2" from the wall, if not a little more. Additionally, stools should usually have "ears" that extend beyond the edges of the casing by this same amount, or more if the style dictates.

The edges of the stool can have any profile you like, depending on your style. 1/4" roundover on all edges is a common look, others prefer more of a table edge type ogee. Usually you want a profile that goes well with the rest of the trim, or the rest of the decor.

You would likely want to leave a 3/16" reveal on bottom where the stool attaches to the jamb, rather than trying to make the stool flush with the jamb.

And please shim and secure the side jambs to the rough opening instead of simply relying on the nailing fin to keep the windows true. Windows move in and out on the nailing flanges, but shimming and securing them will help you flush the jambs up with the surface of the wall and will assure that they stay there.