i just got my first fireplace insert, an american eagle from around 1980 i am guessing. more on that later. right now i am just researching all the steps i need to take and things i need to acquire to get this thing in, which will happen sometime this summer. here is an easy first question. my chimney is massive masonry lined with red brick, and the opening at the top is 17 inches square. i see rockford chimney supply sells an 18 inch top plate. will this be big enough to securely cover the opening? that is, with only 1/2 inch of overlap on each side, will there be enough surface area for the silicone to hold it down and make it water tight? i benefit from your experience.