A. You don’t say if this is a manufactured unit or a custom installation, but I assume you are talking about a complete unit. This type of bay window usually comes with a plywood top and bottom panel and a nail fin all the way around. If the installation is exposed to the weather, you will need to add a sloping roof on top of the unit.

In order to properly install a bay window unit, you need to remove enough siding to allow the nail fin or brick mold to be attached to the wall framing. If you are adding a roof, remove enough siding above the opening to allow the roof framing and roof-to-wall flashing to be installed.

Normally you will have a roof with 3 surfaces, one running from the wall to a line above each window. The two roof planes above the side windows should have step flashing woven into the roof shingles. Before each course of shingles is attached, a piece of metal flashing, folded at 90º, is nailed to the wall. The shingles are then laid for that course, covering the flashing that is on the roof. This is repeated until all shingle courses are attached. The final step is to install a piece of roof-to-wall flashing across the top and extending over the step flashing which was run up the walls. Be sure to nail only on the wall, and use flashing that is bent at the appropriate angle for the roof/wall intersection.

Now you can replace the siding, bringing it to within about 1/8th inch of the window trim on the bottom and sides of the unit. As you work your way along the roof, leave ½” to ¾” spacing between the roof shingles and the siding. Finally, fill the gap between siding and window trim with a good quality siliconized acrylic caulk. You should not need to do any caulking above the window. If you are getting water coming in from above, it is most likely that the flashing was not installed properly. Rather than trying to patch a bad installation, remove the siding above the window and reinstall the flashing as described above.