While visiting a job site in Maryland, some exterior details caught my eye. And not really in a good way. Years ago, when I was a framer, I bumped up against the task of framing a room over a bay window. At first, this seems like a simple little task that should take no more than half an hour. Then the head-scratching starts. At what angle should the hip rafters intersect the bay walls? If they are run straight to the exterior wall, as in this photo, then the roof pitch is different on each side of the hip rafter. Cutting the rafters to bisect the walls at a 45 degree angle, as with a typical hip roof, doesn't work either—the hip rafters can end up bisecting each other before even making it to the exterior wall.
I've seen guys burn through large piles of lumber trying to frame one of these 'simple' little roofs. In the end, they frequently do what we see here: run the hip rafters 90 degrees from the outer bay wall straight back to the house, which results in a multi-pitch, blocky-looking roof. The roof is not as simple as it seems because it's not based on a rectangle. It really should be a small section of a conical roof, like an octagon.
Rather than burning through a virtual lumber pile, I asked for authoritative help on the topic. In this installment of Mystery Inspector, I deputize some design professionals and dub them the Elevation Enforcers. Both architects confirmed my suspicion, and gave additional insights about the design elements of the exterior elevation on this 'traditional' design.
The first architect gave me a 5-point list:
1. Buy Marianne Cusato's book Get Your House Right—best $15 you'll spend this month.
2. The inspector is correct. The hip rafter of the bay roof should be set at half the angle of the corner. On a standard 90 degree corner the hip is set at 45 degrees. On this bay the corner is at 135 degrees, therefore the hip should be set at 67.5 degrees.
3. Façades as human faces is always a strange, out-of-scale idea. The oval window eyes with the bay window nose is a bit too obvious.
4. Bays that include floor space on the interior are difficult to trim on the exterior. It's a scale issue. In this case the eave needs less fascia and overhang and more trim, as in a well designed dormer eave. The sill and floor horizontal trim line should be beefed up.
5. Last, if the bay was painted the beige trim color instead of the white window color, it would soften the lack of detail of the bay.
This kind of thing is so frustrating, especially after doing it for centuries!
The second architect gave Mystery Inspector a vocabulary lesson, a spanking, and additional info on how to size the walls:
This is technically not a bay window, but an oriel, so Mystery Inspector gets a swat on the nose with a rolled up newspaper. A bay has a foundation on the ground while an oriel is a cantilevered protrusion from the wall usually at an upper story. An oriel should always have a visible structural support such as brackets or corbeled masonry, and this oriel has neither.
The oriel window in the photo should be roofed as a hip (as the Mystery Inspector correctly points out), but this would result in only a half slice of a hexagonal pyramid if the oriel walls are of equal length; in the photo they are not. The framing shown is incorrect in that the front of the oriel has a shed roof while the sides of the bay simply fill in the resulting gaps.
The oriel should have equally pitched roofs meeting at a hip that will result in a linear connection with the wall of the house (as opposed to a point), but this linear connection will be a shorter dimension than the length of the front of the oriel.
Ideally, when all sides aren't equal, the length of the front of the oriel will be determined by doubling or trebling the window of the angled sides without increasing the size of the window unit. The oriel in this picture has a large picture window in the center. While this is common practice (and acceptable by today's standards) it is not how a bay or oriel has been traditionally proportioned or fenestrated.
David then also recommended a book to Mystery Inspector: Traditional Construction Patterns by architect Stephen A. Mouzon. He says it is "a very helpful illustrated field reference that explains the do's and don'ts of traditional architectural detailing."
Is there anything right with this picture? See something that we missed? Make a note of it in the comments.
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