drawingboard lessons in residential design by mike maines
here are many reasons you might want to put dormers on a is often the most practical solution. Large, horizontal shed dormers roof. Perhaps you want greater access to daylight and views, can significantly alter the original look of the home, however, so T or you need a little extra floor area. Or maybe you just think sometimes they’re best reserved for the back of the house. Large that well-designed dormers add character to the house. gable dormers are another solution, but their scale can quickly When the primary goal is simply to add usable floor area while overwhelm the lower floors of the house, creating a top-heavy look. preserving the lines of the main roof, a well-designed shed dormer That said, should a dormer become large enough to be considered
3 2 ft. 10 ⁄8 in. B
5 C 6 ft. 1 ⁄8 in. A
D 3 ft.
8 in.
E
3 ft. 6 in. 3 ft. 6 in. A Spacing C Height There are no hard-and-fast rules when it When the house’s facade is tall, the dormers already have perfect classical details, you comes to dormer spacing, but start with can be tall. Similarly, when the facade is have to use the house as a reference. For geometric patterns. For example, these short, the dormers can be short. However, example, on a bungalow, the dormer may three dormers happen to look best if height shouldn’t be a control in itself; rather, look best with a little extra width to create evenly spread across the roof, but often dormer height should always be driven by more horizontal proportions. the end dormers can be cheated a bit to the size and scale of the windows to be used the outside. in the dormer. E Main-house reference On many houses, you can ignore the base B Roof slope D Width of the house and focus simply on how well The most conventional approach is to match In her book Get Your House Right, the dormers look on the roof. Because the house’s roof slope. Here, that slope Marianne Cusato suggests that the total the dormers are in a different plane from is 10-in-12. However, certain designs can width of a dormer be the same as that the main-house wall, and because the roof support various slopes. When the dormer of the windows, including trim, on the eave provides a strong horizontal break, gable is inspired by a classical pediment, main house below. This is a noble goal on alignment with elements below is less the pitch can be lower than the house roof, straightforward, classical architecture. When important than the massing—that is, how down to about 5-in-12. adding a dormer to a house that does not the dormers “feel” on the roof.
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3 2 ft. 10 ⁄8 in. 3 ft. 6 in. a cross gable, it will hold its F I 1 ft. 11 in. own as a major element of the F Placement G home’s design. Dormer placement is driven by Appropriately scaled gable many factors. Ideally, the window 1 dormers bring in light while 3 ⁄2 in. heads would align with any gable H adding charm and interest windows on each end of the house, to a large or plain roof. If and the ceiling in the dormer designed well, they don’t com- would be in plane with the other 3 ft. 1 pete with the lines of the main ceilings on the interior. From an 4 ⁄2 in. exterior point of view, it’s a massing house; instead, they reinforce, decision, and one best viewed in complement, and enrich the three dimensions. The “center of 8 in. house’s personality. gravity” of the dormer—generally about two-thirds of the height of the cheek H Trim size Show restraint wall or half the overall height of the dormer— As a rule, I scale the trim down by 10% to 25% By far, the most common mis- should land near the center of the roof. It’s OK compared to the roof trim on the main house. take people make when adding to move it down a bit, but don’t move it higher. I Trim profiles dormers is to design them too G Rake overhang This house has simple eave trim: a vertical 1x8 big. It’s understandable to max- When there is little or no rake overhang on the with a gutter that creates a basic cornice. It imize the bang for your buck, main house, detail the dormer to match. When doesn’t make sense to use gutters on dormers but big dormers quickly com- the house has a larger rake overhang, reduce because any rain is directed into the gutters promise the design, especially the overhang depth on the dormer by 25% to below. To create a different yet still appropriate on smaller houses that have 50%, unless your intention is to draw attention cornice molding, use a flat fascia board set on a low eaves. To avoid this mis- to the dormer roof. 40° to 45° angle. take, think of the window as the main reason the dormer exists, and provide the mini- 12 44 in., a minimum net-clear opening of mum framework necessary 10 5.7 sq. ft., a minimum width of 20 in., and a around the window for struc- minimum height of 24 in. This does not mean ture and insulation. L that an opening of 20 in. by 24 in. meets Here, I’ve designed gable dor- 1 4 ⁄2 in. code. For example, a 20-in.-wide window 5 in. mers for a 1940s Cape—a style has to be at least 42 in. high to meet the of home that often calls for the 5.7-sq.-ft. requirement. Also, because only half 3 addition of dormers. The upper ⁄4 in. of a double-hung unit can be opened at a time, the net-clear opening is drastically diminished— floor is to be finished as living J down to 4.2 sq. ft. for the window in the space, so light, views, and egress K drawing. Fortunately, manufacturers make are important factors driving its M casement windows that look like double-hung redesign. It’s always important 1 1 ⁄2 in. units for this purpose. For this project, such to factor in egress when design- a casement window would have a net-clear ing dormers because the size of 2 ft. 5 in. opening of 8.4 sq. ft. 2 in. many dormer windows won’t 3 ft. 3 in. meet code for egress. 4 ft. L Gable material If the house details are simple, as they are here, The house has low eaves, J Window size matching the house siding is the conventional simple trim, and an asymmetri- When sizing windows, choose units that reflect approach. If the house has more elaborate trim, cal front facade. It’s in a historic the first-floor windows but are scaled down or the dormers are particularly small, or if you district, so the dormer additions by 10% to 25%. Keep the aspect ratio—the want to add some visual interest, the gable need to be done carefully to proportion of a window’s height to its width— and cheek walls can be finished and painted to enhance, not detract from, the the same, or make the windows slightly squat. match the trim. This makes the entire dormer a home’s character. trim element. K Egress If any dormers are in a bedroom, remember M Sill height Mike Maines is a designer and that at least one window in each bedroom must Visually, less is best. Holding the sill at least 2 in. builder at EcoCor (ecocor.us) in meet code for egress. An egressable window off the roof keeps the sill above snow and ice Searsmont, Maine. Drawings by has a distance from the floor no greater than and creates a vertical dam against blowing rain. the author.
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