Q. My attic is not tall enough for living space, but I use it for storage. I thought about adding a small roof dormer for light and to improve the appearance of my home. How do I design one?
Adding dormers to your home can provide natural light in the attic area, and dormers look attractive from the street. You mentioned “a” dormer. Even though one will provide adequate lighting, two or more look much better. Once you are building them, a second or third one is not a lot more work.
Small dormers are sometimes called doghouse dormers, because they are shaped somewhat similar to a doghouse with a gable roof. They have to be built well enough not to leak water, but air leakage is not a major concern. Attics are supposed to have adequate fresh air ventilation, so a few more air leaks will not harm anything.
The first design concern is the size of the dormer. This depends primarily upon the vertical height of your roof, from the gutter to its peak. In Northern climates almost any size works because roofs have a higher pitch for snow loads than do roofs in Southern climates. In either climate, a larger house will have a greater vertical height.
If possible, there should be at least 1 foot of space between the peak of the dormer’s roof and the house’s roof and 1 foot between the bottom of the dormer and the lower edge of the roof. This simplifies installation and allows adequate room for flashing, which can prevent water leaks. Dormers also look much better with some space at the top and bottom.
Another size factor to consider is the size of the windows you want to install. For aesthetics, dormer windows generally should be smaller than the windows on the floor below. Check at your local home center store for custom windows that did not sell. These can be priced at a steep discount.
Once you have selected windows, this will determine the height and width of the front face of the dormer. Next, determine the pitch of the dormer roof, allowing for the 1-foot clearance from the house roof peak. If there is adequate clearance, make the dormer roof the same pitch as the house roof. This will look best.
It is not difficult to build much of a dormer yourself. Most of the work can be done on the ground or inside the attic if there is enough headroom. Unless you are experienced and comfortable working on top of a roof, have a professional remodeler do the final installation and finish work.
The most critical measurement when building a dormer is the pitch of the house roof. The best method to measure the house roof pitch is from inside the attic. Select a point on the top of a rafter and measure the run (horizontal) distance and the rise (vertical) distance to the peak. Use this exact pitch when building the sloped sidewall of the dormers. The rest of the construction is similar to building a miniature framed house.
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