Dormers absolutely make the roof of a house look far more interesting. The good news is you can add a dormer to an existing roof. The bad news is that it's not a do-it-yourself job. Here are some things to consider:
1. Leaks not wanted
The dormer requires the person building it to have a complete understanding of roof flashings. Without this, you'll have leaks in your roof for sure.
2. Real or fake?
If your home has traditional roof rafters and an open attic, you can add a functioning dormer that you can walk into. If your home was built with trusses, you can install a make-believe dormer that looks good from the street.
3. Truss worthy?
In the case of trusses, the dormer will probably be just decorative. You can cut into trusses and modify them to make a real dormer, but it's very complex. Since you can't effectively use an attic space that is filled with trusses, you might as well just create the decorative dormer.
4. Size it right
You should have a plan that's to scale. You want to make sure the dormer is not too small or too large. Nothing looks worse than a tiny dormer on a massive roof.
5. Get the permits
Once there is a plan, any necessary permits must be obtained. Call your town to find out what the regulations are.
6. Tarp time
With a permit in hand, now it's time to get to work. You will need to strip the roofing material in the area where the dormer is going to be placed. This one task would terrify the average homeowner, as all they can think of is what happens if it rains. This is why you should have large waterproof tarps and lumber that allows you to completely cover that entire section of the roof in case of a storm.
7. Double up the rafters
If you're creating a dormer on a roof framed with common rafters, you'll need to double or triple up the rafters on either side of where the dormer will be. Since you'll be cutting into two or more rafters to create the hole for the dormer, the load from those cut rafters has to be transferred to the rafters on either side.
8. Start to build
Once you have the hole created for the dormer, now you have to start to build it. The three walls that make up the front and sides of the dormer will all have angles that complement the slope of your roof. You also have to build a tiny roof on your dormer.
9. Heavy load
The most critical aspects of building a roof dormer are the structural and roof flashing components. The loads on a roof are significant, and you need to make sure that you know what you're doing when you cut into a roof. Add to this the added weight of the actual dormer. The materials used in the dormer could easily approach 1,000 pounds.
10. Solder on
To make the roof connections in a dormer waterproof, you need to know about step flashings, counter flashings, valley flashings, and base, head and sill flashings. You better know how to solder flashings, too, as you'll have to solder the lower corners of the base flashing where the front wall corners of the dormer contact the existing roof.