I never heard of a prefab dormer. Do you have a link?
Ron

I don't have any good links. I saw this in a magazine once. I think most modular home companies offer something like it, usually as an add on to the homes they sell. I haven't found a site that retails just the dormer. From what I have seen, they open the roof, crane up the dormer and seal, then your ready to finish.

We used to pre fab dormers on site and crane up, but as ron mentioned, the framed opening to accept the dormers was calc'd out w/ quadruple dormer rafters each side, header and sill beams to carry the load. Not something you just flop down on the roof where you want it. That would be sweet though!

I have also seen false dormers but they still need the extra structural support to carry the load.

Ron and Sierra- thanks so much for the info.
A couple more questions:
1) can an existing roof be retrofitted to accept a prefab dormer?
2) Is it more cost effective to beef up the exsiting roof and add a prefab dormer, or is just stick building it going to be the same? It seems you would have less time with an open roof with the prefab route. True?
3) sierra- where did you get your dormer? Can you just buy the dormer alone, or is it part of the package with a modular home? If you don't mind telling a ballpark price.

Thanks again. This site is th best!

Hi dogridge, i shouldn't have used the term pre fab. We framed our dormers on the ground while the roof was being stacked out including the framed opening for the dormers. In that sense they were pre fab but just built on site.

Question #1- yes the existing roof can be retrofitted to accept the dormer but should be engineered according to the dormers makeup in order to get the dormer rafter ,header, sill sizing needed to accept the load.

Question # 2- I couldn't tell you the cost differential, but it makes sense there would be a little less open roof time with a pre fab, although it doesn't take a lot of time to stick build a dormer depending on size, roof type,(hip over straight gable),and details. I would think that the stick built being more labor intensive would still be less considering the mfr'ing and delivery of a pre fab unit, but both have to have the same m.o. for framing out and readying the opening on the existing roof.

You need to take into consideration with the, built it on the ground scenario, is that you will need a crane to get that puppy up where it belongs. They are not cheap.
The most cost effective way to build it, is to build it in place, on the roof. If you feel the open exposure is a liability, hire a professional to do it. They will get it weathertight in 2-3 days, max.
Ron

As Ron states, you are probably better off building it in place. If weather is a concern, there is a method of framing the dormer opening w/o removing the existing roof sheathing, leave some underlayment, then frame and somewhat weathertight the dormer cutting the roof sheathing out after the fact. You will still get a little wet in places, but not as bad as opening up the roof beforehand. Using this method is a lot more involved and effort, so i would try to tackle this when you have a window of decent weather and competent carpenters, open it up and build it in place, getting dried in asap.

I would certainly leave it to the pros. And with the drought were in I probably don't need to worry too much about rain!! Does anyone have any idea what this would cost? It would be about 8 ft. long and 3-4 feet wide. I assume it would be more than just the square footage. Are we talking $2000, $5000, $10K? I am mainly doing this for a closet in the bedroom. My other option is to use the undereave space for a closet and finish those areas out. Thanks for all of your useful info!

Best just to get estimates from locals in your area. You might even try a competent handy person vs. a contractor to shave costs, but when you are dealing with structural issues such as the dormer opening, it would be wise to get that engineered and permits if required in your area. It might require the services of a licensed contractor as handyman services in your area might be limited.Whoever you get, make sure they are heavily insured for personal injury as well as liability for damages that could occur on your project.

I assume from the discussion that you have a roof pitched in 2 directions, and a room underneath which lacks closet space. What you are thinking to add is a 'shed dormer'. The other kind of dormer is often called a 'dog house dormer'.

Raising a roof to add a dormer changes the dynamics of your house. The rafters which run from your ridge to your floor hold your house together. Your actual ridge may be only a 1x board which rafters on both sides of your roof are nailed to. It is not designed to hold up the weight of your roof.

Right now you have a structurally strong triangle - rafters and attic floor. You are thinking to make a uneven 4 sided box - rafters at different pitches, carrying different loads, a wall and the attic floor. The shape is very unstable, so the framing has to be different. Usually the ridge needs to be able to carry the load of the new roof, something it wasn't designed to do.
Please get an engineer to advise you.
Secondly, no existing house is perfectly square. A prefab dormer would need to be adapted to the specific conditions of your roof, even if it's only 1/2" out of true. If the shed dormer is not fit to your roof, it will leak, you will have rot.

I see your point about changing the structure. My plan was to make a short dormer about 1/3 the length of the roof section. I agree that building a shed dormer the entire length of the roof could be dicey, but what about just a small one?

Looks like prefab dormers are more common overseas. Velux has one but I didnt see any pricing info.

https://www.google.com/search?q=prefabricated+velux+dormer+windows&rlz=1C1GGRV_enUS761US764&tbm=isch&source=iu&ictx=1&fir=L8Ks6tHjxL2XdM%253A%252CLcsYJZO3Q-3ktM%252C_&usg=__0weIEktKE6yKDYFY3RdD1S2W6F4%3D&sa=X&ved=0ahUKEwiq89qMzZzbAhWE71MKHXlzBN4Q9QEIUzAB#imgrc=_

They maybe more popular overseas in 2018 but this post is 11 years old and untouched until you brought it back. Why?

Because it shows up in Google searches for dormers and I have a life outside of a Houzz forum where I'm not worried about the age of a post.

"I have a life outside of a Houzz forum"

Very happy for you. And the Velux are windows, the op was looking for a complete prefab dormer.