designing a roof for new addition

[Nate]

Let's take a typical bi-level house, say 26' x 40'. Attach an addition of 16' x 40'. The addition would be parallel to the existing house - i.e., the 40' dimension of the addition is on the same axis as the 40' dimension of the existing house.

The existing roof is pitched at 4/12.

Here's a line drawing (only 4K). There is no roof depicted yet on the addition. (The arrow points to the back yard)

The best solution so far, avoiding any dead valleys and resultant leak problems, would have the roof of the addition at 90 degrees to the main roof, and therefore higher than the main roof (by ~ 2') like this:

I see that the addition roof now looks much more massive. Outside of some practical construction problems with the meeting area there right at the main peak, what other flaws are in this approach?

(The rafters would be about 22', if the base of the right triangle is

20+1' and the height is 7', I wonder if I'd need trusses...)

The thing that nags at me is that this basic problem of extending a bi-level/ranch must be encountered all the time - but I can't see a good, basic solution, unless the whole addition comes off at ninety degrees to the main house.

[Goedjn]

A 40' x 42' footprint is going to be hard to cover without looking massive, and it means that your rooms are going to be abnormally deep. The typical modern solution is, indeed, to stick the addition on at 90 degrees. There is, however, no particular reason that you couldn't do what you've drawn, with the addition being higher than the original roof. It makes a potential leak-point, but no more so than your average dormer.

You could also go with an asymmetric roof, just reducing the slope of the back roof to cover the extension, but if pitch is synonymous with slope (something I've never been sure of), then your roof is pretty flat already. A third option would be to extend the front-facing plane back another

7-8' so that it's centered.

Is it safe to assume that you don't have the space to build the addition at the end?

--Goedjn

[longshot]

In my opinion you should make the addition a smaller width to maintain the same peak height, this will add a vertical line on the sides & be more aesthetically pleasing, not to mention it won't have to be an exact match to the old siding the is probably faded. you lose a little sq footage but it will look cleaner. just my 2 cents.

Rob

[tbasc]

The front surface of the existing roof could be continued up & back to clip the new roof. The new roof could begin at the rear wall of the addition & form a clearstory at the existing ridge. The new roof could be run from rear eave to existing ridge and covered with single ply roofing. The Eave line of the addition could be set lower than the existing to give more rise. Send the fee to Goedjn

TB