STRUCTURAL FOR DORMER ADDITION

[Supun93]

I'm designing a dormer addition for a existing house. The existing roof has 2x4@24" spacing with a ridge board. Dormer Roof has to be 5/12 slope and the roof rafters will be removed where dormer comes.it is starting near to the ridge board. I suppose i need to add a ridge beam to support the additional loadings or is there a way to avoid using a ridge beam? It is safer to have a new ridge beam to support it but I need to know if it is possible to do it without a ridge beam.

Dormer is 12' long. House width is 29' and dormer is going to be 14' long. Thanks

 

[phamENG]

First: decide how long your dormer is. 12' or 14'. Either way, though, that's more than a 'typical dormer'. Guessing it's a shed dormer. That'll need a ridge beam. There are other ways to analyze it, but they're tricky and the connections are very critical. Ridge beam is easier to design and generally a more robust option in place. Best to do a ridge beam.

2x4 @ 24"? Wow. Hope it's a small roof...

 

[DoubleStud]

I have a feeling it is a small dormer so you need the ceiling to be as high as possible. So you will need a ridge beam. You can use collar tie or ceiling joists and get away not using a ridge beam but your ceiling will be lower.

 

[gte447f]

Like DoubleStud said, if you have rafter ties (e.g., ceiling joists) at the new dormer, then you shouldn't need a ridge beam at either the new dormer or the existing roof. However, if the new dormer has a vaulted/cathedral/high ceiling, then you will probably need a ridge beam at the new dormer and at the existing roof.

 

[HouseBoy]

Not sure I agree with others here. Ceiling joists MIGHT be helpful (for avoiding a structural ridge) BUT... The connection from the the ceiling joist to the new rafter AND the old rafter will likely be a high tension force. If the existing rafters are 2x4 @ 24" o.c. then connecting the ceiling joists back to the existing rafters will be tough. MY guess is additional reinforcing to the existing rafters would be needed although it might be hard to fit the needed member in the limited space available....

Regarding the ridge beam (IF one is needed) - Designing the ridge beam may be the easy part. Tracking the load down to adequate support might be a bit tougher.

How old is the house?
Can you post a section of the existing and new?

 

[gte447f]

Houseboy, I don't quite follow what you are saying about attaching new ceiling joists back to existing rafters, but regardless, I agree that 2x4 @ 24" existing rafters are probably a problem, if for no other reason than the the existing roof span (29 ft roof width divided by 2 = 14.5 ft rafter span), unless there are intermediate purlins possibly supporting the existing rafters.

 

[HouseBoy]

gte - I'm just guessing what the existing building section looks like such that they want a shed roof dormer.
Usually when I get a request for something that sounds like what the OP has, it is a shed dormer on a cape code style house (built in the 1940's or 50's).
The attached sketch is what I'm referring to.
A sketch from OP would help.....

 

[gte447f]

HouseBoy, OK, I understand what you meant now. I was picturing a gabled dormer. Yes, a sketch from the OP would help.