help with ridge board / beam design

[zarch]

i have a gable roof on my house. if i added a gable dormer to the attic to align with the existing ridge, do ineed to reinforce the existing ridge beam? and, what if i want to have structural valleys for the dormer addition? does that effect the existing ridge?

 

[Jpurr]

What is the current construction? Is there a bearing ridge beam or are there collar ties?? The load is the same adding just few lbs for the knee walls (if I am understanding your question correctly) but if you have a non bearing ridge beam with collar ties, you should not break the collar ties as this will load the ridge beam.

I know the answer is not very clear, but it really depends on the exact current roof load path.

 

[bylar]

Are you sure the ridge is a ridge beam?

When you have opposing rafters and the horizontal loads taken by the ceiling joists the ridge is not a beam. the collar ties are not there to take the horizontal loads do to gravity loads. There is a great deal of question as to the value of collar ties but I belive the general opinion is that they are primaarily used to prevent a spreading of the rafters due to wind loads. if you check the design of rafters without ceiling joists and without a structural ridge beam you will find they are overstressed in bending at the collar ties.

That said, after you have verified the the ridge is actuallity a ridge beam then you must calculate any additional loads to the beam in the manner you construct the dormer and determine if the beam needs to be strenghtened.

 

[CSEllc]

Without getting to much into specifics since I don't know yours.....If you are adding a small dormer....or several small dormers over the entire length of the roof you usually can get away with designing the ridge board as a ridge beam.

If the dormer(s) are large....you will need to reinforce the ridte board to have it behave as a ridge beam.

The best thing to do is hire a local structural engineer to evaluate you condition.