Dormer on existing house

[JStructsteel]

I have a client who wants to add a full length dormer out the back of his house. Normal gable roof, 6:12 pitch. Right now its rafters with ridge board, collar ties. Plan was to add a ridge beam, support each end at the gables. Just wanted to see if there is a better way, I cant see not having a ridge beam.

Thanks

 

[dik]

I'm not really a wood guy, but I'm pretty sure it requires a ridge beam... is this dormer what they often call a 'shed dormer'?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JStructsteel]

Dik, yest a shed dormer off the back of the house. I agree it needs a beam. Just wanted to see if any other options for the roof

 

[dik]

Not easy ones that I'm aware of.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

Does it look like this?

Does this help?

BA

 

[dhengr]

JStructsteel:
Unlike the typical symmetrical (both sides) gable framed roof, where the horiz. rafter reactions are essentially equal and counteract each other at the ridge board; with your new roof, you have to pay particular attention to the different/unequal horiz. rafter forces, as well as their gravity load reactions.

 

[JStructsteel]

Baretired, thought was to goto the end of the gable each end, that way a post could be put in the end wall.

dhengr, good point. couldnt the dormer roof act as a brace/deep beam to spread the load out? Even a normal ridge beam design I would suspect out of plane loads, that for the most part are ignored.

thanks

 

[BAretired]

In the sketch below, the cross section is shown as a line diagram from a to f. Member de occurs only at the ends.

The triangle in red represents a vector diagram of forces acting at the ridge. Force AB is the gravity force at point c, which would normally be carried by a ridge beam. Without a ridge beam, AB is resisted by diaphragm forces BC and CA.

In the existing structure, members ae (the floor joists) act as a horizontal tie to resist the horizontal component of the rafters, as mentioned by dhengr. If member de is removed throughout most of the length of the ridge, member ae at each end must carry the horizontal reaction of the two sloping diaphragms.

dhengr, good point. couldnt the dormer roof act as a brace/deep beam to spread the load out?

Yes, but the deep beam reactions would need a tie member at each end to provide a horizontal reaction at each end of the building. And Diaphragm abc was not built to act as a diaphragm, so it may need strengthening.

A new ridge beam bearing on a post at each end is probably the simplest solution.

BA

 

[JStructsteel]

Thanks for everyones input. A ridge beam design is in the works.