Shed Dormer on House w/ Exist Gable Dormer

[Matt PE]

I have a client that wants to put a shed dormer on the rear of an existing 2.5 story brick house to convert the attic into living space. Dormer is to run the entire length of the building (21 feet). Plan is to cut out the rear of the roof, anchor a sill to the existing brick, install a 2x6 wall ~ 5 ft tall atop the brick, and then add the new 2x12 rafters at 3:12 slope.

There will be a ceiling joist, but I'm not convinced it is low enough to be effective. I would prefer a structural ridge here. Drop in an LVL ridge beam; everyone sleeps at night. Client is OK with this and actually prefers a structural ridge for the same reason.

Here's the twist:

There is an existing gable dormer on the front of the house.

In my opinion, the existing valley rafters and cross ridge are structural members, i.e. the reactions need full support. That brings a point load in dead center of my proposed structural ridge.

I can get a reasonable size LVL to take the point load if the rest of the roof around it is "conventionally" framed. (Existing ridge is a rough cut timber; 100 year old house.)

If there were no point load, I can get a reasonable size LVL as a structural ridge to carry the uniform load coming from the front portion of the roof, which I now think I need to carry.

The point load on top of the uniform load kills the beam and pushes me to a very deep LVL or even steel, which is not feasible here.

Thoughts?

Has any one run into this situation?

 

[jhnblgr]

What’s the plan view look like?

 

[Ron247]

I never like answering something like this without a fairly clear idea of what is proposed. So here it goes.

Is the picture the concept of the final product or the existing condition?

anchor a sill to the existing brick, install a 2x6 wall ~ 5 ft tall atop the brick, and then add the new 2x12 rafters at 3:12 slope.

How are you going to "anchor" to a brick wall? How is the brick wall built? Plan view of existing framing?

In my opinion, the existing valley rafters and cross ridge are structural members, i.e. the reactions need full support.

The valley rafter is generally load bearing. I call the valley rafter the single beam that runs on a 45 degree angle that the valley jack rafters attach to. What are you calling a "cross ridge". Do you mean the ridge board for the front dormer? If so, it is a point load that appears to need support by the main ridge area. Typically, that would be a vertical stiffleg for support.

I can get a reasonable size LVL to take the point load if the rest of the roof around it is "conventionally" framed. (Existing ridge is a rough cut timber; 100 year old house.)

How big is this rough cut ridge? Is it a ridge beam or ridge board?

Has any one run into this situation?

We all have clients that want a $10 modification for a $100 headache.

 

[Matt PE]

What’s the plan view look like?

It is just a box. 33'-2" in the direction perpendicular to the ridge and 21 ft wide int he direction in and out of the page of the original sketch.

 

[Matt PE]

I never like answering something like this without a fairly clear idea of what is proposed. So here it goes.

Is the picture the concept of the final product or the existing condition?

Sorry. Sketch is proposed. Right side shows the proposed shed dormer with raised section. Left side shows the existing roof with the existing gable dormer coming in at midspan.

How are you going to "anchor" to a brick wall? How is the brick wall built? Plan view of existing framing?

Yes, I hate brick. This detail needs attention for the uplift. Still working on seeing if the roof is feasible.

The valley rafter is generally load bearing. I call the valley rafter the single beam that runs on a 45 degree angle that the valley jack rafters attach to. What are you calling a "cross ridge". Do you mean the ridge board for the front dormer? If so, it is a point load that appears to need support by the main ridge area. Typically, that would be a vertical stiffleg for support.

Yes, we are using the same terminology for the valley rafter. I'm calling the cross ridge the ridge beam for the front dormer. I agree that is applying a point load to the proposed ridge beam. Client does not want a column. Besides, nowhere to land it.

How big is this rough cut ridge? Is it a ridge beam or ridge board?

3"x8" It was probably intended as a ridge board, but it is actually working as a ridge beam to carry the point loads from the front dormer. I'm always amazed that these old houses are still standing.

We all have clients that want a $10 modification for a $100 headache.

 

[Ron247]

If the existing hip rafters or the hip jack rafters have vertical supports for them, they are most likely holding up the main ridge where the front dormer ridge frames into it. If they do not, then I am surprised that everything is currently intact unless you are not in an area with a considerable snow load. I have seen some really bad framing practices survive in Roof Live only areas.

What do you intend for the ceiling joist at 7'-9" to frame into on the left side if that is typical front dormer framing?

Is the brick wall you mention a multi-wythe wall or is it a veneer?

 

[jhnblgr]

What depth of LVL can you get to work and why isn’t it feasible? Seems like you have the head room if you plan on having that ceiling joist. Could also look into glulam beams. How did you figure the loads for the beam, any possibility you didn’t accidentally double count a load?

What if you add a dormer to the other side and run the beam right through giving different loads paths. Likely a terrible idea.

 

[Matt PE]

What depth of LVL can you get to work and why isn’t it feasible? Seems like you have the head room if you plan on having that ceiling joist. Could also look into glulam beams. How did you figure the loads for the beam, any possibility you didn’t accidentally double count a load?

What if you add a dormer to the other side and run the beam right through giving different loads paths. Likely a terrible idea.

I can get a quad 18" to work.

This needs to be lifted into place by hand; can't get a crane in. That's about 700 pounds of engineered lumber.

 

[Matt PE]

If the existing hip rafters or the hip jack rafters have vertical supports for them, they are most likely holding up the main ridge where the front dormer ridge frames into it. If they do not, then I am surprised that everything is currently intact unless you are not in an area with a considerable snow load. I have seen some really bad framing practices survive in Roof Live only areas.

What do you intend for the ceiling joist at 7'-9" to frame into on the left side if that is typical front dormer framing?

Is the brick wall you mention a multi-wythe wall or is it a veneer?

The existing hip rafters do not have vertical supports. We are in Upstate NY, so we get snow once in awhile This is the second attic I've been in in this area with valley rafters framed this way, and I am honestly not sure why this hasn't collapsed. Century old buildings, both of them.

I will need to run some member across at ceiling elevation there (in and out of the page) to collect the ceiling joists in the gable dormer area. That will be a difficult connection, all the more reason why I want a structural ridge to carry these loads to the walls. I don't think I can 100% count on conventional roof action in this area.

Multi-wythe brick wall.

 

[gte447f]

The existing hip rafters do not have vertical supports. We are in Upstate NY, so we get snow once in awhile This is the second attic I've been in in this area with valley rafters framed this way, and I am honestly not sure why this hasn't collapsed. Century old buildings, both of them.

Well, I assume in the current configuration (i.e., before making the renovation you are currently considering) the point load at the ridge from the front dormer is resisted (i.e., "propped up") by the conventional rafters on the back slope of the roof which I assume are tied at the tails by ceiling joists.

 

[XR250]

I can get a quad 18" to work.

This needs to be lifted into place by hand; can't get a crane in. That's about 700 pounds of engineered lumber.

What about (3)20" LVL or (2)24"? Who cares what it weighs. They can put it up in pieces. Done all the time.
I usually put the new ridge under the existing ridge or on top of the ceiling joists. Hang the ceiling joists and then use a pony wall on top to support the existing ridge.

 

[Ron247]

I am trying to picture the original load paths since you will be altering that by any modification.
Does the right side currently have a stiffleg near the brick wall like the left side? Is the stiffleg isolated to the hip rafter or is it part of a kneewall?
What do you know about the floor framing under the stiffleg? Is there a load carrying beam there?
I assume the right side has rafter framing on the same pitch as the left side.
Is there a ceiling joist or tie at the top of the brick walls or at the top of the stifflegs currently?

Thought: Leave the original ridge and install 2 beams (or more) offset a little from the ridge. Cross fasten them to each other laterally. Install them as LVLs so you can bring in the layers and attach them together in place? I am trying to not cut the building in half to replace the ridge configuration and keep the point load mags down. I am scared of those "highly ductile" 100 year old brick walls.

 

[jhnblgr]

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[Eng16080]

I'd probably use a new ridge beam as others have suggested, either LVL or steel.

However, I think it's possible that the gable dormer can be used to resist the outward thrust at the left end of the roof rafters. It would need to be detailed such that there's a valid load path, but I think it's logical overall. If you think about it, for the roof to spread, it would need to slide over the gable dormer or push it over. That seems unlikely (even without any real detailing).

 

[Ron247]

Based on the age, I assume the roof decking is plank. Is that correct? If so, how close are the planks to each other?

 

[XR250]

I'd probably use a new ridge beam as others have suggested, either LVL or steel.

However, I think it's possible that the gable dormer can be used to resist the outward thrust at the left end of the roof rafters. It would need to be detailed such that there's a valid load path, but I think it's logical overall. If you think about it, for the roof to spread, it would need to slide over the gable dormer or push it over. That seems unlikely (even without any real detailing).
View attachment 14808

Well, the Devil is in the Details. Not sure how to justify this even though, in reality, it would likely work. Even if you could get it to detail out, they would be too complex for the avergae framer so you might as well go the safe route and install the ridge.