Brick Structure - Converting Roof to Structural Ridge

[scootermcrad]

Hey guys! Been awhile since i've posted. Miss this place!

I have sort of an interesting problem that I seem to be having a hard time finding any answers on. Hopefully there are some roof framers here that can help with some answers.

I'm planning an addition to an existing garage and a portion of our home. Part of this addition is re-framing an existing flat roof as a structural ridge gable roof (no ties) with a 12-12 pitch. The building structure is FULL BRICK on three sides, and one side stick framed. The roof is just a flat roof (with a very slight pitch built in) so there's not much of a side load to consider with a flat roof. The footprint of the structure is 24 x 14 (ish) to the outside of the structure.

For the new roof structure, I plan on using a 24 foot Glulam beam supported half way truss (longest span being 12 feet) sitting on two brick wing walls. Rafters will sit on top of the laminated ridge beam (beam will be exposed). My problem comes in at the top of the brick walls itself. What do I need to do for a top plate on the perimeter walls? The ridge beam supports the rafters and roof and thus decreases the side load on the walls themselves as do the end walls, but what should I be considering here? What is a typical approach in brick construction for a tall gable roof? Is this as simple as lagging a 2x6 top plate to the top of the brick structure and framing the rafters with a standard birdmouth as usual? Double top plate the same as any other standard exterior 2X4 or 2x6 constructed wall?

Hope that's all clear. Any info would be appreciated! Just never dealt with a full brick constructed wall before.

Thanks guys!
S

EDIT: Here's a picture of a basic structural ridge roof layout for reference (found in a google search). Just replace the stick constructed walls with full brick constructed walls.

 

[Chris705]

Scooter, to summarize your post...you are going to support the ends of the structural ridge with wood columns that will bear on the brick walls below? You will frame in the gable ends with studs & rest the rafter ends on the existing (solid, not veneer) brick walls with out raising any of the brick walls? If I translated what you wrote correctly then I would recommend using pressure treated sills where wood framing members come down to meet the brick. A single plate would be adequate, you are not trying to tie the walls together as you would in wood framed walls. I would use tapcon screws to secure the plates or if possible 1/2" threaded rod to get down three or more course of brick using epoxy to secure. Your roof will be high and subject to lots of wind loading and tying it down has me concerned. It sounds like you have the structural ridge figured out size wise? Your ridge will have a center column splitting the 24' length roughly in half? Will that column be braced with a wall or tie beam of some sort? That will be a very long column and un-braced would need to be of a pretty good size.

 

[TommyK]

Need more info on exactly how the existing brick walls are constructed but if they are un-reinforced I would not do what you are contemplating without having an Engineer look at it.

 

[dutchgray]

That above, if single skin of brick without pier's could be dodgy.
I would use 2x4 pt plate on walls, screwed down, and strapped with some metal fixed on the inner face of the brick for about 3'.
Birdsmouth on plate and ridge. Usually tie with joists across wall plates otherwise roof will push walls in and some ties up near the top.
Personally I would build the gables up in brick and tie to the roof with some metal straps, but there is no reason you can't do it in timber.
I would do the ridge so it didn't need a post, if that's means a steel so be it.
Up spec the wood, its the cheapest part of the job.
This is what I would be doing this side of the pond, 12 years in construction, nearly all masonry built and I have never used a truss.

 

[Charles (in GA)]

I seen a few old buildings of solid brick blow out the sides as the roof overstressed the walls, due to collar tie failures (building abandoned and roof leaking, rotted, etc.) I cannot imagine having a roof like that without collar ties.

Charles

 

[Chris705]

A ridge beam doesn't allow for outward force, hence no need for collar ties. All the dead load forces are downwards. Original roof load was on the walls so he knows they will support safely. Wind loads will be new.

 

[scootermcrad]

Some really great responses here! Thank you everyone!!!

Scooter, to summarize your post...you are going to support the ends of the structural ridge with wood columns that will bear on the brick walls below? You will frame in the gable ends with studs & rest the rafter ends on the existing (solid, not veneer) brick walls with out raising any of the brick walls? If I translated what you wrote correctly then I would recommend using pressure treated sills where wood framing members come down to meet the brick. A single plate would be adequate, you are not trying to tie the walls together as you would in wood framed walls. I would use tapcon screws to secure the plates or if possible 1/2" threaded rod to get down three or more course of brick using epoxy to secure. Your roof will be high and subject to lots of wind loading and tying it down has me concerned. It sounds like you have the structural ridge figured out size wise? Your ridge will have a center column splitting the 24' length roughly in half? Will that column be braced with a wall or tie beam of some sort? That will be a very long column and un-braced would need to be of a pretty good size.

Basically, yes. Correct.

It's a little more complicated than this, but when you simplify the loading situation, that is correct. The structure is full brick, two passes (whatever that's called in masonry terms), one end open and will be stick framed. This end will tie the walls together. The opposite end is MOSTLY brick. It's attached to the house which is stick framed with brick veneer. So YES. The ridge beam will be supported at either end by a post. The load on the "closed" end post will actually be tied into the existing adjoining house structure.

The reason for being supported half way, is probably more for aesthetic reason than anything else. There are two "wing walls" at the half way point. No idea why they are there, but they help support the walls and are part of the brick structure. Once the flat roof is gone, there will essentially be a structural ledge on top. With a full cathedral ceiling it will look strange, so the plan was to frame in a nice arched truss that will sit on top that will bring some character to the space as well as offer support for the ridge beam. The ridge beam will be slightly oversized to share the load, but I figured if there was going to be something there, it may as well help support the ridge as well as offer mid-lateral support to the walls.

A ridge beam doesn't allow for outward force, hence no need for collar ties. All the dead load forces are downwards. Original roof load was on the walls so he knows they will support safely. Wind loads will be new.

Also correct, and thus the reason for going with a structural ridge.

I have zero doubts about the walls holding the loads and I'm capable of doing the calculations here, but being that brick structures are generally out of my wheel house, I just needed to know what the best practices are for top plate treatments.

Sounds like treated top plates fastened accordingly is the answer? Double them up, or no?

 

[scootermcrad]

This is similar in construction, just using a different style ridge beam and obviously not brick walls. Nothing new here. This type of construction has been done forever.

 

[scootermcrad]

OH! I meant to share this also. This was the only top plate detail I could find for a brick structure. It basically backs up what's been mentioned here, but can't seem to find any details beyond this. Especially more recent construction standard details.

 

[matt_i]

The battle of evermore is to tie dissimilar materials together, strongly, and without encouraging rot or corrosion.

Ideally you'd have something that could tie several courses deep. Metal rods or metal flat strapping come to mind, but what is the condition of the bricks themselves? Hollow or solid, or too early to tell...

Most "modern" brick seems to be veneer, so its not really structural, there's a timber framed wall right behind it to do the actual heavy lifting and then its wood-to-wood connections.

 

[scootermcrad]

The battle of evermore is to tie dissimilar materials together, strongly, and without encouraging rot or corrosion.

Ideally you'd have something that could tie several courses deep. Metal rods or metal flat strapping come to mind, but what is the condition of the bricks themselves? Hollow or solid, or too early to tell...

Most "modern" brick seems to be veneer, so its not really structural, there's a timber framed wall right behind it to do the actual heavy lifting and then its wood-to-wood connections.

It's too early to tell, at this point. If I had to guess, I would say there are 3 holes in each brick, but they may be solid. I won't know until the roof is off. The condition of the bricks and (and morter) themselves is fantastic.

There is no structure beyond what I can see. However, the interior will be studded out as if the walls were a veneer. 2x4 construction.

 

[Chris705]

Single treated plate sounds adequate to me.

 

[scootermcrad]

Single treated plate sounds adequate to me.

Awesome. Thank you for the input. I very much appreciate it!

With a little luck, someone already went through the effort of installing a top plate properly between the flat roof and the top of the wall and will be able to re-use it. I won't know until work begins. It's a strange building, so we'll see.

 

[kbs2244]

I am a big fan of ridge beams roofs and the vaulted interior they give you.
There is no need for top of the walls joists since any roof stress is straight down.
(This is the same as a truss type construction.)
Any rafter stress is in tension vs compression as in "normal" construction.
Doing the lower end of your joists as in your illustration will work fine.

 

[scootermcrad]

I am a big fan of ridge beams roofs and the vaulted interior they give you.
There is no need for top of the walls joists since any roof stress is straight down.
(This is the same as a truss type construction.)
Any rafter stress is in tension vs compression as in "normal" construction.
Doing the lower end of your joists as in your illustration will work fine.

Thanks kbs!

 

[pmiranda]

Dumb question, how are the rafters attached to the ridge beam? I imagine something like a joist hanger, but designed to prevent the rafters from pulling out at the bottom and pushing out the side walls, right?
That center support for the ridge beam will be nice peace of mind for the end walls, since the side walls will only be carrying about half of what they used to, all things being equal... which they aren't since I imagine the new roof is a different weight.

 

[scootermcrad]

Dumb question, how are the rafters attached to the ridge beam? I imagine something like a joist hanger, but designed to prevent the rafters from pulling out at the bottom and pushing out the side walls, right?
That center support for the ridge beam will be nice peace of mind for the end walls, since the side walls will only be carrying about half of what they used to, all things being equal... which they aren't since I imagine the new roof is a different weight.

Yeah, figure the side wall will carry about two-thirds the original weight of the current roof, since I believe the current flat roof to also have two layers of roofing material and oversized structurally already. I believe the current roof is built from 2x12 southern yellow pine and 3/4" plywood or even planks, based on other parts of the home. HEEEAAAVY!!

Rafters sit on top of the structural ridge and would use mending plates, or similar type of connection. I need to look at code to see if they will also require hurricane ties. I'm pretty sure they require them in my area.

I honestly need to look at the best method for securing the rafters to the ridge beam. On my list of things to figure out.