I need to temporarily shore a roof while I re-frame the ceiling. Rafters are 2×8 @ 12″, 6:12 slope, cement tile, single story house. There is a structural beam (2-2×12) that has 2×4 supports to the ridge about every 48″. I will be removing the 2-2×12 and replacing it with a 3×18 glulam (based on an engineers calculaitons). I plan to put in two rows of shoring, like a-frames, keeping the center area open to allow the beam change. How do I connect the shores to the rafters? Here’s a pic of the existing condition:
Do it right, or do it twice.
Replies
Was having a hard time picturing exactly what you were talking about until I opened your picture. Wish more people would include picks in their inquiries like that. It made your question a lot easier to understand.
I'm still not sure I understand what you are wanting to do with the two rows of a-frame shoring just yet though. Are you wanting to tear out the existing ceiling and carry the ridge load on the A-frame shores all the way to the floor or are you talking about creating temporary A-frame type trusses in the attic space to carry the load out to the walls?
As I look at this, the rafter connection at the top wall plates is where things are going to try and come apart first when you remove the existing bracing. The way that ridge load is going to want to succumb to gravity is to push down at the peak and out at the walls. If it can't move the connection at the walls, the next thing it will try to do is move that outward thrust up the rafters until it finds a vulnerable point that will succumb to the force and buckle upward. If you can find a way to counter the outward thrust at the walls and hold it down as it tries to buckle the rafters upward I believe the existing 2x8's at 12" o.c. will hold the dead roof load just fine.
Would it be possible to cable across the entire attic at your ceiling joist height and tension the rafters together low down near the walls to prevent them from spreading out and letting the ridge drop? You might be able to use the existing rafters as your temporary ridge shoring if you could counter their desire to spread apart at the base with the tensioning cables. In fact, If the connection between the joists and rafters can be made solid enough the existing ceiling joists would essentially do the same thing.
You might ask your engineer if he thinks a few lag bolts at the joist to rafter connection and some cross bracing between them at strategic points in the middle to resist any tenancy the rafters might have to buckle upward would be an adequate temporary measure. This would essentially render the rafters and joists as temporary roof trusses that could carry the dead load of the roof without a central support.
At 12" O.C. you may even have enough rafter strength that the cross bracing would not be required. I can't imagine the need for compensating for a large snow load in central Texas but that is the only reason I can think of for needing 2x8's @ 12" O.C. at that span. You have a pretty solid framed roof there Elcid.
Edit: never mind - just caught the concrete roof tile - there's your reason for the tighter than average spacing.
I'm not running any numbers on this, just thinking through the load transfer from the peak down. Again - your engineer is probably the guy I would ask about this first.
Kevin Halliburton
"I believe that architecture is a pragmatic art. To become art it must be built on a foundation of necessity." - I.M. Pei -
Edited 7/16/2003 3:05:49 AM ET by Wrecked Angle
i agree that you need to keep the rafters from spreading at the wall plates.
cabling it all up seems like a bit of a faff though what about using a number of holddown straps like this: http://www.screwfix.com/app/sfd/cat/pro.jsp?id=18170&ts=41673
they have a 5 tonne breaking strain and should be fiarly easy to fix.
i used one to gradually pull the sides of a collapsing shed together or time until it was more or less where it should be and then proped the ridge up.
just a thought
aleks
Elcid -
First question I'd have is why is the existing beam being changed?
Given that there's a good reason, ....
I'd first frame some collar ties across above the existing ones, the ones that appear to be resting on the existing beam. You could do this with truss framing hardware or get a nailing spec from your engineer. They'd be placed high enough to get the supports close the ridge and leave just enough room between the two rows (see below) to work to install the new beam.
Then I'd run a continuous member just under these new collar ties and place pole shores from there to solid bearing ... I don't mean the floor below, I mean onto the ground if that's possible. You've got a heavy tile roof dead load. Since you've got engineering expertise on this project, I'd suggest sketching out something like this and have him/her provide location for the supports, member sizes, et. al.
Again, I don't think I'd put that kind of load on the floor joists if that's what the floor framing below is. Then again, the floor might be strong enough to hold it ... If so, spread the load on the floor as well with a continuous plate of some kind.
Pole shores can be rented, at least around here in the Seattle area. They can be jacked up to put some pressure on the underside of the roof to take the load off the beam. Last thing you need is to have any sag in the existing structure when you remove the existing beam.
Dennis in Bellevue WA
[email protected]
The existing 2-2x12 ceiling beam, +/- 12 ft long, is being replaced with a 25 ft glulam so that all the interior walls and center column can be removed to enlarge/open up the kitchen-dining-bar into one large room. Currently there are three columns: one at each end (which will remain) and one in the middle...the middle on gets removed. And the flat ceiling gets changed to vaulted, so all the ceiling framing gets removed.
I'll talk with the engineer again. High collar ties might work, and they would provide working room below for getting the glulam in place. Cables or low collar ties would block lifting room for the glulam. Dennis, it's a concrete slab on grade, so I plan to drop temp posts onto the slab with a plate to spreads the load a little. If I put in a high collar tie, then I have a flat surface for the shoring...that solves my initial question. I couldn't figure out how to attach the shores to the sloping rafters so they would stay in place.
The deal I have with the engineer is that he will design the new framing, make a site visit after all the sheetrock is gone to verify conditions before starting work, and visit again when the framing is up before the sheetrock is replaced. I found him through two or three referrals from architects I know. Got lucky too...the job is 30 miles out of town, but he happens to live a mile away and commutes every day, so he'll be stoipping in on his way to work which cuts out the mileage & time costs.
Do it right, or do it twice.
Edited 7/16/2003 8:19:49 AM ET by ELCID72
ELCID,
Good photo -- that helps se the problem. Everything you've said makes sense.
Here's another way to look at it;
As I see it from my limited perspective (through a 19" monitor), the key issue here is not spreading the exterior walls. So given the constraints of the job, all you really need to do is hold the ridge in place for a little while as you do your work.
This is going to sound weird, but it worked, and a former employer of mine actually got an engineer's stamp on it.
We ran into a similar problem for different reasons.
We put an LVL beam up under one set of rafters, about a foot or so downslope from the ridge. The LVL had an 8/12 cant strip on top of it so it would sit plumb.
We then set collar ties above the LVL, locking the opposing set of rafters in a little better to the rafters supported by the LVL.
We then set three posts underneath the LVL, taking the weight off of the existing ridge.
All we did was make the weight of the ridge a little bit cantilevered onto one set of rafters.
It worked great. In your case, it only has to work for a few days.
As for floor load under the posts, if you are on a slab, no big deal, if you are on a joisted floor maybe you could just get under the house with some jacks?
Like I said, it's a weird idea, but it worked. And as you know from Murphy's Laws Of Combat Operations, "If it's a stupid idea, and it works, it ain't stupid."
Besides, you already have an engineer. <G>
DRC
ELCID,
Follow up from my comments this morning;
I should probably let the coffee sink in before I type. I should have written "the key issue is to not let the walls spread."
I got thinking today about that displaced structural ridge we put in.
We also put kickers from the bottom of the LVL to the opposing rafters to keep it from rolling.
DRC
That's not wierd or stupid -- at least I hope it's not, because it's very similar to what I'm doing. I have to replace 7 out of 8 rafters due to termite damage on the East side of the ridge, and code requires a 2x instead of a 1x for the ridge, so that gets replaced, too. I'm going to put a temp beam about a foot down on the good (West) side to hold it up. Fortunately there isn't a whole lot of weight involved. The East side was tile, which I've torn off, and the West is just roll roofed.
-- J.S.
You're right that you didn't see me complaining about his fee. Two reasons: he's doing something I can't and it covers my big butt, and I don't like people trying to negotiate my prices so I felt that he priced it at what he thought was reasonable. Ya know, we're not talking about a simple cooktop installation here :)
BTW under additional services, the hourly rate for a principal (and he's the owner) is $125/hr.
Do it right, or do it twice.
Hi Elcid ....
Understand the reason for removing the beam now, thanks for the explanation.
Vaulted ceiling? .... At what elevation will the new ceiling be placed, then? Would collar ties be in the way of placing it at a higher elevation or will it be exposed below the new ceiling?
Whew....! Makes my little remodel project look rather puny by comparison (grin)
...........
Dennis in Bellevue WA
[email protected]
What you see in the pic is a view from the garage attic looking over the kitchen with an existing vaulted ceiling, and the dining room with a flat ceiling. The DR ceiling isn't visible because it's hidden behinbd and below the Kit ceiling. Since it will all be one room with one continuous ceiling plane, and since the existing 2x12's have to come out, I plan to demo the entire ceiling of both rooms, re-frame to match the vaulted profile you see in the pic, then apply new sheetrock and finish it as one new ceiling. That way I have plenty of access for the beam change, and all the sheetrock will be installed and finished at the same time, which should look better than trying to get the two halves to line up.
BTW anyone want to guess what the engr is charging for this? Here's the scope per his agreement: "Provide structural design and consultation for a new roof support beam at the interior renovations and header beam at the exterior walls. This is to also include two site visits during the renovation. Included in this proposal is structural design and consulattion, two site observations, misc wood framing design, roof beam design at interior renovation, header beam at exterior wall." Sounds like he said the same thing twice, doesn't it? The exterior wall header beam design refers to the part of the job where I will be installing a 9 ft wide patio door unit into an existing exterior wall.
Do it right, or do it twice.
Something like that would be about $350 for my engineer.
I think, personally, you're going about this the right and proper way - gut everything down to where you can re-build and get it right. Makes for one helluva lot of work but you'll have a nicer end result.
Do the joists/collar ties that show in the picture remain and the new beam gets installed under them in the same location as the existing 2x's? There is some concern about the thrust of the rafters against the exterior walls, no doubt. If these existing members are doing their job resisting this lateral force of the roof framing then what I suggested should work fairly well. The easiest way to explore this situation is to make a sketch of the roof structure and examine the forces at play here. If the lateral forces are being adequately resolved by those existing ceiling joists or collar ties *and* you can get adequate connections to the temporary collar ties under the shoring posts, I think it would work.
But as with all things, remember that advice is worth what you pay for it (grin). All I've seen is a picture of your framing situation so ..... Standard disclaimers apply! (hehe)
...........
Dennis in Bellevue WA
[email protected]
I don't plan on taking out anything that isn't in the way, so the higher collar ties will stay...partly because they're not in the way and partly because they are helping the structure...and every little bit helps.
I'm not too good at examine the forces at play here so that's why I'm buying the engineer's time and knowledge. My examination shows that the roof is still above the top plates, and that's all I can tell. I need to talk with Ralph's engineer...this is costing me $750.
Do it right, or do it twice.
Edited 7/17/2003 8:08:16 AM ET by ELCID72
@ $75/hour that's only 10 hours of time for your engineer. I don't consider that totally unreasonable considering he's paying E&O insurance, office overhead, etc., etc. It's his neck on the chopping block if anything goes wrong. Remember, .... you get what you pay for.
Good luck with your project. Sounds like you've got quite a job on your hands!
...........
Dennis in Bellevue WA
[email protected]
I guessed between $80 and $100/hr for your engineer with a couple/three days worth of work but I was speculating that he was going to be drawing up a set of plans for the "miscellanious wood framing" in there as well.
Pretty uninformed guess but I wouldn't hesitate to spend the money you're spending to get that load calculation done right. Think what it might cost if it wasn't done right.
I didn't hear you complaining and he's doing the job so it sounds like you've reasoned that one out already yourself. I would be very interested in your engineer's take on all of our ideas if you end up running them by him.
It's been a good head scratching exercise. Thanks, the brain needed it. :-)>Kevin Halliburton
"I believe that architecture is a pragmatic art. To become art it must be built on a foundation of necessity." - I.M. Pei -