Roof question

[supton]

So it looks like I might get some roof work done next summer. Don't know all the details yet but there is a chance that the sheathing will come off. If that does happen, is there value in removing the rafters so as to go from 2x8 rafters to whatever is in vogue now? I think they are 2x8's, certainly not any larger. The roof is part of a cathedral ceiling, so insulation is critical in order to keep a cold roof.

I forget what the contractor said when we spoke last spring; he might have talked about using spray in foam for insulation. I'm a bit leery of that, seems any time something new comes out for an idea of "the best way to do things" it gets contradicted a few years later. But I'm guessing foam would allow me to retain the existing 2x8's and get a good R value. But I'm not sure what cost and performance tradeoffs are.

FWIW the ridge is sagging so there will be a fair amount of jacking and reworking involved. I'm expecting all the sheetrock to have to be replaced regardless of what happens.

Just looking for feedback, want to know my options so I don't get pushed into a bad decision.

 

[emmett442]

I don't think upsizing rafters to gain R value will be cost effective at all. You'll never make the cost back through energy savings.

If the ridge is sagging, it's probably because the walls are spreading, pulling the ridge down. If so, the rafter ties (if it has them) were probably installed too high or the structural ridge was undersized and you'll basically need to reframe the whole thing to correct it.

 

[supton]

Originally Posted by emmett442
I don't think upsizing rafters to gain R value will be cost effective at all. You'll never make the cost back through energy savings.

If the ridge is sagging, it's probably because the walls are spreading, pulling the ridge down. If so, the rafter ties (if it has them) were probably installed too high or the structural ridge was undersized and you'll basically need to reframe the whole thing to correct it.


No rafter ties, can't be added (1.5 story house), but there should be a way around the sagging. Somehow. But you have the idea. If it needs to be reframed, in part or in whole, is removing 2x8's making sense or not?

The contractor had an idea that I think would do the bill. Basically: since the ridge is sagging because the wall is kicking out, if he pulls it all straight (pull and jack ridge), then puts down 1/2" plywood and screws it down, then overlaps again with 1/2" plywood and screws that down, then the roof itself becomes structural--the plywood won't let the roof move. [Right now, as it sags, the roof is moving in all sorts of directions. The plywood will resist movement in the x-y directions, and by doing so, should result in no z deflection either.]

 

[PimTac]

Two layers of plywood is a lot of weight. If the walls are kicking out then something is amiss in the general framing plan of the house. I wonder if some remodeling occurred in the past and a crucial wall or bearing wall was removed without compensating for it?

Edit: I would not put two layers of plywood up. It's totally unnecessary for structural strength.

 

[Wolf359]

Originally Posted by PimTac
Two layers of plywood is a lot of weight. If the walls are kicking out then something is amiss in the general framing plan of the house. I wonder if some remodeling occurred in the past and a crucial wall or bearing wall was removed without compensating for it?

Edit: I would not put two layers of plywood up. It's totally unnecessary for structural strength.


Ditto about the two layers. Saw lots of homes with sagging roofs. Basically it was the weight of the snow. A few years ago we had 100+ inches of snow in the area within a month. All that extra weight caused lots of roofs to sag that never sagged before. Home inspectors consider that a cosmetic issue. You could jack it up and fix it though as it might scare future buyers away, but no need to go too crazy.

 

[andyd]

The ceiling joists are your collar ties up stairs. Deleting them for a cathedral ceiling is a mistake.

 

[mattwithcats]

Use GAF DeckArmor instead of tarpaper under the shingles.

Cost is the same, $30 for 3 square of tarpaper, $100 for 10 square of DeckArmor..

Very tough, my cats been using the remains of a roll as a scratching post...
Not shredded yet...

https://www.gaf.com/en-us/roofing-p...oof-deck-protection/synthetic/deck-armor

 

[supton]

Originally Posted by andyd
The ceiling joists are your collar ties up stairs. Deleting them for a cathedral ceiling is a mistake.

It would never have had them. It was always a 1.5 story house. Balloon frame. 1st floor ceiling is 3/4 of way up studs, then in the upstairs the rafters come down and sit on top of the sill plate. Always a cathedral in the upstairs (otherwise it would have been just an attic, which it might have been, then got finished off, who knows).

It was just poorly built, eons ago. Its moving slowly, but considering all the sheet rock joints now need to be redone, and would only have to be redone again in another decade, it seems wise to fix.

Not sure why everyone is saying it's too much plywood. It's only 1" of plywood being thought of--instead of the nominal 3/4" that would be used. Right now it has 1" thick planking on it. I realize that plywood is heavier, but it can't be that much heavier than 1" thick planking. [And it's 1" thick as I can tell where a patch job used 3/4" plywood]. Hmm, quick look shows 600kg per cubic meter for plywood vs 450kg per cubic meter, so it's appreciable. If I'm doing the math properly, that turns into 28lb per cubic foot and 37lb per cubic foot. At 1" thick that is 2.3 vs 3 psf. Snow load is what, 20 to 40 psf?

 

[supton]

Originally Posted by mattwithcats
Use GAF DeckArmor instead of tarpaper under the shingles.

Cost is the same, $30 for 3 square of tarpaper, $100 for 10 square of DeckArmor..

Very tough, my cats been using the remains of a roll as a scratching post...
Not shredded yet...

https://www.gaf.com/en-us/roofing-p...oof-deck-protection/synthetic/deck-armor

Is DeckArmor different than snow and ice shield? I had that installed on the whole roof, and it's failed after 10 years (ice dams). Only last year did I read that it was a bad idea to do shield on the whole roof (traps moisture). This roof side just has bad ventilation, although by now who knows what is wrong up there. [It does have the small vents, and a ridge vent, but who knows what lurks in there.]

 

[PimTac]

What are the dimensions of the cathedral roofed area?

Not having collar ties may be the problem.

Also in reference to the thickness of the sheathing, the weight transfers all the way down to the footing. If there is a weak point anywhere along the building then that will present itself.

 

[Triple_Se7en]

People that insulate their rafters at the roofs always seem to eventually encounter sagging. Trapped air that sees sudden, drastic changes in temperature and humidity indexes cannot breathe and that makes wood bow.

Cathedral ceilings in homes are nice to look at. I cannot think of another reason why I'd want one thou.

 

[Wolf359]

Originally Posted by supton
Originally Posted by mattwithcats
Use GAF DeckArmor instead of tarpaper under the shingles.

Cost is the same, $30 for 3 square of tarpaper, $100 for 10 square of DeckArmor..

Very tough, my cats been using the remains of a roll as a scratching post...
Not shredded yet...

https://www.gaf.com/en-us/roofing-p...oof-deck-protection/synthetic/deck-armor

Is DeckArmor different than snow and ice shield? I had that installed on the whole roof, and it's failed after 10 years (ice dams). Only last year did I read that it was a bad idea to do shield on the whole roof (traps moisture). This roof side just has bad ventilation, although by now who knows what is wrong up there. [It does have the small vents, and a ridge vent, but who knows what lurks in there.]


It's two different things. In the past you used tar paper, either 15 or 30 pound, but deck armor replaces that. Then it's ice and water on top of that, normally you just use 6 feet of it from the edge. It's normally just 3 feet from the heated space, but because the edge isn't heated, it's easier to just do 6 feet.

Anyway with a lot of snow, there's sometime no other solution besides getting the snow off the roof which is why roof rakes are also recommended.

 

[supton]

Originally Posted by Wolf359
Anyway with a lot of snow, there's sometime no other solution besides getting the snow off the roof which is why roof rakes are also recommended.


Yeah, I'm just going to rake it this year. I got tired of that and had the roof replaced ten years ago, but since it's ice damming now, I don't have a choice. I think I'll build some scaffolding or something, as a rake to go up two stories is an unwieldy beast.

Originally Posted by Triple_Se7en
Cathedral ceilings in homes are nice to look at. I cannot think of another reason why I'd want one thou.


After having one I never want another. Unfortunately in this case I can't go back.

 

[emmett442]

For those mentioning collar ties, they don't come into play here. Collar ties aren't meant to resist outward movement at the low end of the rafters. Rather, they are to prevent separation up at the ridge.

After re-reading the OP, it's likely that the house in question should have been framed with the rafters strapped to a structural ridge BEAM, transferring roof loads through the end walls, essentially "hanging" the rafters from the ridge. I can only guess that either beam is too small, causing the sag, or a beam wasn't even used, pushing on the walls.

Either way, it'll have to be reframed.

I still don't see much of a benefit in up-sizing rafter size.

 

[PimTac]

Originally Posted by emmett442
For those mentioning collar ties, they don't come into play here. Collar ties aren't meant to resist outward movement at the low end of the rafters. Rather, they are to prevent separation up at the ridge.

After re-reading the OP, it's likely that the house in question should have been framed with the rafters strapped to a structural ridge BEAM, transferring roof loads through the end walls, essentially "hanging" the rafters from the ridge. I can only guess that either beam is too small, causing the sag, or a beam wasn't even used, pushing on the walls.

Thanks for that info. I used the wrong terminology.

 

[supton]

Originally Posted by emmett442
For those mentioning collar ties, they don't come into play here. Collar ties aren't meant to resist outward movement at the low end of the rafters. Rather, they are to prevent separation up at the ridge.

After re-reading the OP, it's likely that the house in question should have been framed with the rafters strapped to a structural ridge BEAM, transferring roof loads through the end walls, essentially "hanging" the rafters from the ridge. I can only guess that either beam is too small, causing the sag, or a beam wasn't even used, pushing on the walls.

Either way, it'll have to be reframed.

I still don't see much of a benefit in up-sizing rafter size.

Thank you, that answers my question.

Don't know what's under the ridge of the roof. Kinda doesn't matter--they did hips on the ends, so all the force goes onto the rafters; and with no rafter ties, all it takes is 70 years for gravity to do its toll. I think the hips are original so they aren't at fault. Honestly, I think the prior owner weakened the wall when he put in some awning windows; before it was probably moving, but more slowly, but with the loss of several studs it was just enough to accelerate the movement.

Maybe one of these days I'll open up the ceiling and see what lives up there for a ridge. With hips I suspect there is a board, but who knows what they did. It was built on a budget, that I can tell, and renovated the years went by.

 

[hemitom]

if you want to make your roof solid with no movement , you don't need two layers of plywood.
take construction adhesive apply it to the roof trusses then screw down the plywood , you will have one solid surface that will not move, but if you ever have to remove the plywood be prepared for a monumental task.
However if your walls are kicking out at the sill plate you have other issues.