Roofing question

I am extending the length of my back porch and therefore the roof over it and am thinking about the roofing(asphalt shingles).
I figure I can tuck the last new row of shingles up under a row of the existing house shingles. But I'm wondering about a good way to weave the new rows of shingles on the new part of the porch together with the rows of shingles on the old part of the porch.
Does anyone know a good technique or a good reference book that might cover it?
Thanks.
RonT
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Two problems- doubled starter course, and nails.
Normal practice is to double the first course for consistent thickness, avoiding gaps in coverage, whatever. You're talking adding a course under that.
The existing shingles are nailed down, making it difficult :') to slip shingles underneath.
Dunno what you mean by "weaving"- is there a valley?
New courses you're talking will start from bottom (I hope), and you're hoping that x courses will sync with the existing. Maybe.
Do you want this to look good, or like an obvious cheapie job? The old shingles will _not_ match the new.
Not being a pro, nor Holiday Inn Express resident, I can see a real mess, and redo, in the offing. IOW, I'd tear the old off.
HTH, J
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Hey, events that test the lateral force resisting system are typically high magnitude low probabilty events like earthquakes or hurricanes. So just because it has been standing for 50 years doesn't mean that it isn't worth strengthening _if_ the hazard is high. Hence my preface "if you are in a high wind or seismic area."
Yours, Wayne
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Well, if the winds and sheer force are great enough to rip off T&G, a bit of ply over it ain't gonna stop it.
Harry K
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It's not a matter of ripping off the T&G boards, it's a matter of deforming the framing so that each rectangle formed by two joists and two of the T&G boards becomes a non-rectangular quadrilateral. This only requires that the T&G boards slide relative to each other. Plywood would resist this deformation much more than the individual boards do.
Yours, Wayne
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It's safe to assume that the guy's framing DD's house didn't use 32" long scrap to sheath the roof, so it's not just two _rafters_ involved, probably more like five or six or more. At the forces required to make a, what?, 30' roof start to rack appreciably, the whole roof would come off in a piece. Other failure modes would be involved way before roof racking became an issue.
I understand your point about plywood adding to racking resistance, but there's no indication in this instance - even if in a high wind area - that the roof would benefit in any significant way from the additional layer of sheathing. On the other hand DD's wallet would be significantly lighter.
R
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I'm not proposing that the plywood should or shouldn't be added, or saying that it is or isn't worth doing. I'm mainly pointing out that reroofing provides an opportunity to improve the racking resistance of the roof structure, and that standards for roof construction have changed since the house was built in the 1950's. So I think it is worth checking what the current design loads are for that area, and then evaluating whether the strengthening is worth the cost. You are probably right that it won't be worth it, but neither of us can say that definitely without knowing the design seismic and wind loads for the OP's location.
Yours, Wayne
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Right, I was referring to the whole system deforming, so that "each" rectangle deforms, together.

The longer the ridge is, the lower the resistance to lateral forces will be. As for uplift, that's a great point--increase the racking resistance enough and you can be sure that the uplift failure mode will dominate. So _if_ the OP is in a high wind or seismic area, and _if_ the OP wishes to take the opportunity to strengthen the roof structure, then it would also make sense to add hurricane clips to resist uplift.
Yours, Wayne
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OP here -
On the rare chance that a hurricane or earthquake hits western NY, either one of them can have my house.
Thanks for some interesting reading, but I won't be paying anything extra for hurricane clips or rack-prevention plywood.
Heck, I don't even want to pay for new shingles! <g>
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Which doesn't happen. Deformation happens locally and then spreads when the initial resistance is overcome. By the time the whole roof is involved you've got one hell of a big and airy skylight.

Maybe you should review that sentence. How can the resistance be lower with a longer ridge? More rafters are involved, more points of attachment, more moments resisting racking. It can't be lower. If you meant proportionally lower, well, I don't buy that either as the roof is probably of a piece, but for argument's sake, exactly how short/narrow of a house do you think DD lives in?

You can always reinforce something to make something else the weak point. That's not the issue. The issue is whether it's money well spent and whether it's a real risk.

IFF (if and only if) that is the case, the hurricane clips should be the first thing installed.
R
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Engineering practice is to treat conventional roof rafter and ceiling joist construction as if it were a simple flat diaphragm (joists plus sheathing), as far as calculating its lateral load resistance. That is certainly easier to visualize and analyze.
For lateral force applied in a given direction, the only part of the structure that can carry that force to the foundation are the walls parallel to the force. So the diaphragm has to carry the lateral force to the end walls parallel to the force. The diaphragm is modelled as a deep beam spanning between those end walls. The longer the distance between those walls, the greater the span of this "beam", and the less the resistance.

Absent an analysis of a conventionally framed roof with T&G board sheathing, that seems quite reasonable, that the uplift capacity is the weakest link. Uplift on conventional roof framing puts both the sheathing-rafter nailing and the rafter-plate toe nailing in withdrawal, so that probably won't work very well. :-)
Yours, Wayne
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Wayne Whitney wrote:

rectangles are deforming, something else is happening to the house under it. Hurricanes take the roof OFF, and take them apart after they are off.
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That's not the concern. The concern is the house and or roof twisting in a high wind or quake and buckling, because it is not stiff enough. There are two popular methods of making a house stiffer without adding significant mass. One is to sheath with barn board or similar on a 45 degree diagonal. The other is to sheath with plywood or OSB.
    Una
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