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Default Roofing Question

On 2008-04-06, RicodJour wrote:
On Apr 6, 11:52 am, Wayne Whitney wrote:

If you are in a high wind or seismic area, you may wish to consider
going over all the T&G 1x with 1/2" plywood to improve the shear
resistance of the roof. Of course, the wall sheathing would be more
important to strengthen, but it is presumably not accessible.


At $45/sheet? You're pretty free with the guy's money. The house has
been standing for 50 years, so there's little benefit to be gained
from an extra layer of sheathing. I also don't like adding weight to
a roof unnecessarily.


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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On Apr 6, 2:35*pm, Wayne Whitney wrote:
On 2008-04-06, RicodJour wrote:

On Apr 6, 11:52 am, Wayne Whitney wrote:


If you are in a high wind or seismic area, you may wish to consider
going over all the T&G 1x with 1/2" plywood to improve the shear
resistance of the roof. *Of course, the wall sheathing would be more
important to strengthen, but it is presumably not accessible.


At $45/sheet? *You're pretty free with the guy's money. *The house has
been standing for 50 years, so there's little benefit to be gained
from an extra layer of sheathing. *I also don't like adding weight to
a roof unnecessarily.


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


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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Default Roofing Question

On 2008-04-07, Harry K wrote:

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.


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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Default Roofing Question

On Apr 7, 12:09 am, Wayne Whitney wrote:
On 2008-04-07, Harry K wrote:

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.


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.


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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Default Roofing Question

Wayne Whitney wrote:

On 2008-04-07, Harry K wrote:



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.



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


Looks like an answer from an old engineering text. If the freeking
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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Default Roofing Question

On 2008-04-07, RicodJour wrote:

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.


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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Default Roofing Question

On 2008-04-07, RicodJour wrote:

On Apr 7, 12:09 am, Wayne Whitney wrote:

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.


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.


Right, I was referring to the whole system deforming, so that "each"
rectangle deforms, together.

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.


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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Default Roofing Question

On Apr 7, 11:42*am, Wayne Whitney wrote:
On 2008-04-07, RicodJour wrote:

On Apr 7, 12:09 am, Wayne Whitney wrote:


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. *


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. *


Right, I was referring to the whole system deforming, so that "each"
rectangle deforms, together.

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.


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


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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On Apr 7, 11:42 am, Wayne Whitney wrote:
On 2008-04-07, RicodJour wrote:

On Apr 7, 12:09 am, Wayne Whitney wrote:


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.


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.


Right, I was referring to the whole system deforming, so that "each"
rectangle deforms, together.


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.

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.


The longer the ridge is, the lower the resistance to lateral forces
will be.


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?

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.


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.

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.


IFF (if and only if) that is the case, the hurricane clips should be
the first thing installed.

R
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On 2008-04-07, RicodJour wrote:

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.


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.

IFF (if and only if) that is the case, the hurricane clips should be
the first thing installed.


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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Default Roofing Question

Harry K wrote:
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.


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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