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Home > Archive > Building and Construction > May 2006 > Hip Roof Framing/Engineering
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Hip Roof Framing/Engineering
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| TVeblen 2006-05-15, 5:21 pm |
| A few times now I have been in new framed houses with hip roofs that have
double LVL hip rafters. The last one had 2- 14" LVL's! I've been a framer
(many moons ago) and we always used a single 2x for hip rafters, provided
the jack rafters laid face to face on the hip and the walls were tied
together by ceiling joists. The Mass building code states: "At all valleys
and hips there shall be a valley or hip rafter not less than 2 inches
nominal thickness and not less in depth than the cut end of the rafter." So
the old ways still meet code. I asked the framers why the huge hip rafters,
and they said the architect had it on the plan.
So why the huge hips? What conditions would justify building the hip rafters
like a beam? Anyone know?
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| Bob Morrison 2006-05-15, 7:21 pm |
| In a previous post TVeblen wrote...
> A few times now I have been in new framed houses with hip roofs that have
> double LVL hip rafters. The last one had 2- 14" LVL's! I've been a framer
> (many moons ago) and we always used a single 2x for hip rafters, provided
> the jack rafters laid face to face on the hip and the walls were tied
> together by ceiling joists. The Mass building code states: "At all valleys
> and hips there shall be a valley or hip rafter not less than 2 inches
> nominal thickness and not less in depth than the cut end of the rafter." So
> the old ways still meet code. I asked the framers why the huge hip rafters,
> and they said the architect had it on the plan.
> So why the huge hips? What conditions would justify building the hip rafters
> like a beam? Anyone know?
>
In the past, hip ridges have generally been undersized for carrying their
tributary share of the roof load. This is particularly true if there is
no ceiling framing to resist the horizontal "kick" produced by loading the
rafters vertically. A stiffer hip ridge reduces the outward thrust on the
corner.
One could analyze a hipped roof as part of a rectangular dome, but the
analysis gets a little complicated and tension forces in the wall top
plate can get pretty high -- I did this once just for a learning
experience.
As a result, it's better to have a stiffer hip ridge -- the design is much
easier and its cheaper to put in a bigger member than it is to run a 3D
dome analysis.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| RicodJour 2006-05-15, 7:21 pm |
|
Bob Morrison wrote:
> In a previous post TVeblen wrote...
They should be designed like beams because they are beams.
[color=darkred]
> In the past, hip ridges have generally been undersized for carrying their
> tributary share of the roof load. This is particularly true if there is
> no ceiling framing to resist the horizontal "kick" produced by loading the
> rafters vertically. A stiffer hip ridge reduces the outward thrust on the
> corner.
>
> One could analyze a hipped roof as part of a rectangular dome, but the
> analysis gets a little complicated and tension forces in the wall top
> plate can get pretty high -- I did this once just for a learning
> experience.
>
> As a result, it's better to have a stiffer hip ridge -- the design is much
> easier and its cheaper to put in a bigger member than it is to run a 3D
> dome analysis.
In addition, the IRC requires hip rafters to be designed as beams if
the roof is less than a 3/12 pitch.
R
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"Bob Morrison" <SpamFighter@junk.com> wrote in message
news:MPG.1ed29523e340a606989b61@news.west.earthlink.net
>
> In the past, hip ridges have generally been undersized
> for carrying their tributary share of the roof load.
> This is particularly true if there is no ceiling framing
> to resist the horizontal "kick" produced by loading the
> rafters vertically. A stiffer hip ridge reduces the
> outward thrust on the corner.
What you say is true but a true hip is self supporting, a 1 x 4 will work.
Of course both legs have to have lateral support.
An old formula I used to use. As you can see, I'm from the old school and
retired some 15 years ago. Anyway, with an off pitch hip and you want the
side cut of the jack rafters. Using the framing square, take the length of
the regular (common) rafter and the run of the opposite common rafter. Cut
your cut on the length for the side cut of the jack. The side cuts of the
hip are just the opposite. Both found with the framing square.
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| TVeblen 2006-05-16, 9:21 am |
|
"Bob Morrison" <SpamFighter@junk.com> wrote in message
news:MPG.1ed29523e340a606989b61@news.west.earthlink.net...
> In a previous post TVeblen wrote...
>
> In the past, hip ridges have generally been undersized for carrying their
> tributary share of the roof load. This is particularly true if there is
> no ceiling framing to resist the horizontal "kick" produced by loading the
> rafters vertically. A stiffer hip ridge reduces the outward thrust on the
> corner.
>
> One could analyze a hipped roof as part of a rectangular dome, but the
> analysis gets a little complicated and tension forces in the wall top
> plate can get pretty high -- I did this once just for a learning
> experience.
>
> As a result, it's better to have a stiffer hip ridge -- the design is much
> easier and its cheaper to put in a bigger member than it is to run a 3D
> dome analysis.
>
Hey Bob, thanks for the reply
I can understand the need for load carrying capacity for hips in designs
with cathedral ceilings (the rectangular dome) or in low slope designs (less
than a 4 pitch). No problem- they are now beams just like a gable ridge w/o
ceiling joists. But I'm talking about your traditional framed triangle here,
where the ceiling joists are framed to pick up the outward thrust on all 4
sides of the box, and the roof has a 12 pitch. In a traditional gable roof
there is hardly a need for a ridge beam at all if the rafters are
face-to-face and the out-kick is secured by the ceiling joists. Why would a
hip, framed the same way, be any different? Where does the load on the ridge
come from if the walls are secure from kicking out?
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| Wayne Whitney 2006-05-16, 11:21 am |
| On 2006-05-16, TVeblen <killtherobots@hal.net> wrote:
> But I'm talking about your traditional framed triangle here, where
> the ceiling joists are framed to pick up the outward thrust on all 4
> sides of the box, and the roof has a 12 pitch.
How exactly can the ceiling joists be framed to pick up the outward
thrust on all 4 sides of the box? I live in a 1908 hipped roof house,
and the ceiling joists are all parallel--it's not entirely clear to me
what is resisting the thrust from the hipped portion of the roof.
Cheers, Wayne
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| Glenn 2006-05-16, 12:21 pm |
|
"Wayne Whitney" <whitney@post.harvard.edu> wrote in message
news:slrne6jn12.2al.whitney@pizza.private
> How exactly can the ceiling joists be framed to pick up
> the outward thrust on all 4 sides of the box? I live in
> a 1908 hipped roof house, and the ceiling joists are all
> parallel--it's not entirely clear to me what is resisting
> the thrust from the hipped portion of the roof.
>
> Cheers, Wayne
Simple. Just a couple ways. Run a 1 x 4 brace diagonally from one wall to
the other with it recessed into the bottom of the joists. Nail the hell out
of it.
Use a solid bridging on the outside joist to the wall and tie that with your
stiff back from one side to the other
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| Bob Morrison 2006-05-16, 12:21 pm |
| In a previous post Glenn wrote...
> What you say is true but a true hip is self supporting, a 1 x 4 will work.
> Of course both legs have to have lateral support.
>
>
That's what I meant when I said a hipped roof could be designed as a
rectangular dome (or at least part of a rectangular dome). As a "dome" the
hip member can be subject to both bending and axial stresses. A 1x4 may
have worked, but only because the building was small and the hip may have
been vertically supported somewhere in the middle, thus reducing the span.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| Bob Morrison 2006-05-16, 12:21 pm |
| In a previous post TVeblen wrote...
> Why would a
> hip, framed the same way, be any different? Where does the load on the ridge
> come from if the walls are secure from kicking out?
A conventional gable roof forms a true triangle when the ceiling joists
are included.
A hip has non-linear force lines. That is, the rafters on either side of
the hip are not in a straight line. This has two effects: axial forces
are generated in the hip member, and no direct way to transfer the
horizontal tension at the bottom end of the rafters other than through the
wall plates.
If you look carefully at old houses with hipped roofs you will often see a
sag in the middle of the hip rafter. It is usually not real noticeable,
but it's there. As soon as there is vertical deflection the forces start
to rearrange themselves into new load paths. This is one reason why this
type of roof typically doesn't fail, but it still isn't properly designed.
Finally, if you apply wind loads to a hipped roof you can get stress
reversals due to uplift on the roof. A conventional hipped roof cannot
resist these forces with "dome" action and it will fail.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| I do agree with you in saying a 1x4 isn't practical. That was just an
illustration but in housing, the main stress (properly installed, the only
stress) on the hip is tension pulling from the corner. Except for that, it
is just another ridge holding two opposing rafters (in this case, jacks)
apart.
"Bob Morrison" <SpamFighter@junk.com> wrote in message
news:MPG.1ed389e3aadf4c7b989b62@news.west.earthlink.net
> In a previous post Glenn wrote...
>
> That's what I meant when I said a hipped roof could be
> designed as a rectangular dome (or at least part of a
> rectangular dome). As a "dome" the hip member can be
> subject to both bending and axial stresses. A 1x4 may
> have worked, but only because the building was small and
> the hip may have been vertically supported somewhere in
> the middle, thus reducing the span.
>
> --
> Bob Morrison, PE, SE
> R L Morrison Engineering Co
> Structural & Civil Engineering
> Poulsbo WA
> bob at rlmorrisonengr dot com
| |
| TVeblen 2006-05-17, 9:21 am |
|
"Wayne Whitney" <whitney@post.harvard.edu> wrote in message
news:slrne6jn12.2al.whitney@pizza.private...
> On 2006-05-16, TVeblen <killtherobots@hal.net> wrote:
>
>
> How exactly can the ceiling joists be framed to pick up the outward
> thrust on all 4 sides of the box?
What we do up here is frame the ceiling joists traditionally (from the front
to the back walls), but at 32" before the side walls those joists end with a
double joist. "Cripple" joists (32" long) are run perpendicular off these
doubles. The hip rafters tie into these cripples and the top plate of the
side walls. When we install the wood strapping inside the house we run each
piece down each cripple joist and then across all the traditional joists,
ending on the nailer attached to the top of the partition walls. Up in the
attic we install 1/2" plywood, orientated to run long over the traditional
joists, that run out over all the cripple joists. Everything is nailed
together like a big "shear floor". Some guys just strap the joist tops with
wood or lumber straps, some build "strong backs" or a sort-of "box beam"
onto the double joists, but this way doubles as attic floor sheathing so it
gain points with the owners.
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| TVeblen 2006-05-17, 9:21 am |
|
"Bob Morrison" <SpamFighter@junk.com> wrote in message
news:MPG.1ed38c8860d7ae7c989b63@news.west.earthlink.net...
> In a previous post TVeblen wrote...
>
> A conventional gable roof forms a true triangle when the ceiling joists
> are included.
>
> A hip has non-linear force lines.
Ah. That makes sense.
So how do you calculate the size of the "beam"? Same as a traditional
cathedral gable (tributary area x design load, etc), or is there a factor
that represents the partial linear force that could be used to minimize the
size of the beam?
The reason I am concerned about the double LVL hips is that if you are going
to vent that hip, assuming a 2" slot for ventilation on either side,
traditional ridge vent products don't really fit correctly, the span is too
great. The roofers fudge it and the potential for nuisance leaks occurs. It
would be far better if we could use one LVL, but if you assume the hip
rafter is carrying the full roof load like a cathedral ridge beam then that
is not practical.
| |
| Bob Morrison 2006-05-17, 11:21 am |
| In a previous post TVeblen wrote...
> Ah. That makes sense.
> So how do you calculate the size of the "beam"? Same as a traditional
> cathedral gable (tributary area x design load, etc), or is there a factor
> that represents the partial linear force that could be used to minimize the
> size of the beam?
In general most people ignore the axial force in the hip. The hip "beam"
has a triangular load on it and most software available these days can
design this type if member very easily.
> The reason I am concerned about the double LVL hips is that if you are going
> to vent that hip, assuming a 2" slot for ventilation on either side,
> traditional ridge vent products don't really fit correctly, the span is too
> great. The roofers fudge it and the potential for nuisance leaks occurs. It
> would be far better if we could use one LVL, but if you assume the hip
> rafter is carrying the full roof load like a cathedral ridge beam then that
> is not practical.
>
Why are you venting the hip ridge? The only reason to do that if the
house has a cathedral ceiling.
If you have to vent the hip, then the top of the rafters can be set about
an inch above the top of the hip beam and the sheathing cantilevered over
the ends of the rafters to close up the gap. This will allow the use of
typical ridge vents and is about the same detail one would use for a
regular ridge beam with rafters hung from the beam.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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