How does a hipped roof work?
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- čas přidán 10. 03. 2021
- How does a hipped roof work, and how do you support the purlins and hip rafters?
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In this video, a Structural Engineer explains how to support a hipped roof, showing how the hip rafters, purlins and ceiling joists work together to support the roof, and what structural aspects to look out for.
#roof, #roofing, #structure, #engineering
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I'm a chartered civil engineer with a specialism in structural engineering. None of the things I say or teach in my videos should be construed as 'advice', and you should always have your designs checked by a competent, experienced, registered or chartered engineer. - Věda a technologie
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Very impressive presentation and sketch-up. Thanks Robin. A couple of questions and discussions:
1- The support of the purlins and loads on the hip rafters. The end purlin is supported by the hip rafter. Therefore, when sizing the hip rafter, the loading will need to cover the jack rafters as well as the end reaction of the purlin. Considering a 3-4m purlin supporting all the dead/live loads from the common rafters, the point load transferred to the hip rafter will be considerably high.
Then, the end reaction of the hip rafter itself will be transferred back to the end of the ridge board. Then it will be passed on to the 1st pair of common rafters. So the question is, why we normally won't thicken the 1st pair of common rafters at the hip? (sorry for the long winding questions and it would be much easier to talk through a sketch board).
2- I know you have another video about loft conversion. However, my question here is about loft conversion with large dormers (a lot of home owners would prefer a dormer which is as wide as the span of the ridge board). For the hip roof structure in this video, I don't see any way the dormer conversion can be done without installing a new steel ridge beam with additional support at the hip (can be a column or triangle frame). This is a result of undermining the integrity of the triangle which is formed by a pair of common rafters and a continued ceiling joist. Just wonder if you have any better alternative or I missed anything?
Many thanks for the video as well as reading my questions.
Cheers,
Rick
nice overview Robin, in other similar projects rafter collar ties are place at 1/3 to add extra support to the common rafters and hip rafter
Love This!
Thank you 💙
Hoping you have more on different roof structures!
Helpful video . Thank you for making this
Great video Robin, very helpful 👍🏼
Glad you found it useful, Cory 👍
Love this ! Nice to here from a proper expert , really good vides on your channel 🫡
Great video! This is a common type of residential wood roof trusses here in the US. I would be curious to see a wood truss design with a vaulted ceiling. That is often a tricky design because, without the collar ties to tie the system together, it is hard to transfer the horizontal shear force to the top of the walls while maintaining stability.
Great suggestion! Check out this one for the analysis of a collar tied roof: czcams.com/video/JbRUdP2AYKg/video.html
Even better the second time around! Just in time to design a hipped cut roof, so many thanks Robin 👍🏼 perfect learning and revision material!
Thanks Cory - great to hear it!
@@RobindeJongh very helpful. Would the hip rafter typically be doubled up to account for the higher loading and point load from the purlin, as well as the first ‘truss’ supporting the hip rafter at the top?
Thanks for this very helpful!
Great explanation and visuals - thanks.
Thanks Wayne. Glad it was helpful!
Would be interesting to see you run through the calculations for a traditional roof. Thanks for the video. Your sketchup stills are impressive.
Thanks Andy!
Hi yes I too would be interested in a run through for the calculations including connection designs for this hipped roof.
Great explanation thank you!
Will you be doing loft conversion for hip roof in future?? my roof is the as this except it has long brace from front to back. apart from that its all the same. brick pillars, wall bearing spine down the middle. thank you for your post.
Hi and many thanks for the professional explanation
My question is how the roof load will transferred to the walls?is it distributed transferred along the wall length and points loads applied in the corner transferred by the hips?
Thanks
What way does it work then when converting a hipped roof loft? In terms of the purlin support walls, they would protrude into the available room space could they be replaced with vertical timber supports every 600mm along the purlin?
Or even cut the purlin wall support back flush with the edge of the purlin to reduce protrusion into the room?
Or replace with a steel beam somehow?
Would be very interested to see a video on how this hipped roof structure could be converted and maintain both structural integrity and the hip design. You see an awful lot of videos on CZcams of hip to gable loft conversion but never any hip loft conversion dealing with the purlin support walls problem.
Loving all your videos have been so informative, keep up the awesome work!
Great suggestion - watch this space!
Hi, i have a bungalow with roof like this. I am now looking to make use of the space. Can you explain how a loft floor would be installed. Thanks
Hi, good video, ex joiner and building surveyor. I usually put a small pice of timber nailed under the hip where the purlins meet so the purlins can join / exert equal and opposite force and usually a plywood gusset attached to the outer face of the last two commons so that the larger hip plumb cuts have something to nail to otherwise the bottom of the hip plumb cut is dangling where it meets the ridge. My question is I’ve always found the unsupported purlins which span from the short length of hip to hip at the left and right sides abit odd as they add extra weight to the hip with no supporting masonry pillar underneath. Is this arrangement providing support to the hips via equal and opposite force where they join to the supporting main purlins and thus don’t need a pillar under them ?
I have a room in roof hipped roof that needs to be removed. What would be the safest sequence in which to remove the members?
Hi Robert, super enjoying all your videos, very clear with no room for guessing.well done. Question: can a hipped roof be converted to livable space? Can the traditional roof way apply in the same way? Many thanks, Cesar
Hi Cesar. A hipped roof can be converted with steel beams.
Hi. very much enjoying your videos. I'm a joiner by trade but now a building surveyor and often we see hips which are clearly sufficient for purpose having been there 50 or 100 years without significant deflection, yet they often don't calculate. can you explain what's going on at with the magnitude and direction of forces at a hip. I'm aware that the purlin load is transferred to the hip which is more efficient being in compression (hence the smaller section) but I think it would make an interesting video and I always learn something from your videos. Also if you could explain how then loading of a hipped roof translates to the forces on a beam below. Thanks again for your content.
Calculations account for more loading than is likely received, there’s also a factor of safety, snow is rarely in such quantities as accounted for and is short term, there’s also a lot of load sharing between the members, timber quality is perhaps better than assumed in the design codes, older roofs that failed have probably been replaced!
Great presentation many thanks Robin. Funny I was never a fan of the hip roof from the issue of maintaining the hip tiles. I have realised recently though that they avoid all the issues of bracing and restraint strapping of a gabled roof, so provide a wind proof structure at the expense of a lot of joinery. Just one minor question: For the overlapping ceiling joists, for the central overlap presumably a bit extra overlap timber should be allowed such that there is say at least 6 inches ( if not a bit more) of timber beyond the last nail, to stop the nails splitting out the end of the timber? At the wall plate end assume best to use purpose made straps/ties to fix it all together. Thanks again.
Thanks John - very good points. Definitely a bigger overlap for the ceiling timbers than what the graphics indicated is important to allow the nails enough end distance. I'm not a roofer but it seems to me that the tried and tested methods are best to stick with, so either purpose made connection plates (warrantee'd by the manufacturer) or else the right number of nails as specified in college textbooks or by experienced joiners!
@@RobindeJongh Thanks Robin. I've been wondering what the terminology was for the overhang so it's "End distance". Agree on using manufacturers purpose made brackets at the wall plate rather than making something up. I hadn't thought about the wall plate corners before, that was an interesting point.
Hi Robin, should purlin supports mirrored / symmetrical?
Wondering if it's feasible to have both purlin supports positioned opposite eachother, but slightly off of the structural mid point?
Thank you for all the great content you upload !
Is it ok to replace a couple sagging ceiling joists if i do them one at a time
Have an older home with hip roof. 2 x 4 construction on rafters and ceiling below. House is in northern USA, so snow load happens every year. Currently no Purlins are present. We are replacing shingles on this home and notice that the rafter are bowing. Can we jack up rafters to take out bow and install a knee wall on all 4 sides instead of Purlins. Concern is 2x4 ceilings supporting weight possible wall cracking below in living area. One side of house has 1 wall underneath as other side had 2 walls underneath to disperse the weight. Thoughts on knee wall?
Hi Dee. I recommend that you have a structural engineer round to look at this.
Hello from Erie , PA USA
I have a section of my 1913 cape cod's hip roof attic (2x6 rafter) that'll be converted to living space . I have a supporting wall 12ft from one side and 16ft from the other . The floor joist (2x6) for the attic connect at the support BUT at the exterior walls they sit ~5inches below the top plate and are nailed to vertical stud . Because I'm converting this section to living space I've added 2x10 collar ties (7'6" ceiling , originally 2x4s but moved up 3.5') . I plan to sister the joists with additional 2x6 on each joist + blocking at mid span as I want to avoiding losing height. The rafters and joist run in same direction .
Do you think this attic floor will hold up as I believe the floor acts as the rafter tie in some way ? Its currently not sagging or bouncy . I'll be reinforcing the roof rafters to the top plate with Simpson Strong rafter ties and attach the sistered floor joist to outer wall with 2 Spax Powerlags 3/8 thick screws . Ill be removing the 42 inch knee walls after all this . Does this sound like it'll work ?
Thanks !!
Hi. I'd always get a structural engineer to take a look if you're doing structural modifications.
Random question but I'm a roofer and just started doing jobs for my self and I'm struggling working out the sqm or m2 (not sure if there the same thing) of a 4 hipped roof and to then work out materials for the roof eg batten felt tiles. Could you shed some light on this please or anyone?
Robin, I do have a question...I have an existing structure that is a 4-way hip roof over a square building and has no ridge. Basically a pyramid. The owner wants to add 10 feet to the back of the structure with a gabled roof. The existing structure is 24'x24'. Adding 10' in the rear would effectively require a 22-23' ridge extending back to the gable. The owner would like to remove the jack rafters on the back to provide full access to the attic. My assumption is that the two hips in the back would need to be beefed up somehow to prevent bowing caused by the pressure of the side jack rafters. I was going to install a 24' carrying beam along the back where the existing back wall will be removed and installing the ceiling joists on top of and perpendicular to the beam. Do you know how the back hips would be beefed up to reinforced them once the back jacks are removed?
Hi Scott. This would need a structural engineer to produce a design as it's not an easy answer.
I'm watching this because I'm researching about installing a skylight in an end-of-terrace on the hip end, which would put it at the top of the stairs internally.
Could be fairly straightforward unless the purlin is in the way.
Do you know how you would vault a hipped roof? I've seen ones that seem freestanding. Are the ceiling beams (the white ones) necessary for the support of the roof?
Hi. You can do this with a structural ring-beam or a steel moment frame that goes up to the hip/ridge, or a cranked steel ridge beam.
What kind of siding do you use?
What software was used in this video at the beginning?
For the purlins get notched around the hip at the bottom so they touch then nailed together
Hi Robin. I’m looking at converting my bungalow loft which has a hipped roof, 45° pitch and a 4’ ridge. The purlin props are at 90° (or not in some cases) and strutted on to the internal walls for support. Providing I make sure that they still strut down to an internal wall, or a beam fixed across the internal walls, is there any reason why I can’t have them in a vertical position? Rafters and joists vary between 360 and 400mm - 1935 built brick bungalow. Many thanks.
Hi Mike. You need to resolve all the horizontal and vertical components of the force, which is why the props are in at an angle.
Thanks for the reply Robin. How would you suggest to overcome this? I was advised by a builder to fit a props vertically on to a different internal wall to take the weight of the purlin and rafters and then remove the angled ones. From what you are saying, I gather this won’t be sufficient?
Hello
Nice Video !
Btw , what sofware do you use to draw this in 3D ?
Hi - glad you like it! The software is SketchUp Pro
I would have thought the top of the hip rafter was supported by the last pair of common rafters. There will only be outward thrust at the bottom of the hip rafter if the connection came loose or if the ridge lowered?
Hi Tom. Yes the hips are supported at the top by the triangulated common rafters and ceiling tie. However, for equilibrium I believe it would also need support at the bottom, both horizontally and vertically.
Hi mate. Is there a way of removing the ceiling joists to make my hip roof vaulted. My cieling is only hipped on one side and spans apprx 2.4 metres wall to wall so its not big. Thanks
Hi Wayne. You would probably need a steel frame and ridge beam to do this.
@@RobindeJongh im a joiner by trade but not a roof joiner. Where would i place the steel frame
Can you cut one of the hip rafters to build a wrap around dormer? Can the weight be somehow redistributed allowing to have full height in all the dormer? This would be as an alternative to hip to gable, so modifying only part of the roof. In the case I refer to the purlin on that side is already partially cut and removed by a previous building project (staircase to the loft)
This is what we have in out loft conversion. It's quite common in our area on semi-detached houses because planners don't like a full hip to gable.
@@tallerlaura thank you. Could you upload a link to a photo or to a similar finalised loft?
Can you do a video if the building is l shape with a hipped roof
Hi Robin. Just a couple of quickies...
(1) the tension in the bottom ties. Obviously a builder would ask how many nails are needed. If you had say.. A tension force of a 10kN. When you split the bottom tie in half, do you usually specify the nail fixings etc?
(2). The pier walls. Have you done this on many projects retrospectively? With wind loading, is that a concern at all? Would you specify any proprietary fixings to ensure the wall stiffness is at its max or if its a loft conversion, would you instead design steel supports spanning from eaves to internal wall, then design a stud wall / timber post to support the purlin?
Cheers! And great videos as always.
Hi Mark. 1) yes, would need calculations to ascertain fixings if the ceiling ties were not continuous. 2) Do you mean take the pier walls out?
I have known structural engineers give the shearing strength of nails for fixings but in reality what is usually specified is "adequate fixing" which means someone getting busy skewnailing with a nailgun.
Could I dl the sketchup model?
Where do I can get the hight of roof
How is the peline determined?
Can you have a hipped roof like this without rafter ties?
How would it a hip roof without ceiling joists, to prevent thrust, be designed? Is this possible?
Hi Michael. Yes it's possible - usually you would provide a ridge beam and support the ridge with a post or steel frame.
I have done this two ways, supported a structural ridge from a post or structural stud wall on to masonry, and the other, where the areas floor is to be lined you can provide a stressed skin to the ceiling joists.
can outside wall be removed on hip roof with porch
Hi Kenneth. You will need to have a structural engineer look at it. If just a door opening then you may just need a lintel.
Could a hipped roof be braced in such a way to lift it off of cinder block walls so that it wouldn't collapse on itself? Would like to raise the height of my walls without the expense of building whole new roof structure.
That could be done in theory. It would need some careful structural design.
This video fundamentally misunderstands how hipped roofs work:
1) the purlins are not 'supported' on hip rafters - but actually form a 3-way compression joint (c-c-c) with the hip rafter. The hip rafter is predominantly a compression member rather than a flexural or bending member.
2) the purlins are not 'ties' - ie tension members - they actually form what's known as a 'compression ring'. The opposite of ties.
3) having said the above, the jack rafters on the hip tying to ceiling joists at eaves are actually a fudge: they don't 'tie' back to anything other than the plasterboard! However, historically it does seem to work (all hail all those who wryly say 'this house will be held together by the plasterboard'). If I design a hip roof from scratch, I normally nog a few ceiling joists back and use galvanised straps over the top (a roofer will tell you that's overkill!)
The force diagram for the cross section is correct - and helpful for understanding. But the similar action of the hip rafters in conjunction with the purlin is missed.
Hi. I’d be very interested to read any references you have on this?
The purlins should be positioned so they are acting at 90 degrees to the rafters. They are less effective they way you have shown them. If you have two rows of purlins you can often reduce the rafters down to 75x50mm which allows the purlins to be located higher up the hip and create more area for fixings. Also, traditionally there would be dragon ties across the wall plates at each corner to help restrict spread. This is particularly relevant if there are openings close to the edges of the walls. Another very well presented video though.
Hi Paul. Thanks for your insightful comments. Glad to have you aboard!
On my roof there is a support beam from the purlin to the middle joist. So at an angle. One on each side of the roof.
My question is, can I remove these 2 support beans?
There are supports under the purlins, going vertical to a beam that sits on top of the joists.
also do you have a video regarding the angles where the purlins meet on the hip rafters, like the person commented, placed at 90 degrees to the rafters with notches out to allow for the angle at the corner
The Perlins shouldn’t be perpendicular they should be 90 degrees to the rafter. The way it’s drawn shoes the rafters load is hitting on a corner of the load taking timber namely the perlin
Hi Jerry. I have seen it done both ways. For the vertical purlin for example see here: nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-7-joints-and-connections/
Anti typhoon roof.
Thanks for your explanation .