How does a room-in-the-roof truss work?
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- Опубликовано: 13 мар 2022
- A structural engineer explains how an attic truss loft structure works.
👋Download the annotated drawing and 3D model here: geni.us/attictrus
How does an attic truss (room in the roof truss) work, and is it an option for your loft? In this video, a Structural Engineer explains how an attic truss works, showing how to brace it and why this is necessary.
#loft, #attic, #structure
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How a dormer loft conversion works: • How a dormer loft conv...
Quickframe software used in this video: quiksoft.co.uk/software/quikf...
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This is some of the most professional educational content I’ve seen on YT. From one engineer (Geotech) to another, thank you so much for sharing your knowledge. Your on the fly sketching is A1. These videos should be a module in Civil Engineering undergraduate degrees! I love the blend of theory, loadings, design codes and practical considerations. Would you consider an update to this video to show some possible details for adding a dormer to this room in roof structure? I have an idea what might be required but it would be great to watch you discuss it.
Thanks Gavin! Very much appreciated, and glad you're enjoying the videos. I'm hopefully looking at dormers soon 👍
Thanks for the discussion Robin
Keep up the good work, and you get another coffee seeing as your last one went down so well. Really appreciate you enriching the internet... we need a better signal to noise ratio here... :)
Thanks Clark! 👍☕
Great video, many thanks Robin. The analysis software is very interesting as I struggle to figure out with some members particularly at the top what is happening with those, and see you actually got the highest load there.
Minor points: most loft conversion video on RUclips seem to gloss over bracing and strapping, and remove them. Is the following all true for your example:
1. Longitudinal bracing should extend to touch the gable end inner leaf (being a top builder you made your building exact multiple of 600mm so you didn't need any overhang)
2. There should also be straps into the blockwork for the gable so for a particular wind direction for the gable on the lea side of the wind that will be being sucked outwards putting the strapping under tension.
3. The diagonal bracing should go up to the ridge brace at the top, and at the bottom down to the wall plate with some overhang and be nailed to the corner of the wall plate. Personally I don't like that bit of the standard and it seems a bit poor. Mitek South Africa had a better looking solution of a heel shelf fixed between the last two trusses. The brace can be securely fixed flat to that shelf. The two trusses then need blocking either side on the opposite side to the heel shelf. In the gable end case masonry "box ends" if I can describe like that (or a raised parapet gable) would provide the best blocking? Stupid question though: would UK BCO reject such solution because they wouldn't understand? Need a report/analysis from top structural engineer like yourself I guess to get that passed. And find a builder prepared to go non standard. It would be of course a lot more work, the shelf being held in with small strong brackets.
The other silly thing is the plasterboard walls inside would appear to assist the diag bracing? I guess building regs wouldn't let you use T&G instead of plasterboard because of the fire risk? (so it's effectively internal sarking)
One interesting thing on insulation: Robin Clevett has just done a video where he runs the insulation continuously from eves to ridge, then installs a non structural dwarf wall.
Note how Robin chooses his truss pitch to minimise cutting and wastage on the insulation sheets.
I would love to see the two of you design and build a house. That would be truly awesome.
Here Robins video on insulation (also see preceding videos of his "big build" playlist)
ruclips.net/video/5BoJNWGljVE/видео.html
Great video - I would love to know how Robin has supported this, as there are some large spans involved.
@@RobindeJongh Have a look at Robins earlier videos in his "big build" series. The floor chords from memory are Mitek posi joists, wood and steel lattice. Note also in the top of the roof as there is so much headroom he fitted wooden lattice sections. Think those were premade off site. All from memory so watch the videos for best answers. Try numbers 16&17 for the floor and 26 for the lattice ridge.
Great video Robin. I have been looking through YT for a week or so looking at what would be possible in our 1970s build house that we wish to complete a loft conversion into, for office space and storage space. I saw this video which shows the exact roof construction we have and am therefore wondering if the current structure would be feasible, (if the current dimensions for the build are; rafters - 125 x 30, bottom cords - 180 x 30, vertical ties and collar ties - 125 x 30 ) for a loft conversion as is, or if steels or Flitch beams would need to be added along with new joists to build the loft floor onto. The existing bottom cords / floor joist are supported on a load bearing wall at mid point (front to bac) one one side of the house and at each third distance in the other side of the house. Internal load bearing walls are 20cm, External are 30cm block, basement walls are 50cm stone, built on a 70 x 70 cm concrete foundation. Thanks Robin for any thoughts you might have.
Great explanation thanks. What would be the implications of using Light Gauge Steel framing?
Hi Robin great video, very informative as this is pretty much exactly the roof structure I have in a new build. I have a question for you if you don't mind - can the collar ties in this setup typically bear any weight? such as if I were to use them to attach a pull-up bar or attach a 30kg punch bag from them. Or should they be left alone completely to do their job without any weight bearing from below
Super helpful video, thank you for creating and sharing! What are some solutions if the stairs run perpendicular to the trusses down the center of the room?
Hi. You would need structural advice if you are cutting through trusses to form a stair opening.
I have a garage getting built with attic trusses. My question is will 2x4 8 ft high walls be sufficient enough to hold the weight of the trusses and all the weight of material items within them.
If so would you need proper engineered 8 inch thick footings for the structure. My builder has just an engineered slab with thickened edges. Thank you!
Very interesting information about attic trusses. I have wondered whether the large diagonal bracing member can be removed by forming a shear wall where the upright members are by the use of 2 sheets of overlapping ply, glued and screwed to the truss members and also by the provision of a line of diagonals in the upper void to stiffen that section. With the main truss members being quite big sections they should be quite rigid and, if not sufficient, the underside of the sloping ceiling could also be ply boarded or the insulation infill be considered as taking lateral forces between the truss members and making the completed structure stiffer.
Have a question about my attic conversions witch was carry out already would you able tell me your opinion or it safe structurally if I send you a drawing (have a bad feeling that something is not right) thanks
Really interesting video. I'd be interested to hear what load combinations you consider when working on projects like loft conversions.
Hi Paul. The load combination shown in quikframe demo was just some quick loads added for the demonstration, so that would have looked slightly awry to your trained eye! This is bs5268, so allowable stress code, so everythings 1.0. The if wind is significant I may look at that too in combination with and without imposed load .
@@RobindeJongh Thanks! 👍
Great video Robin! Please what’s your take on creating a dormer on this type of roof? Also is it possible to create a domer without removing the entire roof truss. Thanks
Hi. Yes, I've done this before. You would need some steels to support the trusses before you cut them to form the dormer.
Hi Robin, very interesting videos. I am constructing an attic roof very similar to the one you are showing, I am an old school carpenter and use traditional methods. My span is 6.3m, 45 degree pitch and 7m long, I have 7x2 rafters and 7x2 joists at 400mm centres. 3.750m between the vertical upright studs/braces. May I ask you how you if you think this will be sufficient and also do you think I should bolt together the rafters and joists at wall plate or do you think substantial screws would be ok? many thanks for any feedback you are happy to give.
Love the video! We have recently moved into a Dormer Bungalow with Attic Trusses they are spaced at 600 centres. Have you got any videos which show how you would go about adding a small dormer say to span 1200 over two trusses and also how you would go about creating a long box dormer spanning the length of the roof?
Great suggestion 👍👍
So, could you use these trusses laid directly on top of an existing garage to create a second floor and put dormers in the center either end? i.e. to create two rooms or one large bedroom with ensuite?
Can you do a video on how a trussed timber roof should get braced please 🙏.
Hi Robin, great video as usual.
I thought that I was fully subscribed to the channel but the setting must of been changed unbeknownst me. I had to allow notifications in order to change the subscription bell from "personal" to "all". I can turn of YT notifications again on my phone after changing that setting as you can get inundated with notifications. I've noticed it happen to a few channels I follow. I wouldn't of known your new vid was there only for your email.
I'm interested in the attic roof aspect of things. How would you tackle one that is not tied into the floor joists? I.e for a story and a half building where you can't run a cross member from wall to wall and your trying to maximise head and floor space.
What are the rules of thumb for the un supported spans on each timber spec code. Does the data sheet for the timber give that type of info?
Hi. For a roof truss not tied at the base, you would usually need to have a collar tie about a third of the height up, or a ridge beam.
Another useful video :). Can attic trusses be retrofitted to replace an existing structure? The loft I’m looking at converting (1880s terrace in Truro) has 2x2 rafters, non breathing membrane under the old (refitted at some point) slates, and saggy cross beams etc., and I think just going for a new truss structure that meets current regs and allows for decent insulation etc will be a better bet than working with the old….
Hi Paul. Yes, they can be retro-fitted. It may be a pain though, as you might need to remove your ceilings!
What type of app are you using for building
Hi Robin, great videos as always, one question though, am I wrong in my thinking that diagonal bracing shouldn’t be a mirror image but opposing? Ie. High to low on each side alternating? Or is that not strictly necessary?
I’m converting a 1976 Bungallow loft at the minute and I’m a little concerned that there is No bracing at all anywhere so figuring on adding some. (Trusses are 400mm centres)
Hi Rob. Thanks for your comment. The NHBC has a guide in section 7.2.9 of the guide.
Do you have recommendations on modifying a tie/hanger ie remove one a nd trim out for access ope
Hi. I have just done a project like this, and it took a surprising amount of analysis to verify it. I would go to the truss designer and ask them to provide a design, and if that doesn't work, get a structural engineer to check it and provide a solution.
Thanks for your videos, i love how informative they are.
I have bought a house which has this type of roof structure.
I can see though, that the "joists" are not 50mm*200 but 38mm*200, and spaced at 60 cm.
I was thinking of using this room as a guest room, so putting in a sofa bed for 2 people which would not be used often.
I'm just worried that those 38 mm joists could not support the weight of 2 people + a sofabed + some flooring and eventual storage.. any idea?
Hi Tony. Usually if you have this kind of truss it is specifically calculated to bear the weight of a room in the roof. Get in touch if you want me to check the truss out for you.
The studs wall can be moved close to the eaves if is was made prefabricated ?
Hi. Moving the uprights closer to the eaves would mean making the rest of the section sizes larger.
Pre-fabrication doesn't change the fundamental properties of the timber, but might improve on workmanship.
What is the roof pitch in this example?
Hi Daniel. 47 degrees.
My attic has only three collar ties about mid height how can I remove them?
Hi Will. Do not remove them - they are structural.
If the span was only 4.5 m and the roof a 60 degree pitch what size timbers would be needed.
Hi Voda. A structural engineer would need to size the timber for you. Let us know if you would like help with this.
Hi All, is it safe to assume you don't need steel beams using this method?
That's correct, no steel beams needed.
Are attic trusses as safe as a regular roof?
Yes, providing they are properly designed for the loads they will take, which will be greater than those of a normal attic. The purpose of attic trusses is to create habitable space in the roof, and habitation implies dynamic loads and greater static loads.
Hi Robin, do you mind doing a video on Chimney breasts in general? How they offer structural support and what calculations are required to justify their removal? - Not much clear guidance out there and most people I speak to just say check it as a vertical wall panel floor to floor. Do you have a patreon account or anything to help support you?
Hi Liam. The chimney video is on the list and I hope to do one when I find time. Here you go: www.patreon.com/robindejongh
@@RobindeJongh Thanks Robin
🏚Get the attic truss drawing and the 3D model I used in this video: geni.us/attictrus
With the ply externally there's also the issue with the ventilation of the rafters. As in counterbattening over the ply will not enable the air circulation under it. That is the main argjment for osb3 or ply to underside. Other than that , great video again
Thanks Ben - great point as always! 👍
The solution would be a warm roof construction with insulation on the sarking and counterbattens over the insulation.
add 20 mm to truss and notch out for bracing then pb flush