How a loft conversion works
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- čas přidán 19. 11. 2020
- A structural engineer explains how a loft conversion structure works.
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How does a loft conversion work, and how do you support the roof and floor once you have cut the diagonal timber struts out of a trussed rafter roof? In this video, a Structural Engineer explains how a loft conversion works, showing how to cut and support the roof and floor, and why this is necessary.
#loft, #attic, #structure
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How a dormer loft conversion works: • How a dormer loft conv...
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*Disclaimer:*
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
Would be great if you could do similar assessment for a older roof style. Like for a 1930s that dont have triangle trusses. Trying to understand how to strengthen the floor for the larger spans. Check that walls will take the loads.
We did one with a ply box beam on a traditional roof with purlins very similar this but didn't need steel which made it easier to get in. Great use of sketch up which makes it very clear.
Great video, really clear, informative and concise. I’m not doing any work in my loft personally but I think it’s important to be educated before talking to any contractors etc. So thank you.
Thank you Robin. This is exactly what I planned to do with my trussed attic. I am also going to try to do a floor over my 2 story high living room & foyer and potentially add ~800-900 sqft to my house. Thank you for educating people out there ;)
Just subbed, I am not a structural engineer, but fascinated by the theory will be binge watching Your calculation vids 👍
Wow, this is the exact video I have been looking for. I have a friend that has a house that is 95% structurally exact same as mine, only difference, he has an above garage loft. I paid almost 1K for a structural engineer to draw me blueprints for modification of my house to be able to have an above garage loft. What he told me is that I would have to replace the entire section above the garage with a (2x6 instead of the existing 2x4?)... This video made it very clear that the existing structure could be bolstered without changing the entire beam above garage. I will have to reassess with my contractor from before. Again, thank you so much for this video I have subscribed and turned on notifications ✌️
Thanks Brandon - really glad to have been of assistance. Hope your loft conversion goes well.
excellent presentation with a very clear description of the necessary pinch points for the conversion
Very educational, always nice to learn how things are done.
Thanks 👍
Fascinating... Have sat up in my huge loft space contemplating the same thing. My trusses are very similar to those in the animation except... My joists have an additional vertical piece from the ridge down to the ceiling joists. My joists and ceiling rafters are all 4" x 2"... without climbing up into the loft right now and measuring I bet they are closer to 100mm x 50mm. Now 'if' I go ahead with my project, it seems to me that doubling up the angled roof or tile supporting rafters is achieved with by doubling side by side the rafters (Don't worry I'd not do anything without consulting a structural engineer and having specs drawn up). Changingh the ceiling rafters to floor rafters it seems are achieved by placing by the side and abutting each with an appropriate length of 200mm x 50mm floor joist. The Dwarf walls on the outer reaches (if that is the correct term are similarly achieve by installing 200mm x 50mm floor joists end to end that is running the length of the house, or as far along the length of the roof as you wish to go. I'd go further and surmise that in fitting the extra roof support rafters and the floor rafters these would need to be fixed not only at the ends, but with a 100mm screw fastening...maybe even bolts, every 600mm along their lengths. Once the lengthways joist are in place and once the first three maybe four floor joists are in place, it seems to me that the other appropriate pieces may be installed truss by truss before removing the sections that provide the current triangulation. In this way, economy of materials can be achieved be re-using in the next section pieces removed from a completed rebuilt section. As well as saving on materials and costs, it means far less timber to be disposed of as redundant and progress can be made as and when, not having to complete a reconstruction/construction all in one go.... I happen to know a building construction engineer, so if I do go ahead with this project, it is he I will consult... I had toyed with the idea of using structural steel, but this keeps all the materials the same with the same expansion characteristics etc... Not only that... With a half competent person who has all the appropriate tools it can be quite easily and safely completed... And hey!... no need to remove a single tile...except perhaps if Velux windows are to be installed.... Have designed and constructed rooms, steel sheds, truss gabled pergolas covered in clear carbon fibre corrugated panels in the past...so confident about my skill set....
Great stuff . Found what I was looking for.
Thanks for the reminder for me to like the video
You are the MAN. I am going to do this myself.
It's also essential to add extra support from your new floor structure to the existing ceiling timbers. Good video!
Yes, the ceiling timbers are also strapped at the beam location.
Great video. Very clear!
What an excellent explanation. Thank you
Nice video and redesigning insight. To me this just emphasizes the importance of considering this open loft space prior to the built. Now you still have studs, may it be vertical, in your living space. And you've had to lift up the floor vertical by x centimeters.
Brilliantly simple explanation
Glad it was helpful! Thanks Terry 👍
Interesting, neighbour of my parent's are converting & I was concerned with party wall as they hadn't informed us. Thanks for the heads up
Great video Robin. Could you do a follow up video and explain how the new design transfers some of the weight that was on the front and back wall plates into the gable walls. Also answer questions like how.to tell if the gable blocks are strong enough and when padstones might be required to sit the steels on. Also other practical considerations. I have seen alternative options for front to back steels that look easier to install but will increase the load on front and back walls. Also any other practical points and considerations etc. I'm thinking about the questions that should be asked before looking for a.contractor that is happy to meet those requirements.
Great explanation and presentation, thank you.
Thanks Alan!
Thank you. Great informative video.
Excellent video!
Very interesting. I have been considering a way to do this in a garage.
great content
Brilliant video. Thank you!
Glad you enjoyed it!
lovely vid, I m working on a similar project currently
💯Check out the fun way to learn structural engineering sovrn.co/1cb6ejt
Great video,subbed.
Thank you very clear. It would be great if you could make a video on a loft conversion with mansard!
Such an Excellent explanation / easy to understand and interesting. Instant subscriber!! I’m renovating my attic and have talked to a few engineers none have come close to simplifying things like this. You’ve really helped me out. Thank you!
Thanks Mark and I'm glad to hear the video helped!
@@RobindeJongh hi excellent videos and illustrations. I’m currently converting a loft . Very old building it has two purlins embedded to both cable ends . The ceiling rafters rest on the purlins . I have created a suspended floor below 10 ft wide the breadth of the fire place . With 3 x 9x 3 100mm into the fireplace wall brick . My concern is removing the ceiling rafters and somehow lowering the to the mezzanine floor I’ve created. The whole width of the building is 24 ft the floor finishes under the purlins . Could i support the roof by fixing it to the new floor bolting it to he purlins then finally bolting it to the roof rafter’s with collar ties at the top
Very informative. We moved into a new build around ten years ago and the builder had already prepared the roof space for conversion, yet didn’t get final planning permission. A bonus for us in money and time. When fitting a velux fold out balcony that spans the width of two rafters, what is needed to alter the structure but maintain structural integrity? I can’t find any information out here that explains this.
Thank you for this video. I've been looking to better understand the "how" of what this is. I'll get a structural engineer to do the calculations, but I feel better informed.
Thanks - that's very much appreciated! Glad you found the video useful!
Thank you
Thanks
Great, Thanks for explaining :) was thinking but wasn’t shore.I am not Joiner, self Trade will do my first big loft conversion for 2 bedroom place :)
On the comments I sow brown is steel, possible use thick timber instead? :)
Many Thanks
As a carpenter in Norway we avoid to convert a self bearing W roof to a A-roof. Something you never mentioned is that the yellow/broun bearing structure must be continued all the way doun to all the floors by building bearing walls that carry the load to the foundation. Another week point would be the pick of the roof, there must be a bearing beam as well suported on boath ends all the way down on gavel walls.
Hi. The brown structure are steel beams which bear onto the loadbearing gable walls.
Would be awesome to show adding a dormer in after doing this conversion
Hi Robin, Thanks for these amazing videos of the lofts, I've been watching your videos for almost a year. I think you have successfully changed my mind. I'm now in the stage of preparing my loft for conversion. Could you please help , should the starting point be calculating the beams? Do I need structure engineer to do that? what is the current most cost effective material; wood or steel? Thanks
Hi Sameh. Glad you liked the videos! Yes you will need a structural engineer for the loadbearing structure. The starting point should be working out your headroom and checking if you have any loadbearing internal walls. Then I would go with steel beams if you have that as an option, because most engineers can design those for you.
Regarding the long pieces you took out from the roof apex to the 'thin' floor - were these in tension?
Is this why it was okay to remove them?
So the roof would not drop at all when you take them out, and might the roof would go up a bit even as it was relieved of its duty of holding up the thin floor (the ceiling of the room below) ?
Great video! I’m just wondering how you would get the large beams up in the attic?
You would probably need to take some tiles off the roof and insert them from scaffolding.
Great video. I am curious though how would you go about similar if you wanted to open up the eaves also? So far it looks like a strong peak beam, so roof off job, any alternative options you can suggest?
Hi Marc. Yes it's quite different for a box dormer conversion. Give this a thumbs up if you would like to see this in another video 👍
Hi Robin please can you make a video how to make a dormer in the attic
Good timing - I've just done a dormer conversion video! czcams.com/video/BnlYpsZQKJI/video.html
Hi Robin, very good video. Very clear instruction. Similar to my current build... Q . My layout is 6m wide x9m long 45 deg roof pitch. No collar ties,instead 2 x rsj installed as ties spaced 3m apart , 6m long installed two rows of block down from wallplate. rafter ties 1/3 down from ridge board. 1.2 m knee wall above with struts every third stud tied back to wall plate. Is this sufficient to prevent wall spread?
Hi Denis. I am not able to give specific structural advice on this channel due to the complexities involved, but I would very much encourage you to get a qualified and experienced structural engineer take a look at that.
Trusses are also tied to each other with a combination of diagonal and horizontal bracing which distribute racking forces. How are these accommodated once the triangles are removed?
Hi Christopher. Good question! With a loft conversion you are adding plasterboard and floor boards. Also there will be rigid insulation between the timbers. Your structural engineer will consider this and make a judgement on whether this is sufficient.
Many thanks for this video. Found it really informative and very useful. I just have a question regarding strengthening the joists
I note that you only strengthen the beams in the middle of the new vertical roof supports
Do you not need to increase the strength of the joists that sit on the walls IE not in the “storage areas?
I’m ready to tackle this project myself as it no longer looks like a “black art”
The original “ceiling joists” which remain in the storage area are suitable for light storage, obviously these are not suitable as floor joists which is why they have been replaced in the habitable area with much larger joists
Great video thanks. looking at you original loft before converted, how much weight of storage could go in there? Also is it ok to put some boards as shelves sitting on the diagonal triangle uprights to take stored items? Hope you can help.
Typically they are designed with the absolute minimum weight criteria, in order to keep costs down, which means no storage.
Is it possible to raise the pitch of an existing roof without tearing the existing roof down and to just modify it?
Thank Crunchie I didn't employ the roofer who told me he could safely take away the rafters to give me more space without having to strengthen the rest of the roof. He said it was "over-engineered"! I believed him but didn't proceed in the end. Robin, thank you.
😅
I have a timber kit house with a truss span of 8.4m. I asked around last year for a structural engineer to start the process of conversion but gave up as I could get an engineer to have a look. I’ve looked at telebeam, ecotruss and the traditional timber way. Telebeam by far is most expensive. However I haven’t been able to get an estimate to do it this way in timber. Would you say timber would be less expensive than the ecotruss way? Cheers
Great video. I’d love access to this drawing please… link perhaps?
Great video Robin, I am looking to create a usable storage area as I don’t have the height for a proper loft conversion. The room will be finished with plaster board etc .Is the steel beam necessary and would we need to build the floor or could we lay the floor into the existing rafters ?
Plasterboard and storage will add extra weight which the roof timbers may not be designed to support. In most cases you would need the steel beams.
Hi Robin
Do have any hip to gable examples
What support the weight of the two new beams from below
if you used collar ties, could you do with out the extra rafter additions?
Question if my house has a Center loadbearing wall do I still need to beef up the floor?
Are those beams tooted into either side of the gables. Thanks for vid. Very informative.
Yes the beams are supported on the gable walls
Can you do one with two dormers added ?
great collection of videos.. Q. in your experience, do uk councils approve planning to raise the height of bungalow roofs? i need and extra 600mm just at the apex. or does it just depend on the council ?
Can you use RSJ’s with a modern timber framed house for a truss roof loft conversion?
Excellent video. Roughly how much does it cost for steel beams and a team to install them with joists abs rafters?
Costs vary greatly depending on size of property, loadings, geography, type of conversion, cost of steel, market forces.
I believe you would also need a ridge board/beam here as once the compression web is removed the tip of the roof is no longer supported.
Hi. Ridge beam not usually needed, but collar ties near the ridge are usually needed.
@@RobindeJongh collar ties would significantly decrease the headroom and I believe the purpose of these is to prevent roof separation under wind uplift. As they are in tension I am not convinced they provide adequate support which should be provided by spanning the ridge board between gable ends
@@RC-jx9nw Hi. Sorry I can't comment on your particular situation. In the example shown in the video no ridge beam is needed as the new steels and loadbearing studwork support the rafters.
@@RobindeJongh Hi thanks for the clarification.
Great video, really clear to understand. One question for me, is this the same method people would use if purely using timber for a conversion? I know almost every conversion uses steals these days and not sure if purely wood strengthening wood satisfy UK building regs. Cheers
For a totally timber conversion you would need a timber beam that could span a similar length to the steel, for which you would need a LVL or Glulam or plywood box beam, or timber truss. I like the box beam or truss method and often suggest that in loft conversions I design.
@@RobindeJongh great thanks, is there ever any way to have 2 joists making up that entire span, so having a join, say over lapping a load bearing wall and bolting the second joist onto the first at that load bearing wall point? Thanks for all the content you are providing, it’s very informative and made easy to understand
@@stilltime4fitness795 two beams bolted together over a loadbearing wall would work fine, if I'm interpreting correctly what you mean.
What do you think of the all steel floor joist and rafter support systems? I think it's called steel truss system. It uses thinner gauge steel, no timber studs, everything bolted together.
Check out this video for my review of a light steel system for loft conversions: czcams.com/video/AJ69IfyhOro/video.html
@@RobindeJongh brilliant thanks. Watched some of their videos. My main objection is one you pointed out, cold bridging but on the dormer construction. I hadn’t considered the areas you pointed out. It’s difficult to see how condensation will not form on the steels somewhere and I don’t know how galvanised sheet reacts to long term condensation.
@@RobindeJongh one thing, this system has to have every rafter trussed, just to carry the roof loads. But this means that every ceiling joist must also have a steel, even though one could probably get away with a truss every other joist and still achieve good support.
is a loft conversion steel beam method the same as making the walls 8 foot high with a new roof
Good informative video. Is the RSJ sitting on the original joists across the full span gable to gable or should it be raised up? I have a span of 6.2m so using 3 pieces bolted together per side. Thanks
It should have a gap to the top of original ceiling timbers, as it will deflect as the weight of the floor is applied to it.
I wonder how much the cost is to renovate an attic like this
What did you use to make this video , love the pictures
That'll be SketchUp (Trimble?), using the styles function to change the line type giving a more 'drawn-by-hand' effect.
Hi! What app did you use to do this?
No discussion of altering the space beneath to support all these beams and joists you added in?
We have a similar set up but with a centre vertical timber stud, we are not planning a loft conversion but one day perhaps a little den for the kids. Would the centre post mean its not possible to remove this safely and support in another manner?
Hi Brian. Anything is possible, but you would need a structural engineer to take a look at it.
Great video. Do you have to use steel if it’s just for a storage space?
I have a recent video on non steel options you may find useful.
how on earth do you get that size steel beam up into an attic? is it in sections and welded?? obviously case-by-case but generally speaking I'd love to know how its done. thx for vid!!
Hi. It's usually spliced and bolted together.
Would value your thoughts re structure design, and we are planning to consult a SE. We have 6mx6m carport, connected to house, same truss system.
There are 90x90x6 SHS SS posts at outside front corners of the fwd truss. I was thinking weld/fix same SHS between these. This would sit under existing fwd truss, the cement sheet ceiling, and a 90x35 batten - thoughts?.
IYO what size ‘brown beam’ should span from front of carport to house? Does it need to be steel, or can it be sistered LVL’s. Also, is a single brick veneer considered for structural support use? Hope I am not confusing. Cheers, Paul 👍
Hi Paul. I wouldn't be able to give you direct specific advice here, but you can get me on the following consultation form: forms.gle/4vfWnEvX14LVoTzc7
what are the stell beams sitting on and how did they get into the loft?
Beams are sitting on padstones in the gable walls. They can be spliced and manhandled into the loft, or inserted through a hole made in the masonry
Do steel beams not have to be used?
I'm curious do structural engineer charge to come look at attic and see if it could be converted? If yes how much? Also could they give estimate for the cost of the project?
Thanks
Hi Daniel. Structural engineers generally always charge for inspections, as they earn their money through hourly rates, similar to how lawyers or architects are set up. They would probably not want to estimate build costs, but once you have a structural design you could send that to builders to get some quotes.
The previous owners of my house have started a conversion but abandoned it halfway , they put floorboards on the floor , there are pieces of wood in place for the plaster boards, some boards are loosely in place should I get a structural engineer in to check it out as I can do the plaster boarding myself, I have an electrician and plumber I can use, I would like to do the work myself to save money, but I want to make sure it's safe to finish the project. Any idea how much an engineer would cost to have a look? Would they guide me as what I will need to do
HI. Definitely get a structural engineer to take a look. I've inspected "loft conversions" and found beams supported on windows and alls sorts of worrying issues. So get a structural engineer out - the fee will pay for itself.
Have you had any experience with using steel c purlins instead of a large rsj? Like the eco trus system?
Yes, a video about this is on my radar.
@@RobindeJongh be interested to hear your professional opinion on this system
See my Eco Trus review video here: czcams.com/video/AJ69IfyhOro/video.html
so where you have the steel beams , did they have to be steel ? or can they be in a thicker wood say ?
Hi Wayne. It's unlikely timber beams would be sufficiently stiff. You could go with a box beam, trusses timber, glulam, LVL.
Can the support beam running the length of the loft be in more than one section? I.e. 3 metre lengths joined?
If it's steel beams then usually yes.
Is it advisable/essential to insert a cross beam from left to right above head height on each rafter, I am flooring my attic, exact same structure as you have described but would love to create a larger room by moving joists. I'm concerned about downward forces pushing the rafters out from bottom (I hope I make sense). I'm no engineer, I'm just a tree surgeon...
Hi - Yes it normally needs tie beams near the ridge.
My loft is identical to this, i want to floor just the center area, by putting 4x2 down on existing joists, so i keep the 11inches of insulation, then loft flooring on top and then build a model railway, does this sound do-able ?
Thanks.
It sounds doable. You will need to have the steel beam support for the joists (in brown in the video) or Glulam/LVL/truss/box-beam.
Is there a horizontal kick on the edge wall now that you have removed the internal diagonals? Does the steel beam need to take that force as minor axis bending?
Hi. The horizontal reaction is resisted by the bottom chord of the truss which is still in place.
@@RobindeJongh where you are cutting out for the stairs. These would be removed? But I assume the trimmer would be designed to transfer this to the ties either side. Also what if there is a windward positive and negative leeward wind loads. The loading would not be balanced?
We had a purlin beam across the length of the top area where your connections are. That’s another way to do it? They were thick beams..
Hi Sean. Yes, can be done that way. You'd need to support the floor however.
@@RobindeJongh thanks for your reply. I guess that depends if there are joists or not.
What supports the new rafters at the apex? As you have removed the webs the old truss can not be considered structural in any way and cannot provide any stability at the apex?
There would be collar ties just below the ridge.
@@RobindeJonghWith a truss roof you will need a beam at the ridge.
@@MrJimjam2011 No ridge beam needed but it would normally need a collar tie near the ridge.
Can you use timber instead of the steel beam and would this steel beam need to sit on a wall, so a wall should be running below the steel or beam
Or can just it sit on top of the joists
Hi Robert. The steel sits on a wall at each end. For timber options see my other video: czcams.com/video/hgH9z_JJDXk/video.html
what program is used with the sketh here, pls? thank u.
It's SketchUp Pro.
Maybe stupid question but why do we need this steel beam? Can we attach those yellow timber stud to the existing horizontal wooden joists ?
Nothing's ever a stupid question, and I'm gad you asked! The timber joists are too slender to span the whole way so need to be supported by the steel beam. The same goes for the rafters.
@@RobindeJongh great video, my question is instead of(brown) steel beams could engineered timber joists be used? . Would this not be a more economical.?
@@patconnell2472 Hi Pat. Engineered timber beams could be used, or timber or steel truss, or a plywood box beam.
Convert truss into rafter?
Does the new joists(green ones) not meant to span the whole width of the house and rest on outside wall plate then the steel beams(brown ones rest on top for more structural strength)?
Hi Marc. No, it's the other way around. The new floor joists span from beam to beam, and the beams span from gable to gable.
@@RobindeJongh Yeah like Marc, I don't understand how the green joists add any strength. Aren't they just increasing the weight on those existing joists? What are they fastened to if not the existing joists?
@@karmabunnysprout Hi Darren. The existing ceiling joists span just 2.5m in this example, and are now spanning 5m. This means they are over stressed. The new floor joists are deep enough to span the 5m. They are supported at each end by the steel beam.
May sound silly but how is the steel beam supported? Surely it is just bearing on original floor joists? Or is it toothed into the external walls? Cheers, nice video and easy to understand visuals, have subbed 👌🏼
Thanks - glad you liked the video. The steel beams are supported at each end by the gable walls. For some larger properties they would need support also over an internal loadbearing wall.
Any suggestions for supporting the steel in timber frame semi D
@@nailbomber86 You may be better off with a glulam or LVL beam, truss or box beam.
Hi Rob,
My loft height from underside of ridge beam to top of loft floor joists is only 235cm. is there any way I could ever get a loft conversion out of this space without lowering the first floor ceiling?
Thanks.
With a rear dormer loft you could have at least half of the space to stand up in.
@@RobindeJongh Hi Rob, I'm having an L.shaped dormer conversion. The first steel can be hidden under the velux side. I was thinking is it possible to use acro props on the first floor then cut the loft joists to install the second steel close to the loft floor lathe then install the new joists with joist hangers and then fix the old joists to the new joists to keep the floor as thin as possible?
I'm designing one as an extension. if starting from scratch and no trusses , whats the best solution
Hi Wayne. This method with steel beams in the floor, or alternatively a steel ridge beam and purlins.
@@RobindeJongh Could we put steel gussets across the rafters at the top just below the ridge, the floor height falls 900 mm below the wall plate, can i email u , dont mind paying for calcs etc
Tasty enough lads to be fair. That said i would have had a continuous double top plate. Also i would have used gluelam beams instead of steel.
Hi if I was to replace 2- 1 3/4x9 1/4 LVLs with a 6x6 steel I beam on a 15ft span would that be sufficient? There was a wall taken down in the center of our house and had an engineer spec the LVLs but just want to go smaller bulkhead or even hide it. There is only 1 bedroom above this small area
Hi Jeff. Generally speaking a steel will be much smaller in depth than any other option. You would need to get it calculated by a structural engineer.
@@RobindeJongh thank you. I had an engineer spec the LVLs that are there. The steel H-beam I’m looking to go with is 2 inches shorter and will fit into the ceiling cavity.
HIi, i am wondering what 3d software yuo used in this presentation?
Hi - it's SketchUp Pro - see here: geni.us/sketchup
this progrsm name?
Thanks for this I was considering buying a house (1st time buyer) on the basis we could convert the loft which from my brief inspection appeared to have this very triangular style truss/rafter style. Robin, would you be able to suggest a rough estimate as to how much a conversion like in this video, might cost? Not to do myself (I'm terribly un-handy), but what might a standard conversion specialist quote? Would be v grateful for your thoughts.
Hi Peter - I'm afraid I can't, as figures vary greatly. You would need to get a quote from a loft conversion specialist.
So the brown beam is a steel beam?
Yes usually > geni.us/structural
My house is 8x11 meters and I have bearing walls in the middle so 4 meters. Do I still need to install steal beams
Informative video - thank you. I have, what is hopefully a quick question: Are all the beams (Green and Brown) steel? Are the studs (Yellow) wooden?
Green are timber floor joists. Brown, steel beams. Yellow timber stud wall.
@@RobindeJongh Thank you. That's a potentially huge steel beam! I assume you the beams are installed in sections? Also, if either end of the house are of a brick wall construction, do you tie it in to the wall... Or is it fine just sitting across all of the original beams?
@@rc21 For a home the size of the example used in this video, beams would be fabricated in several sections with splice connections, and there would need to be an intermediate supporting wall. I saw a loft conversion recently for a building about 15m long using this method.
I used Flitch beams in my loft conversion that I manafactured myself in the loft space, do some research because if your engineer allows it they can be much easier to work with :)
@@CucumberFanatic I've designed flitch beams often. Bear in mind they are a fraction of the strength of an I-beam or C-section for the same weight, so you are going to have to lug a larger weight of steel into your loft.