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An alternative to RSJ in providing joist support


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I will be starting a loft conversion project in a few months and considering if I can avoid using RSJs to provide support to new loft floor.

 

Nearly all loft conversion project video I've watched on youtube seem to use ridiculous sized RSJ sections which seem to be way OTT for the loads involved.

So I am try to figure out why light gauge galvanised steel joists are not used more often in traditional cut roofs loft conversion; it seems to be more popular option where the existing roof is a truss roof.

 

To me there seems to be a lot of benefits in using galvanised joists instead of RSJ:

  • Maximised loft floor to ceiling height as the galvanised joists can more or less be set within the existing floor/joist build up/void space
  • Less manual handling
  • Do not need to be pocketed into party walls; hence lessening the chance of cracking the walls of neighbouring properties 

 

I assume that the price must be the issue but at a glance I cant see there being a massive difference and I will put an estimate together of both methods before I choose which to use.

I think both Telebeam and Ecotruss will do Supply-Only of galvanised joist lengths, and I imagine that there are also some local steel supplier who could be even more competitive on price. .

 

Has anyone considered the above?

I'd appreciate to hear your opinion.    

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UC and UBs are used because engineers and builders are familiar with them. Look at universal columns rather than beams, they can provide the same load rating as a beam but for less depth. 

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4 minutes ago, Conor said:

UC and UBs are used because engineers and builders are familiar with them. Look at universal columns rather than beams, they can provide the same load rating as a beam but for less depth. 

I dont know, personally I cant see that being the reason; or if it is it should not be to my mind anyway!

 

Effectively beefing up the existing arrangement with (numerous and less loaded) galvanised section is far less of a technical challenge than specifying where much fewer and greater loaded UC/UB should be positioned.  

 

I can imagine there may be some situations where the additional loading cannot be carried by the existing load bearing walls  (typical those are the front and rear walls) and therefore transverse RSJs must be introduced to transfer the loading on the party and/or gable end walls 

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why do you need an RSJ? are there no supporting walls downstairs? here the requirement can be as easy as increasing the depth of the joist by gluing and screwing a suitable depth of timber to the top of the existing joist.

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1 hour ago, Annker said:

I will be starting a loft conversion project in a few months and considering if I can avoid using RSJs to provide support to new loft floor.

 

Nearly all loft conversion project video I've watched on youtube seem to use ridiculous sized RSJ sections which seem to be way OTT for the loads involved.

So I am try to figure out why light gauge galvanised steel joists are not used more often in traditional cut roofs loft conversion; it seems to be more popular option where the existing roof is a truss roof.

 

To me there seems to be a lot of benefits in using galvanised joists instead of RSJ:

  • Maximised loft floor to ceiling height as the galvanised joists can more or less be set within the existing floor/joist build up/void space
  • Less manual handling
  • Do not need to be pocketed into party walls; hence lessening the chance of cracking the walls of neighbouring properties 

 

I assume that the price must be the issue but at a glance I cant see there being a massive difference and I will put an estimate together of both methods before I choose which to use.

I think both Telebeam and Ecotruss will do Supply-Only of galvanised joist lengths, and I imagine that there are also some local steel supplier who could be even more competitive on price.

 

Has anyone considered the above?

I'd appreciate to hear your opinion.    

I use cold formed C sections from time to time. Couple of examples.

 

One was loft conversion with rear dormer was really tight for height. Could not go above the ridge line and had to maintain 2.0m ceiling height on the inside. Eventually after exploring a good few options I plumped for a 140mm deep cold formed cee section with insulation on top.. a warm roof so the steel is within the the insulation envelope thus avoiding many condensation issues. In many ways it was the geometry of the conversion that forced this solution, tricky connection details, other ventilation issues and these kept closing down the more convensional solutions you see on many attic conversions.

 

Another example.. Client requirement to minimise disruption to the floor below and maximise attic floor to ceiling height. Need to span some 5.0m with some relatively big point loads from dormers.. and the structural walls below are all in the wrong place. The existing attic joists are 200mm deep with lath and plaster ceilings.. that must be kept intact.. no cracking! The existing attic  joists are supported by transfer beams mid span. See some posts on BH where the roofs have a purlin supporting the rafters. The transfer beam is just like the purlin but at attic floor level. The problem is that this transfer beam has to go as if not it will end up in the middle of the new floor.

 

To overcome this shallower back to back cold formed Cee sections between the existing joists to span the 5.0m are introduced. This works well as the cee sections clear the lath and plaster and leave room to run electric cables and small bore pipes. Also, as the walls below are not in the right position the cees in places function as cantilevers.

 

Now this all sounds awfully complicated but it's not. The sections are straight out of say the Albion / Steadmans brochure. One key here is also to use their standard punching pattern for bolting. Remember that these sections are used by the kilometer in industrial buildings all the time. One thing though is that you have to do your own calcs for these when say using them as cantilvers and off piste applications.. and the calcs can be very lengthy and time consuming unless you are familiar with cold formed steel design.

 

Lastly to get the best out of you local builder it's good to provide the fabrication drawings for your cold formed sections in say the Steadmans / Albion , other formats. All the builder has to do is then send the drawings to the supplier. This is standard practice in the industrial sector. Often a local builder who just does say extensions is less familiar with cold formed steel in these applications , but if you hold their hand a bit they often engage.

 

@Annker Keep exploring this as on option, even if to rule out at the end of the day.

 

 

 

 

 

 

 

 

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3 hours ago, Annker said:

I will be starting a loft conversion project in a few months and considering if I can avoid using RSJs to provide support to new loft floor.

 

What size and span are the joists? Sistering joists with deeper ones done correctly can also decouple floor loads from the existing ceiling joists reducing or eliminating plaster cracking. 

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50 minutes ago, Temp said:

 

What size and span are the joists? Sistering joists with deeper ones done correctly can also decouple floor loads from the existing ceiling joists reducing or eliminating plaster cracking. 

I suppose it's a question of terminology. Some folk think that sistering means doubling up the joists and connecting the two together along their length so you create a joist twice the thickness of the original one out of two bits of timber coupled (bolted say) together (the old and the new).. thus roughly you can double the load without a bad outcome. But remember that the old timber will have creeped a bit and seasoned so will probably pick up a bit more load first before the new "bendy" timber starts to pull it's weight. Something to consider if you have a period ceiling below.

 

Or sistering can maybe interpreted as adding another joist in between the existing? making it a bit higher and thus you create a floating floor..have done this where you really decouple the new floor / attic loads from a ceiling that is sensitive, say an ornate ceiling.

 

There is quite a lot to think about when doing loft conversions. Often you don't want to mess with the floor / ceiling below as you are living there. Now a plasterboard ceiling often has dwangs / noggins so to sister (doubling up) you need to cut them, to get your new timbers in at the same depth as the existing joists.. and refix the dwangs / noggins. A bit of a guddle (faffing about) = £cost. Often you find that there are cables, a few pipes etc and to keep the cost down it's helpful not to have to move them, especially over the consumer board area if a lot of cables run up into the loft.

 

A lathe and plaster ceiling relies a lot on the plaster squeezing up between the lathes and expanding (belling out) to create a mechanical key. Call these the snotters. Would be interested to hear what the local term is in different parts of the UK. 

 

If you knock them off then the plaster will become loose , particularly as the timber lathes will have shrunk over time.

 

Often it's best to have a look at what you have early on; can you move out, do you have period features etc and then see how you do it structurally to accommodate the constriants.

 

 

 

 

 

 

 

 

 

 

Edited by Gus Potter
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41 minutes ago, Gus Potter said:

Or sistering can maybe interpreted as adding another joist in between the existing? making it a bit higher and thus you create a floating floor..have done this where you really decouple the new floor / attic loads from a ceiling that is sensitive, say an ornate ceiling.

 

Yup, that's what I meant. Effectively a new set of deeper joists raised up off the plasterboard so they don't put any load on it when deflected. Only bolted to the existing joists where they are supported on walls not in the middle of spans. 

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Yes I read it that way, the intent is to decouple to protect the ceiling. The term sistering can be interpreted in different ways... then the different types of connections take on a life of their own! All good fun though.

 

Converting lofts can be quite a challenge! .. but satisfying.

3 hours ago, Simplysimon said:

why do you need an RSJ? are there no supporting walls downstairs? here the requirement can be as easy as increasing the depth of the joist by gluing and screwing a suitable depth of timber to the top of the existing joist.

Simplysimon has great point. This can be done provided you have the height to play with in the loft. It can work structurally but here is the rub.

 

While you can as a designer can prove it all works and produce a specification, in real life unless you know the Client is going to need to have you supervise the work on a daily basis as the chances of a builder doing it properly are low.. and that is a safety risk. Thus I often avoid this elegant and economic approach for safety reasons on domestic projects.

 

I do have a pal that wants to convert his attic and this is something we will look at as I know he will be absolutely dilligent and do it correctly.

 

The secret and elegance in Simplysimon's potential solution lies in the concept that the screws only provide the compression of the longitudinal joint while the glue cures. After that the screws become essentailly redundant. For the glue to work the faces of the timber need to be a little rough, flat for a good mating surface and absolutely clean. The glue needs to be a structural glue such as Cascamite two part resin. What we do (and now have) is to create a 2 ply glulam beam and use the same analysis techniques.

 

 

 

 

 

 

 

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6 hours ago, Annker said:

ridiculous sized RSJ sections

If you use an SE he can iterate until the best section is obtained for your purpose, whether that be cost /depth/availability. But usually people want the lowest (or no) fee so they get a safe size

 

3 hours ago, Gus Potter said:

The sections are straight out of say the Albion / Steadmans brochure.

Cold rolled is easier to work with as it can be cut and screwed with everyday equipment, and is lighter to handle.

If you need only a small number, then the local agricultural merchant will have some in stock.

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34 minutes ago, saveasteading said:

If you use an SE he can iterate until the best section is obtained for your purpose, whether that be cost /depth/availability. But usually people want the lowest (or no) fee so they get a safe size

 

Cold rolled is easier to work with as it can be cut and screwed with everyday equipment, and is lighter to handle.

If you need only a small number, then the local agricultural merchant will have some in stock.

Plus One

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Thanks for the response, apologies to all for the late reply!

 

To @Simplysimon and @Gus Potter If you had told me a few months ago can existing joists be strengthen by gluing and screwing addition joists on edge I would have ?. However I have recently discovered this can work in structural terms at least. I watched an analysis of the performance of this connection here https://www.youtube.com/watch?v=rgsdJ4zF8DA&t=148s, Fischer have even developed software to model the performance  https://www.fischer.co.uk/en-gb/service/design-software-fixperience/wood-fix.

Now this connection may perform structurally, however given the likelihood that the existing joisting may have crept and bowed down any timber fixed to those existing bowed joist will assume a similarly bowed top plane; and thence an uneven floor level.

I imagine that may not be an issue if the loft space is converted solely for storage but may be unacceptable if the loft space is converted for habitation and associated floor coverings.  

You could over size the additional planted on timber and plane down to a level datum but that would be a painful job and best avoided if at all possible.

And as as @Gus Potter highlighted with this method there is less opportunity to decouple the new floor above from the old ceiling below and thus reduce acoustic transfer and risk of cracked ceiling below. 

 

To @Temp and @Gus Potter yes you could sister in the C-sections and that would be my preference.

To isolate form the exisitng plastered ceiling 1" packing pieces tacked on to the wallplates at either end and any rising stud walls from below where necessary.

But I would also try to isolate the C-sections completely from the existing joist, The C-sections would be suitable sized to span independently without any connection to the existing joist and in the absence of joist bridging a structural subfloor could provide support to ensure the C-sections remain upright on edge. 

 

However after thinking out how the C-sections would be practically installed I think biggest challenge (and probably the reason that they are seldom use in loft conversion) is as @Gus Potter mentioned; existing joist bridging's, ceiling joist runners and existing services will likely be laid at or on existing joist level.

I imagine it would be extremely difficult to remove those timber members for the ceiling joist without cracking the underlaying ceiling and even if you did managed to remove them what then replaces them to provide the support to the existing ceiling joist, will there even be scope to install anything given that C-sections will also be introduced into the void space.

And then rerouting any services that are similarly within the ceiling joist void space will also need to be done.

 

So to answer the my initial question!

WRT loft conversation: at first impression, construction of an entirely new floor structure and the installation of cumbersome RSJs to provide support for those joists seems much more work than running in C-section joists however being constructed above and entirely independent of the existing ceiling joist arrangement it avoids the awkwardness of working in and around that existing structure.   

    

 

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  • 1 month later...
On 16/01/2022 at 20:09, Gus Potter said:

I use cold formed C sections from time to time. Couple of examples.

Hey @Gus Potter do you have an engineer that you work with and can recommend for this?

 

The typical SE will usually opt for steel beams and timber but I'm keen to work with steel sections on an upcoming build.

 

 

 

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Just visited a mate doing a loft conversation, he has used gluelam beams instead of steel, they are huge 450x150 , but there is no need to noggin out any steel to fit joists, it all nails on the sides with hangers. 

Looked like a lovely job. 

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I am a bit late to this thread but I have done quite a few loft conversions and apart from ridge beams have always avoided steels. I like glulam, easy to fix too but larger than steel. I don’t like sistering ceiling/floors joists and prefer additional larger floor joists in between ceiling joists, that way noise is transfered less into the rooms below.

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3 hours ago, Russell griffiths said:

Just visited a mate doing a loft conversation, he has used gluelam beams instead of steel, they are huge 450x150 , but there is no need to noggin out any steel to fit joists, it all nails on the sides with hangers. 

Looked like a lovely job. 

Sounds massive!

 

How has he fixed the glulam into the walls? And was there any specific reason he didn't go for steel?

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1 minute ago, mrmckenzie74 said:

Sounds massive!

 

How has he fixed the glulam into the walls? And was there any specific reason he didn't go for steel?

They are a bit big, but once in that’s it, with steel you need to fix floor joists into the way of the steel, that’s a pain. 

Or pad the steel out with timber then fit hangers, with the glue lam you cut out a few steps. 

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11 hours ago, mrmckenzie74 said:

Hey @Gus Potter do you have an engineer that you work with and can recommend for this?

 

The typical SE will usually opt for steel beams and timber but I'm keen to work with steel sections on an upcoming build.

 

 

 

 

Gus is the engineer!

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Great comments from all. Interesting to read all the views. For the budding loft converters a few observations.

 

As a designer your starting point is to establish what folk want out of their conversion, what will meet their primary needs and then, aspiration.

 

Next is to get down to brass tacks. You have planning constriants; ridge height, eaves alterations, what you can project from the roof.. dormers or Velux or in some conservation areas.. mme.

 

From a BC compliance point you look early on at how you get a stair up and fire protection. A critical dimension is to see if you can get the clearances for the stair in terms of head height. Lastly you have a look at what kind of headroom you can achieve in the rest of the proposed converted loft. If your Clients are all six foot six plus then you need to say.. hey you are quite "blessed" but you ain't going to enjoy this even if you comply with the regs. On the other hand you can be "blessed" if you are of shorter stature. Everyone on BH is blessed!

 

Next you delve a bit more into how you may insulate, do you need to replace the roof, get any drainage to work and so on. You look for booby traps that could burst the ball.

 

Now have a look downstairs and where you can support load by means of load bearing walls. What are the Client constraints... do they want to live in the house or move out say. This then gives you a flavour of what to explore structurally. Once you have got this basic information you open your SE tool box and see what will work best, not just in terms of pure structural design but also what will best fit the local contractors. Good design is also about designing something that is elegant and buildable at a reasonable cost. That is the art of structural design.

 

Next you iterate and go back to the Architectural side of things and this lets you design Architecturally in the knowledge that what you are doing has a sounds structural footing.

 

Now you may then want to use Glulam beams..maybe an oak ridge beam for a feature if you have enough height, use sistered joists, steels, cold formed steel.. sometimes very little structural "extras" are required and this feeds into the Architectral design. which is the bit you see at the end of the day.

 

In summary your starting point should be as above and as each attic and floors below are often different it is almost impossible to say "this is the way".

 

If anyone wants some pointers then the best thing to do is to post some sketches thah show for example the span, height to the underside of the ridge, walls below. Rough it out on a bit of A4, take a photo and post. Don't worry if they look a bit rough.. you want to see mine!

 

Lastly while some materials may best on paper to use.. say steel, glulam, cold formed steel the design decisions can be driven by the type of builder you have available. Say you know a great joiner that buys a lot of wood and gets a good price. You may want to play to their strengths and use Glulam that they can buy using their regular account, rather than steels that they may have to pay "punters" prices for. At the end of the day this can work out cheeper for you.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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13 hours ago, Gus Potter said:

You may want to play to their strengths

Not just buying power, but familiarity with the material. Steel does mix with timber, but you can't just  shave a bit off, bang a few nails in 'for now' or adjust.

Then there are pad-stones and possibly bolted joints for the steel, so the joiner may want to get a general builder in. In summary I love steel but like timber buildings to be made of timber.

This especially the case if there are hips or valleys....steel is horrible for this, as it is not forgiving.

 

As Gus says 'then you iterate'. Never forget to go back to the original issue and check it hasn't gone off  track....this is worth a lot of money and quality.

Well said Gus. An Engineer (or other professional) who iterates and thinks of the client's needs.

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  • 1 month later...
On 16/01/2022 at 18:58, Simplysimon said:

why do you need an RSJ? are there no supporting walls downstairs? here the requirement can be as easy as increasing the depth of the joist by gluing and screwing a suitable depth of timber to the top of the existing joist.

Not sure this conforms to independant floor requirements in building regs. Had  lot of trouble doing my last loft conversion bco insisted independent floor, insulation then fireboard inserts and sealed with intumescent filler. 

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