SE specified C24 6.1m * 47 * 225mm for garage roof

[flanagaj]

The SE we used for designing our cut roof specified C24 47mm x 225mm for a 5.9m span for a flat garage roof.  The timber which arrived was rather crap, not straight and twisted.  Whilst doing some googling, I looked at span tables and all of them are saying that even at 400 centres the maximum span is ~ 5m.  I have tried contacting said SE a few times and she is on radio silence.  I am getting concerned now.   How do I double check that she is a registered SE as she has made a number of mistakes now.

[Roger440]

Im no SE, but 6m sounds a rather long span for 225mm.

[Oz07]

Yeh I'd probably pause before putting these on. I have a similar issue with 6m garage flat roof. When I eventually found a span table I think it ended up being something like 70x250 or 225 c24 at 450 centres for 6m garage roof span. 

 

I'd want to speak to said SE and query it. 

[JohnMo]

1 hour ago, flanagaj said:

The SE we used for designing our cut roof specified C24 47mm x 225mm for a 5.9m span

Maybe use posi joists - ours are 256mm our kitchen span is about 7m (12 Deg angle) and at 600mm centres

[-rick-]

1 hour ago, flanagaj said:

The SE we used for designing our cut roof specified C24 47mm x 225mm for a 5.9m span for a flat garage roof.  The timber which arrived was rather crap, not straight and twisted.  Whilst doing some googling, I looked at span tables and all of them are saying that even at 400 centres the maximum span is ~ 5m.  I have tried contacting said SE a few times and she is on radio silence.  I am getting concerned now.   How do I double check that she is a registered SE as she has made a number of mistakes now.

 

Before engaging her you asked for a copy of her credentials and insurance right?

 

What does she claim to be on her communications/advertising? If she claims to be a Chartered Structural Engineer then this is the professional body that you can check with:

 

https://www.istructe.org/

[flanagaj]

14 minutes ago, Oz07 said:

Yeh I'd probably pause before putting these on. I have a similar issue with 6m garage flat roof. When I eventually found a span table I think it ended up being something like 70x250 or 225 c24 at 450 centres for 6m garage roof span. 

 

I'd want to speak to said SE and query it. 

I'm concerned.  Said SE hasn't replied to my emails.  Not even the one regarding her issuing the invoice.  She was originally tasked with designing a suspended staircase, but after two months of continually asking, I eventually stopped.  The cut roof / garage was another job I asked her to do.  Her site states she's a civil engineer, but I am thinking I need to validate that is the case.

 

I've just reached out to a local firm for i-joists as I think these are cheaper and I don't need to run lots of ducting.

[flanagaj]

3 minutes ago, -rick- said:

 

Before engaging her you asked for a copy of her credentials and insurance right?

 

What does she claim to be on her communications/advertising? If she claims to be a Chartered Structural Engineer then this is the professional body that you can check with:

 

https://www.istructe.org/

No I did not.  Agreed.   Naive and stupid.

[-rick-]

2 minutes ago, flanagaj said:

Her site states she's a civil engineer, but I am thinking I need to validate that is the case.

 

In that case the reg body is: https://www.ice.org.uk/

 

This is all very clearly stated on the government website. All I'm doing is googling it for you.

 

All these orgs will likely do is be able to confirm that this person is qualified/has active registration.

 

You really need to check the insurance of the professionals use because if something goes wrong that's what you will be relying on. Mortgage companies, buyers will care about the qualifications.

[flanagaj]

3 minutes ago, -rick- said:

 

In that case the reg body is: https://www.ice.org.uk/

 

This is all very clearly stated on the government website. All I'm doing is googling it for you.

 

All these orgs will likely do is be able to confirm that this person is qualified/has active registration.

 

You really need to check the insurance of the professionals use because if something goes wrong that's what you will be relying on. Mortgage companies, buyers will care about the qualifications.

Thanks.  I've been very stupid here.   I've just gone over the T&C's and there is no mention of her active registration.  But I'm assuming that just like an individual cannot call themselves a Solicitor or Architect unless there are registered with the associated body.  The same applies for an SE.

[-rick-]

2 minutes ago, flanagaj said:

Thanks.  I've been very stupid here.   I've just gone over the T&C's and there is no mention of her active registration.  But I'm assuming that just like an individual cannot call themselves a Solicitor or Architect unless there are registered with the associated body.  The same applies for an SE.

 

I'm at the end of my knowledge. Hopefully one of the professionals on here will speak up with more details. Everything I've put above can help you verify Chartered engineers but don't think you need to be chartered to practice so long as qualified and that might be an area that's a little harder to check.

[ETC]

https://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletD2012.pdf

[Mr Punter]

I doubt you will find longer than 6.0m.  Can you break the span with a steel or flitch beam?

[Gus Potter]

2 hours ago, flanagaj said:

The SE we used for designing our cut roof specified C24 47mm x 225mm for a 5.9m span for a flat garage roof.  The timber which arrived was rather crap, not straight and twisted.  Whilst doing some googling, I looked at span tables and all of them are saying that even at 400 centres the maximum span is ~ 5m.  I have tried contacting said SE a few times and she is on radio silence.  I am getting concerned now.   How do I double check that she is a registered SE as she has made a number of mistakes now.

 

42 minutes ago, -rick- said:

I'm at the end of my knowledge. Hopefully one of the professionals on here will speak up with more details. Everything I've put above can help you verify Chartered engineers but don't think you need to be chartered to practice so long as qualified and that might be an area that's a little harder to check.

1 hour ago, Roger440 said:

Im no SE, but 6m sounds a rather long span for 225mm.

 

Hope this helps lift your spirits a bit. The following is a bit of "chat", but also serious at times. 

 

Firstly. Well done you for looking up some span tables, what you are doing is carrying out a "sense check", you've clocked that the joist span looks out with normal expectation. I agree you are right to question. Also @Roger440, no dafty is asking.. and I'm pretty sure @saveasteading, @JpManning et al may bring professional opinion to the table. 

 

The thing that disappoints me most is this radio silence you are experiencing. There is generally no excuse for this. This is a common complaint on BH, Engineers not communicating or just being downright arrogant. 

 

But on the other hand your Engineer may have fallen unwell, this happens more often than you think. I have a long standing Contractor on my books who is mentally unwell, I'm supporting him. His business is suffering, he is a sole company director and employs folk that rely on him for their income. It may be your Engineer is unwell? 

 

That said SE's in particular seem to be very poor when communicating with Clients. It's actually a core objective of the IStructE, ICE to communicate with your Clients. I would try and determine if this lack of communication is due to say ill health. You'll maybe feel bad if you give them pelters and then find out they are on their death bed. 

 

Ok your joist span looks.. questionable. But your Engineer may have taken the view that it's a garage thus, no brittle finishes on the inside, thus we can tolerate extra deflection, so long as the deflections are not too big we can handle that. If deflections get too big then they can introduce what we call significant second order effects. Us folk with knowledge of structural design get wary about second order effects when we relax standard deflection limits. What your Engineer may have done is to just to design the timbers for strength alone, relaxed the deflection limits,  as it's a garage. But if I adopt this approach I'll always tell the Client I've done, why I've done it, get their approval and check to see if these second order effects are significant or not. 

 

Now if you have a flat roof on a garage and you can push the spans. Yes, the timbers may be strong enough (just to not break and fall down) , but if they deflect in the middle too much then we have to be very careful the fall is still enough so we don't get ponding of water or snow melt on the roof as this can be dangerous. 

 

I would try as best you can to establish communication with your Engineer. Then you can ask what they were thinking was when they designed your garage roof. The may say, I relaxed the deflexion limits to save you money, but neglected to seek your approval for this. 

 

 

 

 

 

[saveasteading]

13 hours ago, flanagaj said:

The same applies for an SE.

Even more so. 

What do they call themselves?

Change the name of course.

Be precise with capitalisation.

 

Then we can advise better without assumptions.

 

Eg  J. Bloggs, Structural  Engineer and Building Technician, BSc MIStructE.

 

It is a question their insurer must have asked.

[ToughButterCup]

You might consider asking for evidence of her Professional Indemnity Insurance...... maybe things have got past that stage  - just a thought

[flanagaj]

I have asked the SE for her PI insurance and have finally managed to get her to answer the phone and agree to a site visit.  Builders are not happy with the arrangement of the separated internal (C section) and external (Non equal angle) lintel specification.  The two sections are not jointed together (yes, it mitigates cold bridging), but builders have told me that when they recently did the same thing on another job, the brickwork above the lintel all cracked.

Having been outside to look, it's quite easy to see how the external L lintel just wants to topple backwards into the cavity. 

[Mr Punter]

58 minutes ago, flanagaj said:

Having been outside to look, it's quite easy to see how the external L lintel just wants to topple backwards into the cavity. 

 

I have seen these where they are bolted together on site.  One of the lintels has some tube / chs welded and the 2 are bolted together.  Needs SE design.  If the cavity is 150mm or less you can use the HiTherm lintels.  Expensive and long lead time on some, but good psi / thermal bridging characteristics.

[flanagaj]

1 hour ago, Mr Punter said:

 

I have seen these where they are bolted together on site.  One of the lintels has some tube / chs welded and the 2 are bolted together.  Needs SE design.  If the cavity is 150mm or less you can use the HiTherm lintels.  Expensive and long lead time on some, but good psi / thermal bridging characteristics.

We have used the HiTherm type lintels on the < 2.7m spans, but for the 4.8m / 5.7m openings we had to conventional steels.  Hoping the SE can come up with a solution and I can try and get a metal fabricator to site to perform the required welding.   

[Temp]

A steel midspan might allow the existing timber you have purchased to be used. 

[flanagaj]

3 minutes ago, Temp said:

A steel midspan might allow the existing timber you have purchased to be used. 

That would have worked if it had been designed in at the outset, but that will mean changing the orientation of the joists which in turn will probably mean that the inner leaf steel for the large garage door opening is now not adequate enough to support the loading of the joists that are not resting on the blockwork above.

 

[Tony L]

The calcs need to take account of what's going on top of the rafters, of course. I was having a Gemini AI chat about a workshop with a flat roof & proposed 6.5m span, about a week ago.  Gemini recommended reducing the width to 6m internal so I could use 47 x 220 C24 at 400 centres to save a load of money against the next size up, in C24 spec' (THIS WAS MIS-INFORMATION). The same chat gave this link for a span calculator, which shows that 47 x 220 will not be good enough for a 6m span. https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=0.75&class=C24

 

 

[ToughButterCup]

17 hours ago, Tony L said:

The calcs need to take account of what's  [...]  

 

Gemini recommended   [...]  (THIS WAS MIS-INFORMATION).

 

 

Thanks @Tony L for drawing attention to the erroneous information from Gemini.

 

I use the same AI source a great deal. The answers to my questions are often simply wrong.

But it's correct often enough to keep my custom.

Annoying, but useful because simple acceptance of the answers given is not an option.

 

[JohnMo]

9 minutes ago, ToughButterCup said:

The answers to my questions are often simply wrong.

I find the same using Chat got, can talk utter nonsense and really contract itself - And AI is set to take over the world.  

[Gus Potter]

On 09/10/2026 at 11:48, flanagaj said:

I have asked the SE for her PI insurance and have finally managed to get her to answer the phone and agree to a site visit.  Builders are not happy with the arrangement of the separated internal (C section) and external (Non equal angle) lintel specification.  The two sections are not jointed together (yes, it mitigates cold bridging), but builders have told me that when they recently did the same thing on another job, the brickwork above the lintel all cracked.

Having been outside to look, it's quite easy to see how the external L lintel just wants to topple backwards into the cavity. 

That’s good news the SE is coming to site. It's also very encouraging to see your builder chipping in with their concerns, demonstrates competency. 

 

From time to time I use an unequal galvanised angle to support an outer leaf.  They are prone to torsion and warping. Just as an aside, the angle is galvanised and the internal I beam is not. If there is too much moisture floating around the angle will become a sacrificial anode. Thus we could electrically decouple the angle an internal steel. Or we could paint the angle and make sure the drip above keeps it dry and protects the paint. 

 

I tend to plump for an I beam on the inside, especially if supporting floors, heavy roof loads down the inner leaf for example.  You can use Cee sections, but again they are prone to twisting, torsional effects. Going into a little more detail. An I beam has it's centre of gravity in the middle of the section. There is a also a thing we call the shear centre. For an I beam the shear centre coincides with the centre of gravity.  But on a Cee section the shear centre lies outwith the actual section profile! If you load a Cee section in the middle of the top flange it immediately twists. Now we can actually take advantage of this behaviour by making sure the floor joists for example frame into the web side, the flanges face towards the outside. But this makes it a bit harder to insulate as the inside of the Cee faces into the cavity. 

 

Below is an example where I'm using an angle on the outside and an I beam on the inside to support floor and roof loads. This project had a fair bit of steelwork thus the method I adopted of coupling the angle and internal I beam didn't add much to the over all cost. There are three thermal bridges, one at each end and one in the middle, but the coupling plates are thin and the work was to an existing build. Yes technically, thermally not perfect, but is does the job in the round. 

 

 

Part of the 3D steel model:

 

 

There are three plates called F4. One at each end and one in the middle. The first detail shows the DPC arrangement etc.

 

The next two show a plate called F5 welded to the back of the angle and a plate F4 welded to the I beam. F5 has a slotted hole to allow for tolerance. 

 

 

Below is a diagram of the shear centre of a Cee section: The shear centre is marked SC, the centre of gravity marked C.

 

 

@flanagaj It might be worth you reading around this a bit (including the finish on the angle, galv or painted) and then ask your SE about it when they come to site so they can explain to you and the builder why they have designed what they have and explain to you in lay terms how it works. Well, lets hope they can answer your legitimate questions. 

[flanagaj]

5 minutes ago, Gus Potter said:

That’s good news the SE is coming to site. It's also very encouraging to see your builder chipping in with their concerns, demonstrates competency. 

 

From time to time I use an unequal galvanised angle to support an outer leaf.  They are prone to torsion and warping. Just as an aside, the angle is galvanised and the internal I beam is not. If there is too much moisture floating around the angle will become a sacrificial anode. Thus we could electrically decouple the angle an internal steel. Or we could paint the angle and make sure the drip above keeps it dry and protects the paint. 

 

I tend to plump for an I beam on the inside, especially if supporting floors, heavy roof loads down the inner leaf for example.  You can use Cee sections, but again they are prone to twisting, torsional effects. Going into a little more detail. An I beam has it's centre of gravity in the middle of the section. There is a also a thing we call the shear centre. For an I beam the shear centre coincides with the centre of gravity.  But on a Cee section the shear centre lies outwith the actual section profile! If you load a Cee section in the middle of the top flange it immediately twists. Now we can actually take advantage of this behaviour by making sure the floor joists for example frame into the web side, the flanges face towards the outside. But this makes it a bit harder to insulate as the inside of the Cee faces into the cavity. 

 

Below is an example where I'm using an angle on the outside and an I beam on the inside to support floor and roof loads. This project had a fair bit of steelwork thus the method I adopted of coupling the angle and internal I beam didn't add much to the over all cost. There are three thermal bridges, one at each end and one in the middle, but the coupling plates are thin and the work was to an existing build. Yes technically, thermally not perfect, but is does the job in the round. 

 

 

Part of the 3D steel model:

 

 

There are three plates called F4. One at each end and one in the middle. The first detail shows the DPC arrangement etc.

 

The next two show a plate called F5 welded to the back of the angle and a plate F4 welded to the I beam. F5 has a slotted hole to allow for tolerance. 

 

 

 

 

Below is a diagram of the shear centre of a Cee section: The shear centre is marked SC, the centre of gravity marked C.

 

 

@flanagaj It might be worth you reading around this a bit (including the finish on the angle, galv or painted) and then ask your SE about it when they come to site so they can explain to you and the builder why they have designed what they have and explain to you in lay terms how it works. Well, lets hope they can answer your legitimate questions. 

Thanks @Gus Potter the SE was as much use as a chocolate teapot.  She listened to the builder and basically said "what would you be happy with?.  I too was also asked". I replied "You're the SE, I think you need to tell us".   So apparently, it's 60mm box section welded at 600 centres, to join tHe two sections together.  I'll of course get her to provide a drawing.