Steel frame portal garage.

[Kevan Marshall]

I am in the process of fabricating my portal steel frame garage, architect has mentioned that due to being more than 30 mtrs square and being about 600mm from the neighbouring boundary it needs to have minimal fire rating of half an hour, I was planing to clad with 40mm PIR insulated sheeting but this just falls under the 30 minute rating, purlins are steel and UB’s are twice as heavy as they need to be but that’s engineers for you, does anyone know of anyone that can supply insulated cladding with rockwool instead of PIR or maybe have another solution such as a cement board between cladding and purlins, any help would be greatly appreciated!

TIA.

[Gus Potter]

57 minutes ago, Kevan Marshall said:

but that’s engineers for you

Call your SE and they may fix your dilemma. Why do you think the steels are oversized? 

 

What you are doing is actually quite complex and fraught with difficulty in terms of the fire design. Your SE is best placed to help.

 

 

[Kevan Marshall]

1 hour ago, Gus Potter said:

Call your SE and they may fix your dilemma. Why do you think the steels are oversized? 

 

What you are doing is actually quite complex and fraught with difficulty in terms of the fire design. Your SE is best placed to help.

 

 

7 meter span, stanchions are 254 x 147 x 30, trusses are 203 x 133 x 31,  haunches are nearly one meter long, SE goes crazy with some things and not with others, cross bracing on stanchions states 50mm x 5mm flat bar and roof bracing 48.3mm tube, it’s all about liability and everyone afraid to sign things off unless everything is over the top, steel frame was going to cost me £8800 plus vat plus delivery with this weight of portal frame, when I priced it before same as ag spec it was £3200 all in, I may have to move the garage so it’s one meter from boundary but then it is closer to my house..

[Gus Potter]

On 02/06/2026 at 23:10, Kevan Marshall said:

7 meter span, stanchions are 254 x 147 x 30, trusses are 203 x 133 x 31,  haunches are nearly one meter long, SE goes crazy with some things and not with others, cross bracing on stanchions states 50mm x 5mm flat bar and roof bracing 48.3mm tube, it’s all about liability and everyone afraid to sign things off unless everything is over the top, steel frame was going to cost me £8800 plus vat plus delivery with this weight of portal frame, when I priced it before same as ag spec it was £3200 all in, I may have to move the garage so it’s one meter from boundary but then it is closer to my house..

Hiya. To provide a bit of context, much of my initial SE training and Masters research was on portal frames. I know enough to make a contribution on this type of design. 

 

Here is my offer. You can call me on the phone on (mobile number removed, PM for contact details) for a 15 -30 min chat. It's free for BH folks! This forms part of my pro bono work that is good for my soul, my primary qualification is in Civil Engineering, you work for the public. 

 

Text me first so I know it's you. I'm pretty deaf so sometimes miss calls. I use my own name and can be easily found on the internet.. I get some interesting calls and offers, these range from "massages" to lots of "financial" offers and other "stuff" that actually breaks up my day, hence me filtering calls. 

 

The section sizes you quote might be dependent on your eaves height, wind loading and the types of finishes. 

 

That the best I can do for now.

 

 

[markc]

That’s a fantastic offer from @Gus Potter. I love steelwork, so designed and fabricated and erected my part-portal garage/man-cave. Roof is 100mm kingspan but why o why did I go for valley’s 🙈. They look great but we’re a pain to make work especially keeping headroom to max by putting purlins between the rafters.

[saveasteading]

11 hours ago, Kevan Marshall said:

that’s engineers for you,

You lost my assistance there. Shame as I've designed and built hundreds.

 

You are lucky @Gus Potterhasn't said the same.

 

OK. Its BH, so:

Likely it is  a fixed rather than pinned base condition because of the boundarycondition.so heavier steel.

 

You say trusses. Portal frames don't have trusses.

Plus an agricultural spec is completely unsuitable, so of course this is much more expensive.

 

It's great you are studying  the subject but there is a risk this turns into dabbling.

 

Out.

 

[Kevan Marshall]

Biggest problem is looking for cladding for the side nearest the boundary, the steel portal frame is fabricated and that’s not going to change, I served my time in a blacksmith’s that put hundreds of sheds up over the years and not one has fell down, last shed I fabricated was 80’ wide and 160’ long 25 years ago, I moved on to work offshore and am hopefully getting near retirement, engineers do calculations and that is ok but to put wind bracing pipes in the roof structure that are 48.4 x 4mm tube and cross bracing that is 50 x 5 flat bar does not cut it with me, also I was going to use wooden purlins but he wanted 9” x 4” timber for 4.5mtr bays also seems a bit strange, he then said we could use a 120mm Albion z-purlin at 1.2 mtr centres which seems a little light to replace a 9”x4” timber, z-purlins are not easy to come by in Northern Ireland so using a multi beam 140mm purlin as agreed with SE….

[Kevan Marshall]

On 03/06/2026 at 08:06, markc said:

That’s a fantastic offer from @Gus Potter. I love steelwork, so designed and fabricated and erected my part-portal garage/man-cave. Roof is 100mm kingspan but why o why did I go for valley’s 🙈. They look great but we’re a pain to make work especially keeping headroom to max by putting purlins between the rafters.

Nice work Mark!

[Gus Potter]

On 03/06/2026 at 18:33, Kevan Marshall said:

Biggest problem is looking for cladding for the side nearest the boundary, the steel portal frame is fabricated and that’s not going to change, I served my time in a blacksmith’s that put hundreds of sheds up over the years and not one has fell down, last shed I fabricated was 80’ wide and 160’ long 25 years ago, I moved on to work offshore and am hopefully getting near retirement, engineers do calculations and that is ok but to put wind bracing pipes in the roof structure that are 48.4 x 4mm tube and cross bracing that is 50 x 5 flat bar does not cut it with me, also I was going to use wooden purlins but he wanted 9” x 4” timber for 4.5mtr bays also seems a bit strange, he then said we could use a 120mm Albion z-purlin at 1.2 mtr centres which seems a little light to replace a 9”x4” timber, z-purlins are not easy to come by in Northern Ireland so using a multi beam 140mm purlin as agreed with SE….

You make a good point about the skills you have, I accept that. 

 

The regs have become a lot more stringent particularly in relation to portal frames on or near boundaries. Best thing you can do is to give your SE a call and discuss. 

[Kevan Marshall]

17 hours ago, Gus Potter said:

You make a good point about the skills you have, I accept that. 

 

The regs have become a lot more stringent particularly in relation to portal frames on or near boundaries. Best thing you can do is to give your SE a call and discuss. 

Thanks Gus,

We are going to submit everything to building control and see what happens, we’ll try and get away with 40mm insulated cladding, I’ll ask my architect to ask SE if we can beef up the bracing a little…

[saveasteading]

5 minutes ago, Kevan Marshall said:

we’ll try and get away with 40mm insulated cladding,

As @Gus Potter says there are strict rules on this. Your wall has to stand up against a severe fire inside your building and the structure remain standing, all to stop fire spread to the neighbour.

I expect the Bco will simply refuse you, and await a compliant proposal.

This will be for a much more complex wall, clad inside as well as out, and with certified details. Also the steel usually needs fire protection.

Your SE foundation design also needs to demonstrate that it won't overturn in a fire.

 

Then post the proposals here and we can check they are practical and economical.

I suggest you simply commission your Architect and/or SE to specify it now, to save you several weeks.

Perhaps it was already their responsibility ( we don't know your contract terms).

[Conor]

Can you move another 400mm away from your neighbour?! That would simplify things greatly.

[torre]

On 02/06/2026 at 23:10, Kevan Marshall said:

I may have to move the garage so it’s one meter from boundary but then it is closer to my house..

As @Conor says, moving it just 400mm will be simpler and maybe a bit more neighbourly too?

It's going to be a substantial not very attractive building that it sounds like you might cut corners on and you don't want close to your own house - they probably don't either! 

[Nickfromwales]

1 hour ago, Conor said:

Can you move another 400mm away from your neighbour?! That would simplify things greatly.

This is such a no brainer!

 

@Kevan Marshall, move it over 400mm and save all this feckin about!!!! 🤷‍♂️

 

Iirc, the 1000mm rule would be the absolute dimensions, so remember to allow for overhangs / gutters and downpipes etc. 

[saveasteading]

12 minutes ago, Nickfromwales said:

such a no brainer!

It's a huge difference affecting ease of building and fire specification. Normally reduces foundation size too. Fire still applies though so do check it out.

[Nickfromwales]

1 minute ago, saveasteading said:

It's a huge difference affecting ease of building and fire specification. Normally reduces foundation size too. Fire still applies though so do check it out.

Automatic fire suppression systems aren’t ridiculously expensive (mist vs sprinklers) so if I had a building like this for storing / working on vehicles or storing significant amounts of ‘stuff that goes up real good’, then I’d fit a mist system.

 

For all of my domestic clients projects I specify a 3-core cable from the house smoke detection system to both attached AND detached garages, so anything not smouldering / smoking / on fire is immediately obvious to the occupants, eg it is flagged as early on as is possible.

 

 

Fitting a (wireless) ‘locate’ button in at least one strategic position within the house is also something that I spec (recommend), so basically upon hearing the smoke alarms all going off, every one of them in unison, where you’re possibly awaken, startled and worried, you simply hit the locate button; this action will silence every smoke detector except the one(s) that was triggered, meaning you can then make a bee line for the problem area to tackle the cause.

 

If the house falls entirely silent, that’s your prompt to get your arse out the door, el pronto, to the detached garage / outbuilding where you’ll then hear that one wailing. 
 

Most modern homes now have plant rooms, attic plant areas, detached garages and out buildings, so it’s far simpler to have this ‘locate’ facility imo, so you can find the issue in seconds vs running to every room / space that (you can remember) has one.

 

I recommend installing one inside a kitchen cupboard unit too, as with large homes with vaulted ceilings, if you burn the toast, you then don’t have to climb on stuff to press the ‘hush’ button on the detector (giving a 7 minute mute) as these wireless units also allow you to hush and test.