# Garden Room - Steel Beam Help Please!!



## Churchy00 (21 Jun 2022)

Hi all,

Please be kind, I'm new here!
So I'm designing a 5m x 3m Garden Room.
Looking to put down an insulated concrete base (100mm thick) on 70mm insulation and 100mm compacted MOT1. I want to make the most of the 2.5m permitted development height to put in a gym.

The room itself has changed design about three times as the Mrs keeps changing her mind!!
It's now got two bifold doors - a 4m and a 3m.
The roof is flat (actually slightly angled - 75mm higher on the front than the back). It's a cold roof with 150mm joists at 400 centres with 18mm OSB and an EPDM rubber roof. It's got 100mm PIR insulation and 9.5mm plasterboard.

The shorter bifold has a 4x2 top plate over the top of it, plus a pair of 4x2 (might use 5x2s) timbers attached to that but mounted at 90 degrees to keep the 4x2 top plate timber as straight as possible - this "lintel" doesn't carry any weight at all so merely needs to sit above the bifold for attaching to. My hope is that any "sag" can be countered by attaching this to the roof structure above it.

Now, I'm struggling a bit with the beam over the 4m bifold. At the moment I've designed in a 152x89 RSJ at 4300 long, supported by 300mm at one end on the wooden stud frame (5x2) and the other by a steel SHS post - the RSJ is fixed to this using 2x M20 bolts through a "tab" on the post. The post is currently 145mm square section but I can't help but think that's miles too big for what I need. The post has an offset base plate that I'm planning on bolting through using Atlas bolts into 7N dense blocks (I'd rather not have to put down a concrete pad foundation if I can help it).

I've added to pictures below that will hopefully make things a bit clearer.

Garden Room...





With cladding removed and showing RSJ in place...




RSJ length and support...




Steel post RSJ attachment...




Steel post base plate is 270mm square but offset to be flush on two sides....thus fixing would only be through the two outer edges....





So I have a bunch of questions....hopefully some of you kind souls can help....

1) Is the RSJ meaty enough for the 4m bifold? Any friendly SEs out there willing to advise?
2) What size steel post do I need - 145mm seems too big....how small can I go?
3) Is the M20 attachment method I'm thinking of OK for the RSJ/steel beam connection? Will it need a 2nd plate welded underneath to support the RSJ or are two M20s robust enough for the job? Will I need a 2nd "tab" on the other side of the RSJ effectively sandwiching the RSJ in between and bolting through all three pieces? What size and thickness should the "tab" be?
4) Will sitting the steel post on blocks be good enough - bolted down using concrete bolts? Perhaps sitting on 100mm compacted MOT1 and two blocks thick?

A lot of questions but I'm hoping this all makes sense.
Would be very grateful for some advice here as I need to cost everything properly before the Mrs will approve the construction.
Feel free to ask for additional info if needed and I'll do my best to oblige.

Many thanks in advance...


----------



## Jameshow (21 Jun 2022)

I'd think 100mm post would be fine. 

89mm might do even? 

150mm depth if beam would be fine, given that the roof will have a fair bit of stiffness too? 

How deep will it be?? 150mm? 200mm?

I'd pass your ideas past a steel fabricator - they will know by experience.


----------



## morqthana (21 Jun 2022)

I'd advise a warm deck roof, not cold - avoid all that faffing with vapour proof membranes and ventilation.

And it might be worth forking out for an SE. The weight that that beam has to support is surely trivial - a bit of the roof. Do you even need an RSJ in the first place?

Or approach a glulam beam supplier, see if you can get free advice wrt buying one of their products.


----------



## Darrenb (22 Jun 2022)

You should look up Oakwood garden rooms on YouTube the garden rooms they make are amazing quality and he goes in to great details on all the reasons for everything they do. 

They also use what they call a hybrid roof which makes the most of the headroom whilst keeping under the 2.5 meter permitted height.


----------



## imageel (22 Jun 2022)

A while back I had a single storey extension built on my house with a pitched EDPM roof and 2 2.5m wide by 2.2m tall picture windows meeting on the corner. Because this room and a further two storey side extension had so much glass that to meet the thermal target of the building regs at the time we had to have 100mm cavity and so our structural engineer specified beams the same dims as you suggest 152 x 89 but with a 6mm thick spreader plate welded to the bases to allow for the larger cavity. These two RSJ's were supported in the corner by a 90x90 3.6mm thick SHS post with 270x270mm 10mm thick spreader plates top and bottom, with the bottom plate resin anchored to the foundations.
I did wonder why such a massive section was required for a single storey with a lightweight roof and so asked the building inspector on one of his visits and his answer was surprising - he said they see a lot of wind damage where updrafts peel the roof off!!
So your proposed solution appears sound, just make sure it's all held down tight!
/Ed


----------



## Jones (22 Jun 2022)

Ask a structural engineer it should not cost much as it's very simple and may save you over or under specifying. I would also suggest a warm roof. Venting a flat cold roof is not that effective, fiddly and involves more cost. A warm roof of 150 mm rockwool and vapour barrier is cheap ,easy and with a good U value


----------



## PDW125 (22 Jun 2022)

Quick fag packet calculation and you could dispense with the steel and use 2 No. 220x45 laminated with an 8mm fitch plate and bolted at 400mm centres across the full width of the 5m span. On the bottom of this add in a 95x45 on flat to give a total height of 315mm from the top of the bifold to the top of the timber. I would add a pair of 89x89 RHS posts at each corner, with a welded L section to bolt to the back of the front fitch board. 

I would then use 145x45 C24 joists at 400c set flush to the bottom of the header, and then a 11mm OSB deck. Taper insulation, then 11mm OSB and EPDM over the lot.


----------



## Jameshow (22 Jun 2022)

Could you use 4" sq oak as a post? It might move a bit though? 

Also if you terminated the roof rafters on the beam and had the porch on the front with stubby beams you would be able to reduce the overall height?


----------



## Churchy00 (22 Jun 2022)

Jameshow said:


> I'd think 100mm post would be fine.
> 
> 89mm might do even?
> 
> ...


Many thanks for your reply @Jameshow.
I think 100mm might be the best option for me as it will match (almost) the stud thickness on each side so will be easier to align during construction. The roof thickness (if I've done my sums right) will be circa 195mm, which is the 150 joist, 18mm OSB, EPDM and Trim...though the latter will have negligible weight to it.

I was thinking the same about a steel fabricator...there's a local one here which I'm hoping my be able to point me in the right direction.


----------



## Churchy00 (22 Jun 2022)

morqthana said:


> I'd advise a warm deck roof, not cold - avoid all that faffing with vapour proof membranes and ventilation.
> 
> And it might be worth forking out for an SE. The weight that that beam has to support is surely trivial - a bit of the roof. Do you even need an RSJ in the first place?
> 
> Or approach a glulam beam supplier, see if you can get free advice wrt buying one of their products.


Hi @morqthana - thanks for the reply.
I toyed with a warm roof but I lose too much in the interior height as I need to put in a treadmill for the Mrs who's already pretty tall. This raises her getting on for 8inches off the floor already when she does her stupid hill climb runs.

Also contemplated a glulam - I found a guy on Youtube who fit one over a 4m bi-fold and contacted him for the dimensions - I found I didn't have enough height above the bifold to keep it within the 2.5m permitted development limit...a shame as this would have been easier to work with I think.


----------



## Churchy00 (22 Jun 2022)

Darrenb said:


> You should look up Oakwood garden rooms on YouTube the garden rooms they make are amazing quality and he goes in to great details on all the reasons for everything they do.
> 
> They also use what they call a hybrid roof which makes the most of the headroom whilst keeping under the 2.5 meter permitted height.


Hi @Darrenb - thanks for the pointer. I've actually already been looking at Oakwood YouTube videos a lot over the past couple of months. I contacted Liam there to ask him about the steel - I know he favours RHS 160x80. I'd copied his template in one of my earlier design iterations...until the Mrs changed her mind!! He did say that he has used the same steel over the 4m length but I couldn't find any videos online where it actually shows this...so I'm a bit skeptical. In any case, all his rooms appear to be a single bifold set to the front...I've not seen a double set on one of his that meet at the corner. He also tends not to use concrete bases...rather building a wooden frame raised above ground...and again this limits my headroom. His rooms are lower internal height than what I'm aiming for.

That said, with my current design I can still opt for the hybrid roof solution he uses at a later stage if I want to as it won't affect the joist selection - I'm a bit undecided on cold/hybrid roof type to be honest. He has a video where he talks about a report that he's had done regarding moisture build up and risk of rotting etc and all appears to be good...so I'm still humming and aaarring about that.


----------



## RobinBHM (22 Jun 2022)

You might wish to consider lateral stability.

there isn’t much in your design to resist wind load, just a small section of timber frame walling and the bolt connections between beam and post.


----------



## Churchy00 (22 Jun 2022)

imageel said:


> A while back I had a single storey extension built on my house with a pitched EDPM roof and 2 2.5m wide by 2.2m tall picture windows meeting on the corner. Because this room and a further two storey side extension had so much glass that to meet the thermal target of the building regs at the time we had to have 100mm cavity and so our structural engineer specified beams the same dims as you suggest 152 x 89 but with a 6mm thick spreader plate welded to the bases to allow for the larger cavity. These two RSJ's were supported in the corner by a 90x90 3.6mm thick SHS post with 270x270mm 10mm thick spreader plates top and bottom, with the bottom plate resin anchored to the foundations.
> I did wonder why such a massive section was required for a single storey with a lightweight roof and so asked the building inspector on one of his visits and his answer was surprising - he said they see a lot of wind damage where updrafts peel the roof off!!
> So your proposed solution appears sound, just make sure it's all held down tight!
> /Ed


Hi @imageel - many thanks....this is really useful information.
It's good to know that I'm not a million miles out with my spec. I'll likely opt for a 100mm or a 90mm SHS post. Not sure about the base plate position....I have it offset as it will need to sit up against the concrete base I'm going to lay.
The roof will be fixed down with upside down joist hangers which seems to be the fixing of choice for a lot of these roofs. The stud walls will be Atlas bolted to the concrete so hopefully all that will be enough for it not to fly away.
My only concern now is the connection from the post to the RSJ - I'll need to speak with a steel fabricator about that I imagine. 2x 20mm bolts feels sturdy enough for the roof it's trying to support...but I need some assurance though.


----------



## Churchy00 (22 Jun 2022)

RobinBHM said:


> You might wish to consider lateral stability.
> 
> there isn’t much in your design to resist wind load, just a small section of timber frame walling and the bolt connections between beam and post.


Hi @RobinBHM - thanks for the reply.
Very good point! So, I was thinking that with the joists fitted and OSB fixed to the outside...particularly on the rear wall, this would add enough rigidity to the structure. However, I take your point about the front wall which is mainly made up of bifold door. Any suggestions? Is there not enough stability coming from the rear wall and the stud section at the far end of the 4m bifold to hold everything securely?


----------



## morqthana (22 Jun 2022)

Churchy00 said:


> Looking to put down an insulated concrete base (100mm thick) on 70mm insulation and 100mm compacted MOT1. I want to make the most of the 2.5m permitted development height to put in a gym.


Presumably you can't get it more than 2m from the boundary?

Could you dig down a little more than 270mm?


----------



## Jameshow (22 Jun 2022)

Churchy00 said:


> Hi @morqthana - thanks for the reply.
> I toyed with a warm roof but I lose too much in the interior height as I need to put in a treadmill for the Mrs who's already pretty tall. This raises her getting on for 8inches off the floor already when she does her stupid hill climb runs.
> 
> Also contemplated a glulam - I found a guy on Youtube who fit one over a 4m bi-fold and contacted him for the dimensions - I found I didn't have enough height above the bifold to keep it within the 2.5m permitted development limit...a shame as this would have been easier to work with I think.


Nowt stupid about hill runs. 

Just not today in this heat!!


----------



## Jameshow (22 Jun 2022)

Churchy00 said:


> Hi @RobinBHM - thanks for the reply.
> Very good point! So, I was thinking that with the joists fitted and OSB fixed to the outside...particularly on the rear wall, this would add enough rigidity to the structure. However, I take your point about the front wall which is mainly made up of bifold door. Any suggestions? Is there not enough stability coming from the rear wall and the stud section at the far end of the 4m bifold to hold everything securely?


How about thinking the post down through the floor and anchoring it to either the floor joists or even down into the concrete of the foundations?


----------



## Terry - Somerset (22 Jun 2022)

I can understand the desire from a design perspective to have as slim a corner post as possible. You have obviously considered static roof load, and it seems that you have planned for the possible impact of wind.

But I would be concerned that any movement could risk the bifolds jamming. I would be inclined to take professional advice on the size, material and installation solution for both post and RSJ - under specifying the materials would be difficult and expensive to rectify.


----------



## HOJ (22 Jun 2022)

Churchy00 said:


> I want to make the most of the 2.5m permitted development


Why not just go for planning permission, you've always got your hands tied if you try to work with PD restrictions, and have to engineer a solution that may cost more in the long run, and will always be a compromise, I wouldn't build anything on this scale without it, and certainly not without a SE design input.


----------



## RobinBHM (22 Jun 2022)

Churchy00 said:


> Hi @RobinBHM - thanks for the reply.
> Very good point! So, I was thinking that with the joists fitted and OSB fixed to the outside...particularly on the rear wall, this would add enough rigidity to the structure. However, I take your point about the front wall which is mainly made up of bifold door. Any suggestions? Is there not enough stability coming from the rear wall and the stud section at the far end of the 4m bifold to hold everything securely?



I don’t know TBH, I used to design timber orangeries, make and build. I used to get a structural engineer to do the calcs, generally his solution was that we bolted the posts to the beam at the top, usually that would be a post at each corner and 2 intermediates.


----------



## RobinBHM (22 Jun 2022)

HOJ said:


> Why not just go for planning permission, you've always got your hands tied if you try to work with PD restrictions, and have to engineer a solution that may cost more in the long run, and will always be a compromise, I wouldn't build anything on this scale without it, and certainly not without a SE design input.



A couple of hundred extra mms would make all the difference.

2.5m isn’t much, because that’s from ground level.


----------



## johna.clements (22 Jun 2022)

I would be mostly worried about the wind as others have noted. When it is windy and a door is open (or the glass breaks) there will be quite a large upwards load. Bolting to a 20 to 30 kg concrete block will not weigh enough to hold the steel post down. You need a structural concrete pad.

I would also concerned about how the RSJ will react with one end supported by a stiff steel column and the other end by more flexible timber. The two M20 bolts will not provide a moment connection so the joint will rotate slightly as the timber compresses shedding more load onto the timber. This could result in the doors jamming. Why not have a second column at the other corner, then the corners match. And then make the RSJ longer may have to be slightly bigger for the increased span.

How are you going to put the RSJ up on the timber studwork. If you had to columns it may be easier to erect and fix the RSJ. When you have the goal post up you can build the rest at your leisure. If you put it on the studwork you have to stop whilst the steel is erected then continue with the roof.


----------



## morqthana (22 Jun 2022)

Churchy - what's your relationship with the neighbour in question like?


----------



## morqthana (22 Jun 2022)

The trouble with going for PP is the old "it's easier to ask for forgiveness than permission" thing.



RobinBHM said:


> A couple of hundred extra mms would make all the difference.
> 
> 2.5m isn’t much, because that’s from ground level.


Hence my Q about maybe digging down a little, say so the FFL of the building is at ground level.

And if he gets on OK with the neighbour, and the high point of the pent roof is 4m from the boundary, the edge nearest the neighbour is under 2.5m, and the neighbour isn't going to complain, JFDI is an option. Any subsequent hassle he can claim an ignorant misunderstanding that he thought 2.5m applied to the height nearest the boundary, but with nobody on the ground kicking up a fuss the council have to justify public interest in pursuing it, and they are so short of funds I doubt they would be able to do that. We're talking a small garden shed here, which at its *highest *point would be 4-8" too high, not a house built without PP. If nobody tells the council they won't even know.


----------



## Churchy00 (22 Jun 2022)

morqthana said:


> Presumably you can't get it more than 2m from the boundary?
> 
> Could you dig down a little more than 270mm?


Nah, must be almost at the boundary. Digging down more is still something I'm contemplating. I'm even considering a wooden framed floor...which I steered clear of up to now as it consumes a fair bit of the 2.5m height allowance when you take into account the raising above soil level, joist height and Egger protect boarding (or something similar). That's why I'm favouring an insulated concrete slab...but this makes it difficult for the steel post anchoring.


----------



## morqthana (22 Jun 2022)

OK, so the high point is 3m-ish from the boundary - same principle applies. If you could have the high point as you want, what would be the height of the low point at the boundary?

As for the steel post, what if you welded it to a reinforcing mesh in the slab before the pour?

Another idea - a bit radical and more expensive:

The building only needs the walls supported - the floor joists are only for the floorboards. Structurally there is no reason why you could not excavate deeper, construct a masonry wall to take you up to ground level and thence the walls of the building, so that you step down as you enter the building and have plenty of headroom.


----------



## RobinBHM (22 Jun 2022)

morqthana said:


> The trouble with going for PP is the old "it's easier to ask for forgiveness than permission" thing


If planning is refused, permitted development will still be intact.

I would rather get planning than worry about planning enforcement.


----------



## morqthana (22 Jun 2022)

But what he wants is outside the limits of PD. My theory is that if he's not totally taking the p**s, i.e. only a little bit too high, and his neighbour isn't going to complain, enforcement action is extremely unlikely given the requirement for it to be in the public interest and the lack of funds available to the council to chase after minor breaches where there isn't really any public interest at stake.

I know what I'd do.


----------



## woodwind (23 Jun 2022)

morqthana said:


> But what he wants is outside the limits of PD. My theory is that if he's not totally taking the p**s, i.e. only a little bit too high, and his neighbour isn't going to complain, enforcement action is extremely unlikely given the requirement for it to be in the public interest and the lack of funds available to the council to chase after minor breaches where there isn't really any public interest at stake.
> 
> I know what I'd do.


Yes, but he might want to sell the house in the future!


----------



## Kriggi (23 Jun 2022)

I haven't read through the whole thread so apologies if my solution has already been suggested. When we had a similar construction created the steel guy came up with a neat solution that means the fixing is completely concealed. Unfortunately I don't have any pictures of the steel without the bricks on as the brickies got there before me.


----------



## MikeJhn (23 Jun 2022)

Please take this advice from an old retried Chartered Structural Engineer, your current design although a commendable effort is flawed, spending a few hundred for a professional design would be money well spent, ask you local steel fabricators who they use, but on quick look you will end up with steel goal post and concrete foundations.


----------



## Terry - Somerset (23 Jun 2022)

The cost per square meter for a reasonably well specified garden office or extension seems to be around £1400. Probably a bit less for largely self build, somewhat more for higher spec or if location/access are issues.

Your 15 sqm will cost in the region of £21k based on this. Ask yourself - how likely is it that a professional design could save (say) £1000 (5%) through better design and appropriate material specification.

Although it always pains me to spend more money than I need to, and I would find the challenge of good self design motivating, I think I would reluctantly pay the professional.


----------



## morqthana (23 Jun 2022)

woodwind said:


> Yes, but he might want to sell the house in the future!


a) One of those useless indemnity policies will calm any buyer.
b) If he doesn't move for 4 years it's too late for enforcement action.
c) If he ticks the "right" boxes on the questionnaire, WT* is going to take the trouble to measure a the height of a small shed when looking at a house and start worrying about PP vs PD?


----------



## RobinBHM (23 Jun 2022)

morqthana said:


> a) One of those useless indemnity policies will calm any buyer.
> b) If he doesn't move for 4 years it's too late for enforcement action.
> c) If he ticks the "right" boxes on the questionnaire, WT* is going to take the trouble to measure a the height of a small shed when looking at a house and start worrying about PP vs PD?


Yeah, I can’t see a surveyor picking up an outbuilding slightly outside PD height constraints


----------



## MARK.B. (24 Jun 2022)

My neighbor recently had a complete new roof on the kitchen extension with pitched roof and concrete tiles , the ridge beam(maybe that's not the correct term ) was a rsj .What interested me was seeing him pick up and carry a 150mmx100mm rsj that was 6 metres long by himself ,turns out that the beam was made from some sort of plastic material. He used a Structural Engineer who did all the calcs and stuff to make sure it was safe etc. I am in my sixties and had no problem lifting it up . The Bifold doors had the same plastic rsj over them and the door frame fixed through the top with screws . Might be an option for you in your build.


----------



## baldkev (23 Jul 2022)

In terms of fixing down, get wall plate straps and add an extra bend, so it is cast into the concrete layer ( i didnt draw the dpm, obviously you'll need one and the insulation barrier between screed and walls )
The straps fix down onto the sole plates of the studwork to add resistance to lifting as they'd have to break the straps or through the concrete. Using 18mm ply on the outside of the timberframe glued and screwed is a good idea, with tyvek, spacer battens and insect mesh etc, then cladding or render.
The steels should be galvanised after fabrication and ideally wrapped with insulation. The 90 x 90 5mm posts would be perfect, but to echo above, get a structural engineer and consider planning. Not only can you go a bit higher, but you could go a bit bigger on footprint.
You will need foundations.


----------



## baldkev (23 Jul 2022)

Churchy00 said:


> Nah, must be almost at the boundary. Digging down more is still something I'm contemplating


The ffl needs to be 6" above exterior ground level, with a dpm. If you cut corners, then if, or when, you run into problems with the council / building control, they'll have more leverage to make you rip it down. 
Seriously, get an engineer and apply for planning. You'll sleep easier and if you sell later, its all legal


----------

