Garden Room - Steel Beam Help Please!!

[Churchy00]

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]

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]

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]

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]

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]

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]

Quick *** 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]

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]

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.

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.