# What beam size do I need ?



## JohnT (13 Oct 2022)

Hi All
Workshop size is 7 mtr X 5 mtr with a roof construction of 80mm Insulated panels. A beam across the 7 mtr length wall leave a span of 2.5 mtr. Advice for the size of the beam needed. Headroom is critical so shallowest depth of beam is what i am looking for.
Thanks in advance.


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## Bingy man (14 Oct 2022)

Hi there, I’ve just read your post and although I’m in no way qualified in roofing but I’d suggest you consider steel box section to get the shallow profile your after . I’m sure if you contacted a local steel stockholder they would be able to help. Obviously the weight of the roof and associated timbers has to be taken into account. Hope this helps.


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## Fitzroy (14 Oct 2022)

7.5m is a long span, I think you are going to need a proper sizing calc. I think you'll also find it is deeper than you would like as a flat/low pitched roof will load up the span. Look at Beams - Supported at Both Ends - Continuous and Point Loads and you can play with some numbers. I think the challenge for you will be what roof load you are going to design for. Your panels will be low weight but are you going to design for the snow, or just chance it not happening? A 20cm thick snow covering will put a 2ton load on your roof, playing with the calculator on engtoolbox a 305x127mm RSJ will have 6mm deflection, which is probs about as much as you want.


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## johna.clements (14 Oct 2022)

JohnT said:


> A beam across the 7 mtr length wall leave a span of 2.5 mtr.


I am not sure what you mean by this. Are you going to have columns in the workshop or will it be 7 X 5.

The rule of thumb for starting a design is a span/20 for simple supported and span/26 for continuous. And span / 7 for cantilever. You need to guess the size of the beam to design it quickly by hand. The beams will be simply supported as it is unlikely that a simple bolted connection at the ends will be able to stop rotation (eventually it will but it will be large so after the roof has failed). 

7500 plus 300 (for the width of the supports/2) gives a span of 7800.
7800/20 gives a depth of 390mm to start design.

@Fitzroy 305x127mm RSJ looks like a reasonable out come. A deeper beam would be lighter and so cheaper. 

305x127mm RSJ are around 37 to 48 kg/m depending on the section chosen. You will need a place where a lorry can park to be off loaded and a place with firm ground for a crane. 

The design of the workshop also needs to take into account the wind. 7 x 5m is a big sail to lift up the columns.

I would get a small steel erector to design, fabricate and erect the steel frame. Use my 20/26/7 rule and Fitzroy to get an outline design and budget then get the steel and foundations designed by the steel erector. Then you can finalise the design for the rest of the structure.


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## Spectric (14 Oct 2022)

Is this an apex roof, and why not run shallow trusses across the 5 metre span, you might benefit from man made beams where they are made from sheet material rather than real timber, like I beams or Glulam beams to reduce depth, that sheet roofing is not heavy.


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## Lard (15 Oct 2022)

Spectric said:


> why not run shallow trusses across the 5 metre span


That was my initial thought.

Can I just ask though, if headroom is critical then surely by placing the main supporting beam in the the middle of the 5m span will mean you have a much deeper head-hurting beam running right down the middle of your 'room'. If you could run the supports along the 5m span instead the overall depth reduction would give you improved headroom......or have I missed the point?


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## johna.clements (15 Oct 2022)

Lard said:


> That was my initial thought.
> 
> Can I just ask though, if headroom is critical then surely by placing the main supporting beam in the the middle of the 5m span will mean you have a much deeper head-hurting beam running right down the middle of your 'room'. If you could run the supports along the 5m span instead the overall depth reduction would give you improved headroom......or have I missed the point?


 Maybe there is some constraint on foundations on the 7000 long sides. Purlins spanning to a wall would produce a UDL on a strip footing. The ends of beams and columns would produce a point load.

Hopefully he will log on and give more info.


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## JohnT (15 Oct 2022)

Hi
Thanks for all the responses, apart from things changing I will include a sketch that will make things clear ( I hope ). The dotted line indicates beam positions.
Thanks
John


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## Jameshow (15 Oct 2022)

I reckon you looking at 225mm X 75mm timber. 

It's a standard 9x3 size. 

Can you get the roof panels in 5m lengths if so that will greatly increase their stiffness. 

Others will know..

I did a horse shelter 6x3m with 8x2 and put them other ways so as not to go over 4.8m lengths. Purlins at 1.5m.


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## Pedronicus (15 Oct 2022)

Would the fall of the roof not be dictated by the panel manufacturer rather than the roofer?


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## Inspector (15 Oct 2022)

I'm no structural engineer but looking at these span tables for roof joists (yours will vary), as long as your lintels were strong enough you could skip the midspan beam and use 38mm x 235mm on 300mm centres running from front to back. If you had to go to 38mm x 286mm because of availability you could go to 400mm spacing, possibly 600mm.

I would suggest that you consider a one piece/built up lintel running right across the front rather than using two. More difficult to put up but makes the wall stiffer and because all the studs are the same length, simpler to frame. That was done front and back of my shop because of the number of openings.

Pete


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## Jameshow (15 Oct 2022)

How do I know what size purlins I need?


A purlin is one of the most important tools in architecture. This is a horizontal beam that provides structural support in buildings, typically...




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Look at this - your beams are basically purlins and you need 8"/200mm ones.

Then use your span tables above for the purlin spacing apart which means 3 spans / 2 beams


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## Lorenzl (15 Oct 2022)

This site may be of help?

Span tables and a pay for span calculator


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## Jones (16 Oct 2022)

You can get standard timber sizes in up to 7.2 m long so it may be possible but a look at the span tables shows a max span for 65 x 225 with purlins at 1800 centres to be 4.09m , if you had four at 1200 centres it's up to 4.7m. Basically you will either need gluelam beams probably about 300mm deep or steels. For steel you will need to get a structural engineer to specify for gluelam you might find tables online you can use. It would be a lot easier if you could add some posts or an internal wall to reduce the span, an internal partition would also control racking forces on the walls. The other option is to use three or four rafters on the 5m span and fit short purlins between them.


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