# Forum Home Renovation Pergolas, Gazebos, Strombellas & Rotundas  How to span 7.8M without a post in the middle?

## spleakz0r

Wanting to add an attached insulated skillion roof structure along the side of our house and avoid having an awkward post in the middle of the beam that would make it less usable for parking cars:   
We've looked at roofing panels from places like Versiclad (Spacemaker) and Bondor and it seems achieving an 8m span with insulated roofing panels won't be an issue. 
Finding it very difficult however to find a definitive answer on a suitable beam that would allow for an 8m span between two posts. (Which is probably actually closer to 7.6-7.8m) 
The two posts between the beam would be *SHS DuraGal Plus 125x125x4 and 100x100x4*, concreted 1 metre deep into the ground, so should be sufficient enough to support any type of beam. 
Roof Load Width (RLW) from what I gather is half the width of the roof the beam is supporting, which would be about *3.7 (7.4/2)* at the widest point, reducing to *1.5 (3.0/2)* at the opposite end. So around *2.5-2.7 RLW* middle of the beam. 
Roof structure should quite light, 150mm Versiclad SpaceMaker is 11.15kg/m2 and I don't think it would require any rafters/purlins in our design. 
We're not overly concerned with the type, material, or size of the beam, so long as it's reasonable and will provide us with the span we're hoping for. 
It wouldn't require supporting anything other than the roof panels and possibly the occasional human walking on the top of it for maintenance. 
Researching online I've come across things such as: 
- RHS / Rectangular Hollow Steel
- UB Beam / I Beam / H Beam / RSJ
- PFC / C Beam
- LSB (LiteSteelBeam), think this Lyasight or Spantec now?
- Spantec BoxBeam/ColorBeam
- LVL (would timber span more than steel?)
- Trusses (think these can span up to like 10m and are designed to hold significant weight) 
All with varying depths/thicknesses, sometimes even being two separate beams joined together. 
I've tried reading some Span tables but a lot of them are quite difficult to understand and some don't even have span tables. (RHS for example) 
I guess my question would be, is there a definitive answer somewhere? Or at the very least, would it be possible to have an 8m span? (We're hoping to achieve this with a beam about 200x100mm)  *Would an 8 metre DuraGal RHS 200x100x4 beam be sufficient? This would weigh 143.2kg. Could even go up in thickness 5.0-9.0mm if it would add significant strength. * Aside from a truss, I figure RHS would be the strongest out of a UB Beam / I Beam / H Beam / RSJ / PFC / C Beam as these would all essentially be 3/4 of an RHS? (Or are these designed differently and actually provide more strength?) 
And the LSB-type stuff from Spantec and other manufacturers seems they are just thinner versions of RHS? (Guessing the trapezoid-like shapes in these beams are what give the thinner metal more span.) 
Any recommendations much appreciated. Will be trying to contact a few suppliers soon to try get more answers.

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## METRIX

You really need an engineer to tell you what the right sized beam is, no guessing or "recommended" beam from a forum or friend etc.

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## Marc

There are LVL span tables and some sections 450x63 do span up to 9 meters, however, in your case I second Metrix that an engineer will be able to provide better and specific advice. 
Steel manufacturers don't provide simplified span tables with standard typical loads, and to chose RHS or I beam, you need to make some calculations. 
Engineers usually don't charge much.

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## intertd6

I spanned 7.0m , RLW of 3.5m , 20kg/m2 roof and ceiling, from memory it was F27 HWD 250mm deep X 200mm wide, the computer span table calculator I used is now redundant.
inter

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## spleakz0r

If we're not able to get a definitive span table from a manufacturer, we'll definitely look at getting a structural engineer to spec something for us. 
Surely though, there must be pre-fabricated beams that support this type of span? 8 metres doesn't seem like a huge amount?  *I guess our main concern for the moment would be ensuring we can get a 7.8m span at all, or if we're always going to have to get a support in the middle.* 
This way we can ensure we put our posts in the correct location now, because we plan on having the posts put in before we pour our driveway.   

> I spanned 7.0m , RLW of 3.5m , 20kg/m2 roof and ceiling, from memory it was F27 HWD 250mm deep X 200mm wide, the computer span table calculator I used is now redundant.
> inter

  This is quite a beefy beam. Would this be because it's wood rather than steel?

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## phild01

I recall using a box span table for what I have done. A box beam would be my only consideration for a light roof.structure like that. 
edit: One that I used: ColorBeam-Flat-Roof-Span-Tables.pdf (spantec.com.au)

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## intertd6

LVL is made in long lengths up to ~13m , availability is another thing. There should be span tables online for engineered timber products, I recollect seeing some just lately.
inter

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## intertd6

> This is quite a beefy beam. Would this be because it's wood rather than steel?

  yes , steel is around 8-10 times stronger than timber, so beam dimensions can be reduced accordingly.

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## spleakz0r

> I recall using a box span table for what I have done. A box beam would be my only consideration for a light roof.structure like that. 
> edit: One that I used: ColorBeam-Flat-Roof-Span-Tables.pdf (spantec.com.au)

  Yeah I've gone through some of the Spantec material, looking at their span table for the BoxBeam product, it looks like  2 x B250-20 can give you a 8.28m span at 4.0RLW:  Attachment 129249  
And that's for an N3 wind rating, which probably isn't necessary where we are, so if we dropped to N2 we'd likely be fine with 2 x B200-20. (Our RLW also drops quite a bit, so at the centre of the beam it would probably be closer to 2.7-2.5.) 
What's confusing is that the ColorBeam span tables, which as far as I can see are just a finished/painted version of BoxBeam, look to have a different/worse span table:  Attachment 129248  
In the ColorBeam span table W is the entire width (not RLW which is half), so if we compare the CB200-20 to the B200-20 comparably it would be like: 
CB200-20, N3, could span about 6 metres at a 2.0RLW.
B200-20, N3 could span about 6.92 metres at a 2.0RLW. 
The ColorBeam table is from November 2014 however, BoxBeam table is from 2019, so maybe it's outdated and they've improved something since then. 
Definitely looks like I'll be giving Spantec a call on Monday to get more information and confirmation on these spans; it looks like we may have found our beam solution.  :Smilie:   *It appears that BoxBeam/Colorbeam and all these "LSB" products are very similar to your typical RHS beam, is there any major difference?  * BoxBeam for example, if 2 x 250 x 50 x 1mm/2mm can do 8.28m span at 4.0RLW, wouldn't 1 x 250 x 100 x 4mm RHS be able to get a similar/better span? Would be twice the metal thickness.   

> LVL is made in long lengths up to ~13m , availability is another thing. There should be span tables online for engineered timber products, I recollect seeing some just lately.
> inter

  Might try taking a look at some timber products then. I always thought steel would be superior. i.e. You could span longer or the same distance with a smaller, lighter steel beam as opposed to a timber beam. 
Then obviously there's the fire resistance, weathering, wear and tear and terminate advantages.   

> yes , steel is around 8-10 times stronger than timber, so beam dimensions can be reduced accordingly.

  Ahh, so - I'd definitely prefer a steel product then. 
Even if we  had to buy something like a steel floor truss that has webbing, we'd be fine with this as long as we could keep it at about 200-250mm deep and 100mm thick - we just want to avoid having something ridiculous like these 300-600mm deep beams I've occasionally come across in my research. 
There is this steel truss mob not too far from where we live - https://steelframestrusses.com/ - guessing they'd probably be able to fabricate a suitable truss to be used? A truss would be stronger than a typical RHS I suppose?

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## pharmaboy2

there used to be a onesteel table document available online around 10 years ago 
OK, found it  https://www.libertygfg.com/media/185...in-housing.pdf 
from that your option is way under sized.  For a couple of hundred dollars given your circumstance and the benefits of the lightest option an engineer would be the goto option - engineers often save you money because they will use the minimum needed whereas tables are often over specced to be careful.  he can also discuss the impact of just 1m on your sizings - sometimes quite a small change in span can make a big difference in the weight and difficulty of install

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## r3nov8or

This one is in The Library here... Looks the similar?  https://www.renovateforum.com/f221/s...-tables-99800/

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## Uncle Bob

> This one is in The Library here... Looks the similar?  https://www.renovateforum.com/f221/s...-tables-99800/

  
Good stuff, I forget about our Library.

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