Pergola - Free standing - 6mtr spans

**o_total** · 14th January 2008, 11:57 AM

Hi everyone,

I am new to this forum and after reading throught literally hundreds of posts I am unable to find a match to the work I am wanting to carry out.

I am wanting to build a freestanding pergola, 6m x 6m, with a skillion style roof with a slight slope from front to back supported on only 4 posts (the two front posts at a height of 2.8m and the two rear posts at 2.3m) which are going to be 200x200mm hardwood.

The roof will be colourbond, and the underneath of the roof will be fully enclosed with fibro sheeting. The whole structure will cover a timer deck which is going to be built over a new concrete slab.

The biggest problem I can see with my wishfull design is going to be the 6m spans. I really want to only use 4 posts if at all possible, and this may require me to look at alternatives other than timber. The size of the structural beams I was wanting to use were either 300x50 or 300x75 in either pine or oregon.

The design I have come up with only uses beams with no rafters. Is this design acheivable considering the weight of the roof and the spans?

The design uses all the same sized beams, 4 sides supported in each corner by 200x200 posts. Then 5 beams running left to right 1mtr appart, these 5 beams will be seperated by 1 mtr spreaders beams running front to back making a cris cross pattern when viewed from above. I tried to add some drawings but had no luck sorry hope this is descriptive enough.

Thanks
Brett

**DvdHntr** · 14th January 2008, 12:31 PM

The 5 beams are your rafters in a way. What you have created is a grid pattern and so will not be fully covered in span tables. But referring to the tables for guidance may give you a starting point. The beams that are 6m long will have to be quite large and just check on the maximum span of the roofing and the lining. Also, the structure will be susceptible to wind loading. So I will need to know the details of your location (general), the local topography (on a hill?) and the surrounding structures.

Based on 40ms-1 wind speed for ~ N2

On my rough calculations you will need a larger beam size. A 300x75-F7 is OK for strength but will deflect over 30mm under dead loads. I think you are getting into the realms of steel or larger timber beams (400 or larger)

**rod1949** · 14th January 2008, 02:08 PM

Do pergolas have covered roofs and lined ceilings? I thought they just had a a few sticks across them?

6m spans unless massive steel beams or webbed steel trusses are used it won't stay straight for long. If using timber then it won't be long before it becomes one of those sagging messes that one sees.

**bitingmidge** · 14th January 2008, 02:11 PM

You may just have the odd bracing issue too, oh, and a bit of uplift.

How big did you say those reinforced concrete columns were??

P

**Dirty Doogie** · 14th January 2008, 03:49 PM

360 x 63 LVL hyspan will do for your rafters @ 900 centres. I dont know what you are going to use as beams between the posts. Your 4 posts will have be designed as Canterlever columns so they need to be sunk 1200 mm into the ground.

**Ashwood** · 14th January 2008, 04:09 PM

Re Rod1949's comment, I think the correct name is "gazebo" (has a roof) rather than pergola (no actual roof cover)?

**Termite** · 14th January 2008, 04:24 PM

I'd be inclined to go for a low pitch hipped roof using trusses. Having just a while ago bought 2 - 6.5M x 263 x 65 LVL beams you might agree with me when you check the price of your beams.

**journeyman Mick** · 16th January 2008, 01:33 AM

You can span 6M with a 150mm metal C purlin. You'd need to get it engineered but you could use a couple of them (or possibly 200mm) at the ends and then run 150s as rafters. If you space them closely enough you won't need ceiling battens.

Mick

**o_total** · 16th January 2008, 08:14 AM

Hi all,

Thanks for all the reply guys, looks like I am going to have to redesign my "gazebo" as timber looks to have too many limitations, and I was hoping to avoid using metal if I could.

Maybe I will go back to my original idea, and that suggested by termite and go for a 6x6 hipped roof using prefab trusses.

Thanks again for all the replies.

Brett.

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