Decking/Pergola Advice

1 Jul 2014
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United Kingdom

I am looking for a bit of advice regarding my decking framework and pergola and ensuring that what I have in mind will actually work.

I am building a 4.8m out by 8m wide decking area off the back of my house using 4x2 framework with joists every 60cm. The boards going on top will be 38mm x 200mm smooth decking.

I am then building a lean-to pergola on top of this decking coming off of the house 2.8m out and 5.4m across. There will be a 6x2 on the wall and 6x2s on the face (maybe 2 'courses' to help with lateral support) with the framework being 5x2 with joists (rafters?) every 60cm.

The pergola will be supported by 4 4x4 posts that will be 2.8m out (and the 6x2 attached to the wall). The posts will be concreted into the ground as well as bolted to the decking joists and bolted to the pergola.

I then plan on putting thick ply on top of the pergola and either putting slate or felt or something on top to create basically an outside lean-to with decking underneath it all.

So, I have a couple of concerns.
1. Is 5x2 okay for the pergola, or should I be looking at 6x2 (or even 4x2?). The length isn't *that* great being only 2.8m, but it being covered changes that.
2. Are 4 4x4 posts enough to support the pergola (as well as the 6x2 on the wall).
3. With my plan of putting some thick plywood on the pergola and slating/felting it, how does this affect the structure - wind will still be able to mostly pass 'through' the structure, but updraft may of course be a concern.

Anything I have potentially overlooked?

Thanks in advance.
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A roof would make it a structure and not a pergola, so you have to accoutn for snowloads, look up spans for flat roofs.
A roof would make it a structure and not a pergola, so you have to accoutn for snowloads, look up spans for flat roofs.

Thanks for that. After some more Googlng I came to a similar conclusion.

I'm pretty sure, with snowloads with a length of 2.8m I will need 6x2 rather than 5x2.

The problem now though, is that I'm not sure if the 4x4 posts can also take all of this weight. Especially since with them being concreted into the ground they would need to be 3m long, which lessens the load it can carry. The 4x4 posts will only be like 4ft apart though, which may well carry the load.

Anyone aware of any tools for me being able to calculate the load of the posts?

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