Timber sizes for lean-to pergola

Natterjack · Post by **Natterjack** » Fri Jun 25, 2021 10:43 pm

Hi, I'm looking for some guidance sizing timbers for a lean-to pergola with a solid polycarbonate roof.
I've done a bit of looking around at different projects and come up with a rough design based on a bit of research and a lot of guesswork.
It's a bit of an unusual design as it has to nest under the eaves of an existing building and is bounded on three sides of an existing courtyard.
My timber sizes my be a little OTT, but I'm also planning to hang a swing seat from the main beams, so I wanted to allow a little extra capacity for that.
I've attached a rough draft of the design and I'd be grateful for any advice as I'm a little out of my depth when it comes to sizing up timber.

Overall size is approx. 4.5 long x 2.6m at widest point (slightly tapered) ; height 2.3m to underside.
Pergola is fixed to existing blockwall along one long side and one gable and supported by two 4x4 posts along the other long side.
Rafter sizes 6x2 spacing is 550mm, max. span of rafter is 2.3m, with a 300mm cantilever at the apex.
The rafters are supported off horizontal beams, with a max. span 2.6m - I have used doubled up 6x2s for these as I was unsure about the whole load of the roof being supported off these.
I've allowed for a few extra braces for rigidity - the attached .pdf will explain it a little better (I hope).

Any advice or recommendations would be gratefully received as I'm struggling here.
Many thanks.

kellys_eye · Post by **kellys_eye** » Sat Jun 26, 2021 6:30 am

There are span tables that cover such details - like here:

https://www.timberbeamcalculator.co.uk/ ... 6&slope=15

from which it would appear that you've over-engineered - considerably. Not a bad thing but considered wasteful.

Natterjack · Post by **Natterjack** » Sat Jun 26, 2021 11:13 am

Thanks for the link, I was looking for something like that, I'll take a look and maybe cut the sizes a bit.
This was my first pass at it so assumed I had over sized.
My main concern was the centre span horizontal beam which seems to be taking the load of half of the roof which is why I doubled up with 2x6s.
Also I wanted to hang a hammock chair, or 2 seater hanging bench from the structure - is there any rule of thumb for allowing for this extra load - e.g. go up a size maybe ?

aeromech3 · Post by **aeromech3** » Sat Jun 26, 2021 3:58 pm

When you say " is bounded on three sides of an existing courtyard" are you saying there are walls for the end timber to be secured to, or is it as shown in your Pergola pdf, a cantilever design ?
If so the 4x2 braces need to be about 2/3rds out on the timber beams falling to meet at the posts at 45*, at least in my opinion

Natterjack · Post by **Natterjack** » Sat Jun 26, 2021 9:33 pm

Yes I'm fixing the rafters to an existing wall along one long side at the low end of the roof - the support for the other long edge (the apex side) is an existing block wall at one gable end, and two 4x4 timber posts, one in the centre of the long side and one at the corner of the 'open' gable end.
Sorry for any confusion - I should have shown the wall on my section which would have made it clearer - I have attached a revised drawing here which now shows the blockwall. The 4x2 braces are not there to support a cantilever, they're just in a few locations to square and stiffen the frame up a bit.