Pergola Beam/Header sizing

[ldoodle]

I'm building a corner pergola. The span across the front will be up to 4.5m. Will likely go for 6x6 posts as prefer the chunkier look.

This is the sort of design I'm aiming for (mine will also be integrated into a deck):

Thanks

 

[Deleted member 323070]

There are no tables for specifying such timber members because there are no imposed loads, other than supporting the cross beams.
It might be worth considering if you will ever hang a chair, a hammock or similar from the timbers.
Your corner supports will also reduce the size requirement.

If they were ceiling joists 47mm X 195mm, 44mm X 195mm, or even 38mm X 195mm would suffice.
If you wanted to, you could size it down to 47 X 170, 44 X 170, or even 38 X 170, but that might be pushing it.
It's really your choice.
As it's a one-off, you could use a non-standard size.

 

[^woody^]

4x2 c24 will be fine. There is absolutely no load, nothing like what floor and ceiling joists are designed for. But if you are using 6" posts then for aesthetics, either something the same depth or the next size up would look right for the main beams and reduce the depth for the transverse timbers

 

[ldoodle]

Thanks for the replies.

I'm looking at sourcing materials from UK Timer, who do pre-cut Dried/Sawn British Larch/Douglas Fir Boards, max thickness is 38mm (which is 32mm if PAR - I'm not sure what 'look' to go for yet), but up to 200mm depth. I do like the idea of matching the beam depth to the post thickness.

For the front beam, I am umming an aaahing about doubling up, like this (albeit not that corner support style):

I can then use the same boards in a smaller depth for the rafters. They also do BL/DF posts in both C16/C24 grades and rough sawn/PAR so it all would have the same look.

As a side point - the posts will be bolted to the deck frame (which is likely going to be 6x2 C24) since the fixings will be hidden by the deck. For the pergola joinery I was hoping to use through dowels to avoid visible fixings - is this a good or bad idea for an exterior structure?

The posts will be notched (for both decking frame and pergola beams) so there'll be additional glue surface for the pergola beams.

Thanks again.