What size timbers?

[kayoss]

Hi All

When finished, my new pond will be 4.6m square and fully enclosed on 3 sides. I'd like to fit a pergola over it and cover this with polycarbonate sheet so that I can fit some removable front panels to provide some insulation and keep the wind chill off in winter. However, I'd like to avoid a centre support on the front edge of the pond and, for obvious reasons, couldn't provide a central support in the middle of the pond!

My plan is to provide 3 cross beams (front, centre and back) so that the effective span from front to back will only be 2.3m, support these on upright pillar supports at each end and use 5"x2" timbers for the joists running front to back.

My questions, allowing for the weight of the 5x2 joists themselves and the possibility of 4"-5" covering of snow in the winter, are:
1. What size timbers will I need for the 3 cross beams?
2. Will 4"x4" pillars be strong enough to support them or will I need to use 6"x6" for the pillars?

Any advice greatly appreciated.

Cheers

[eds]

I built a pergola for my next door neighbours (back when I was a garden designer) using 6" by 2" roofing joists. The only posts are two 6" square posts made as part of a barbeque at one corner and the other beams are attached to the house walls. This means it has unsupported lengths of 3.5m and 6m from the house walls. To be honest I only used such chunky wood as it is a 9ft high pergola over a sunken patio and it needed to look really solid. It now has a big wisteria, a number of grape vines, honeysuckle, clematis and jasmine over it and is completely solid. If it were me I'd go with the same size dimensions again, but only because it looks nice and solid; I'm sure 4" square wood would be fine from a structural point of view but might not look thick enough with big timbers for the cross beams.

[Bob Hart]

Hi Bob,

When I built my large pergola over my pond, like you I didnt want a centre support in the middle of the main pond area. I used 3 x 6"x 2" timbers 6 metres long all bolted together. These were supported on 4" square posts. The timbers from from to back of the pond were again 6" x 2", these being 1/2 metre apart and sat on top of the front timber.

I did wonder if this was strong enough, but a couple of years later and with the snow we've had over the past couple of winters, everyhting is still nice and strong.

[kayoss]

I used 3 x 6"x 2" timbers 6 metres long all bolted together. These were supported on 4" square posts.

Blimey Bob, that's effectively a 6"x6" beam! Did you consider using an 8"x2" or maybe 8"x3" so that the extra depth helped prevent any sagging in the middle of the span? If so, was it an availability problem that made you choose the 3@6"x2" option, or does the bigger cross section timber become much more expensive? Must admit I've not priced the pergola timber yet!

I'm sure 4" square wood would be fine from a structural point of view but might not look thick enough with big timbers for the cross beams.

Thanks Ed, I think you're probably right regarding the appearance of the larger posts.

Cheers

[Gazza]

Hi Bob,

I built a pergola that i was going to enclosed a few years ago...........ok more than a few years ago and its not enclosed yet

All my up stands/supports are 4x4 and the cross sections on the top are 6x2 or 3 i believe but these are 6m long as mine is a fan shape and this has held up to all kinds of weather and will be enclosed one day/The 4x4 posts actually sit on the back of the pond and are fixed by a 10mm stud which is fixed into the pond wall and then into the centerer of the 4x4 post and works a treat.

Next time you are about pop over for a look if you like mate

[kayoss]

Hi Gazza

Thanks for the info. According to my calculations, with a radius of 6m the area of your pergola is about 28m2. Just checked a photo of your pergola and you have 11 of the 6x2 beams , so whatever load is applied to roof (e.g. snow) each beam will need to support about 9% of the total load. If we assume a 4" layer of snow weighs about 10Kg/m2 (I haven't got a clue what it actually is!) that would mean each beam needs to support about 25Kg.

The area of mine will only be about 21m2, but I will only have 3 beams, so each beam will need to support 33% of the load. If we assume the same 4" layer of snow on the roof, that means each beam will need to support about 70Kg + the weight of the timbers running from front to back and the polycarb - probably more than 3 times the weight that yours support!

There must be something, somewhere that tells you the safe loads for different sized timbers spanning different lengths!

Cheers

Bob

[Jules]

Bob I did a triangular pergola in Broadstairs and used 8 x 3 timbers as the runs were 6m. I would be happy to use a 6 x 2 timber for the horizontals but would be happier with a 8x2 or 8 x 3 but you may also need to put a horizontal support unfer the middle of your polycarb to help take the load of the snow too?

Jules

[Bob Hart]

I used 3 x 6"x 2" timbers 6 metres long all bolted together. These were supported on 4" square posts.

Blimey Bob, that's effectively a 6"x6" beam! Did you consider using an 8"x2" or maybe 8"x3" so that the extra depth helped prevent any sagging in the middle of the span? If so, was it an availability problem that made you choose the 3@6"x2" option, or does the bigger cross section timber become much more expensive? Must admit I've not priced the pergola timber yet!

I'm sure 4" square wood would be fine from a structural point of view but might not look thick enough with big timbers for the cross beams.

Thanks Ed, I think you're probably right regarding the appearance of the larger posts.

Cheers

Apologies for not replying earlier Bob

The 6 x 6" beam just seemed a decent enough strength and as I was buying 6" beams it seemed a simple and easy solution.