Looking to build a pergola for my back patio. I’d love to cover the whole area (30’ wide x 24’ deep) without any posts in the center. I’ve attached a picture of the space, sorry it’s dark and a mess. Just bought the place and looking to lessen some of the sun without fully roofing it. I also attached a couple pictures of what I’m thinking. I’d love the arched roof, but I’m assuming that would be even tougher.
Hire an engineer.
This is a non-trivial structure that could kill people if it collapsed. It may require trusses rather than simple beams to cover that span. Arched (bowstring) trusses are a possible "stock" solution depending what you want to do for an "arch", but in any case this is not something where off-the-cuff design seems at all advisable....
Typical single-layer arbors can only span about 10-12 feet.
To span more than that will require some kind additional support. You could do it with trusses, but the problem is that it will have a lot of wood and look really "busy" and cluttered. It will look kind of like an attic--not very pretty.
Probably, the only way to do it in an elegant fashion is to have a steel arch and then hang the arbor from the arch. The way you would do this is buy three lengths of stainless steel tubing to span the long dimension, and maybe three more to span the short dimension. You then bend the tubing into a matrix of arches and wherever they intersect you tie them together with stainless steel wire. This will give you 9 tie points that you can hang the arbors from.
To anchor the tubing you will need something like 4" lally columns with concrete footings unless you can bolt the anchor point to a structural member of the surrounding building. The anchor points for tubing are normally steel plates with bolt holes with a cucumber-shaped nose welded into it. The nose has to be welded into the plate at the right angle which depends on your arch. Normally it would be at about a 30-degree angle.
Obviously buying 250 feet of 1.5" stainless steel tubing will not be cheap. You will be looking at thousands of dollars. Also, it will probably cost maybe another $2000 for the weldments to anchor the tubing. You either have to make them out of stainless or paint them. If you paint ordinary steel anchors, they will rust eventually. That's why stuff like that is normally stainless steel, but of course doing stainless steel weldments is not going to be cheap.
So, basically the long and short of it is, that using center posts is far simpler unless you are prepared to spend about $5000+ to build a stainless steel arch, OR you are okay with ugly looking timber trusses.
As long as the roof is “open”, the design is relatively simple. When the roof becomes “solid” it will need to support a snow load, etc.
There are a few issues: 1) size of long span timbers, 2) weight of timbers for placement, 3) connection from timbers to posts, 4) size of posts, 5) connection from post to footing, 6) size of footing.
We design from the top down:
1) We typically design pergolas with large overhangs. It helps aesthetically, but it also reduces the span and the required size of the timbers. I’d install the curved beams in the 30’ direction with 3’ overhang on each side. They’d rest on the support beams in the 24’ direction with 2’ overhangs.
If you install 2x2’s @ 24” oc on 2x6’s @ 24” oc spanning 8’ (This will require 3 - curved beams.) is use 3 1/8” x 9” glu-lams for the curved beams and rest them on 3 1/8” x 9” glu-lam spanning 20’. The beams could be smaller, but would look disproportionately small compared to the span.
2) Unfortunately these beams are to be installed in the backyard. You’ll need an army to haul them to the back and raise them in place. I’d consider hiring a contractor as this is no easy project.
3) I’d use Simpson’s decorative brackets like this: https://www.fastenersplus.com/Simpson-CC44PC-Column-Cap-Black-Powder-Coated?gclid=CjwKCAjwqLblBRBYEiwAV3pCJlm9YS_vdnn_z34mQu6h5nFRzAg_38BEbe-g-J9ZX5Aym0TJT2tgQBoCTEIQAvD_BwE
4) The posts can be 4x4’s, but I’d use 6x6’s just for appearances. (You want it to look sturdy)
5) Again, I’d use a decorative base like this, https://www.fastenersplus.com/Simpson-CB46PC-4x6-Column-Base-Black-Powder-Coated?gclid=CjwKCAjwqLblBRBYEiwAV3pCJoe2u8hb_w1UbG-vW-Vr4YYszND98if6dfJoBkxNvZiwCooVWh_7BBoCypEQAvD_BwE
6) The code allows a basic soil bearing of 1500 psf without a soils “report”. So, unless you live in a marsh, a 2’x2’ footing will be adequate. However, make sure it is installed below the frost line and installed with #4 rebar at 8” oc each way. You can extend a concrete column up 2’ or so above ground level and wrap with brick or stone.
The brackets should keep the pergola laterally braced. However, you could add 2’ long diagonal braces from the column to the beams.
