What's the longest span (in feet) I can have between the 4x4's of a pergola?

Question

I'm building a pergola on my deck in the back yard. The deck is 16 feet long and 12 feet deep (front to back). If I use a 4x4 in each corner, my span from one side to the other , using 2x8 lumber would be almost 16' long. Is that too long? Do I need to place another 4x4 at 8 feet in the center? Thanks in advance!

Answer 1

If you want your pergola to look good a year from now, you need to space your posts closer together.I realize you want it to be as open as possible, however you also want it to look great a year from now.You may want to reconsider your posts. 4" x 4"s are ok for smaller scale pergola, however I would jump up to 6" x6" to give it the right scale. Now you need beams at the perimeter to hold up your framing. Come in 4' from each post on the long sides (16'), and put another posts. You now have cut your span down to 8'. Use 2 layer of 2" x8"s for the long beams (16' span), and also 2 layer 2" x8"s for the 12' span to keep in uniform. Now come in 2' from the corners of the 12' sides, this will give you a span then of 8' here also. You can cover these corners with lattice and they will give the pergola a little privacy. You can put a water fountain in the corner or other plants,etc.

Remember that you need to install some bracing either under the open ceiling framework or on top to take the "shake" out of the pergola. Withour bracing you will be able to push on the posts and actually move this structure. I suggest a full length 2" x 4" diagonal on the top of the structure ceiling and the same the other direction under it. I would also set your corner posts in the ground at least 3'.

The lattice covering on the corners will also help stabilize it and give it some strength. Use heavy duty lattice or make your own out of 1" x 2 " cedar battens.

Put a center roof beam in to split ceiling rafters spacing on 8'. Use a doubled up 2" x 8" and place it in the center of the 16' span. This gives you 2 areas  8' x 12'. Run your ceiling joist ( 2"x8" the 8' way) set on top of your perimeter beams, let end overhang no more than 18". You can take a jig saw and cut angles or scroll work on both ends of ceiling joist to give it some design.

Now install your pergola sun shade pcs. of wood on top of these rafters. You can use 1" x 2" cedar strips spaced on 4" centers. If that is not enough shade you can always go back and put another 1" x 2" in between. If you want a ceiling fan in the center you need to place a pc. of plywood on top of center beam to keep water off of fan motor. Either cover the pc. of plywood with rubber, or prime and paint it with a good exterior paint. Re-coat it every spring.

Good Luck.

Answer 2

2X8 maximum span is around 9 feet unsupported at 16" centers.you would nedd 2X12 to get 12feet

http://www.awc.org/calculators/span/calc...

Answer 3

I think you need to rethink the entire plan.

1. You're going to have a hard time even finding 16' long lumber, even if it were strong enough. To get that long, you'll be looking at big timbers, not framing lumber.

2. Wood will sag badly over the long term with a span that long, even under just its own weight.

3. 4x4 legs are way undersized for the size of beams it would take to span that far. Plus, they will look spindly.

Here's what I'd suggest: Use 6 6x6 posts, one in the center of the 16' side like you asked. Span the 12' distance across each pair of posts with something like a 6x10 beam, then span the 8' between those with your 2x lumber.

Answer 4

as long as its Structual Select or #1 grade yellow pine or better it should be ok

Answer 5

i would use a double 2x10 with either 4x6 or 6x6 post.  the 2x10 beam will span nearly 18'-20'.  most span charts are based off of live and dead floor loads and since you are not installing a floor you will be fine.  but the problem you are going to have in my opinion is that the structure will look out of proportion with such skinny legs (4x4 posts), a large beam and i'm going to assume you are using 2x6 as the crown joist.

is your deck attached to your house? if so, why not bolt or lag a ledger onto you house and nail the crown joist to that no rear post will be needed (no hangers would be required since theres really no load on the joist/rafters) and then span to the beam along the front.

also try installing four post, say one at each corner and the other 4' in. this will provide the added support to use a 2x8 and give the front of the pergola a nice offset window design.  

on most the the pergolas we build we also install a 2x4 on the flat in between the post,  say about 8-12' lower then the beam, this will add a nice valance design.

keep in mind that a fir (not hem fir) 2x12 will max span and 14'6" at 16" on center, a 2x10 is just under 12' and a 2x8 is about 9'6", but these are for floor loads.  if you are using treated material they are usuaslly a mixture like SPF (spruce, pine,fir) or SPY (spruce, pine, yellow pine) so its hard to say what you are getting.