Adding a pergola to an existing deck?

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Forum topic by DJR83 posted 05-14-2018 05:09 PM 1119 views 0 times favorited 11 replies Add to Favorites Watch
View DJR83's profile


3 posts in 820 days

05-14-2018 05:09 PM

Topic tags/keywords: pergola deck structure

I have an existing deck on the back of my house, and I’d like to add a pergola. The deck is 17’ x 16’, and I’d like to build the pergola to roughly match the size and shape.

The attached picture shows a view of the deck from above, along with measurements and placements of the existing support posts and beams. The posts are 6”x6” and the beams are paired 2”x10”. The joists on top of that are 2”x8”, and there’s deck flooring on top of that. There is also a ledger that runs the width of the deck, so it is attached to the house. I don’t want to attach the pergola to the house though.

My idea for the pergola is this:

  • Place 4×4x10’ posts at the same locations as the deck posts. These would pass thru the deck floor and rest on the beams below.
  • For the 4 posts that are at the outer edges of the deck (A, B, D, E), lag-bolt the new posts to the adjacent joists.
  • The 5th post© would be 2 feet in from the edges of the deck, and about right in the middle between 2 joists, so I’m still wondering how to secure it. Maybe a post bracket (?) attached to the bottom of the post and to the beam?
  • Run 2×6 beams between the new posts, either sitting on top of the posts or attached to the sides of the posts with joist hangers, to form the outer edges of the pergola.
  • Run 2-3 additional 2×6 beams across the middle, parallel to the house.
  • Run some number of boards on edge, perpendicular to the house, across the top.


  • Could the side beams that run perpendicular to the house extend almost all the way to the house? That would provide an additional 3.5-4’ of coverage along the house side, where we typically place deck furniture.
  • Are 4×4 posts sufficient for the pergola?
  • Is the placement of those posts OK to carry/support the load to the structure below?
  • Any other suggestions or questions I should consider?

Thanks for any info you can provide.

11 replies so far

View AandCstyle's profile


3283 posts in 3065 days

#1 posted 05-15-2018 09:39 PM

DJR, since you aren’t getting any guidance here, you may be forced to contact an architect or a design engineer. Sorry that I can’t offer anything better.

-- Art

View uptoolateman's profile


61 posts in 878 days

#2 posted 05-16-2018 02:47 PM

There appears to be a lot of notched posts and lack of metal brackets in your pics, I don’t think you would find anybody on the forums that would be comfortable giving advice on this one. There are a lot of codes/rules about notching of posts/beams and the use of galvanized brackets for deck construction.

View DJR83's profile


3 posts in 820 days

#3 posted 05-16-2018 04:19 PM

The deck came that way with the house. :-) We bought it about 18 years ago, and the deck was at least 10 years old then, maybe more.

View JCamp's profile


1211 posts in 1358 days

#4 posted 05-16-2018 05:41 PM

I can’t really help u other than to say that I don’t think I’d try anything on a 30year old deck. Even if it was new there’s a lot there that I either don’t understand or it isn’t ideal.
I would recommend saving up and having a whole new deck built that would b designed for the overhang. Also I’d recommend if there are a lot of codes in your area that you have a contractor do it that is familiar with them.
U might do or hire someone to do a hatchet job for u that’ll work but it either won’t last a long time (if nothing else because the deck is already 30 years old) or it’ll b a issue if u ever go to sell the house and a inspector sees it. I’ve seen inspectors bad mouth good work before let alone bad work
If u want some shade mayb try a couple big patio umbrellas

-- Whatsoever thy hand findeth to do, do it with all thy might

View dhazelton's profile


2839 posts in 3104 days

#5 posted 05-16-2018 08:03 PM

“There is also a ledger that runs the width of the deck, so it is attached to the house.”

A friend asked if I’d help him rip up rotten deck boards and put down new ones for someone. I get there and start helping. Then we’re under the deck where it’s cool and I start looking at things. The outside beam was too far out from the house for the size of the joists they used. The ledger board had ONE SPIKE every for or five feet to keep it attached to the house. There were no joist hangers and only a couple of toenails attaching the joists to the ledger and outer beam. The posts were 4×4s spaced about 12 feet apart and there was no diagonal bracing to prevent racking. I kicked the base of one and it WASN’T ATTACHED to any footing – it was just stuck in the soil and had rotted. The railings were too short to meet code and the space between the deck and bottom rail would let a baby crawl through I asked the homeowner if the inspector caught any of it and he said they paid cash for the house so didn’t want to waste money on an inspector. What could have been a $10,000 bargaining chip cost him about that to rectify everything that was wrong. Assume nothing and have someone look at what you have before you put weight on it.

View Redoak49's profile


4808 posts in 2796 days

#6 posted 05-16-2018 08:32 PM

The only thing I would do is tear the deck down and build it right. There is no way I would build anything on top of it given the way it was built. Sorry…..

View uptoolateman's profile


61 posts in 878 days

#7 posted 05-16-2018 09:23 PM

Looks like they were already using big patio umbrellas from the looks of the bases weighted down with wall blocks. I would be really concerned about attaching anything that would add weight or contribute to wind shear seeing how there doesn’t appear to be a secure connection of the beams to any of the posts. It also looks like the rim joist is straddling a very thin piece of the notched post and the beam and one of the notched posts has 2 rectangular holes through it. I’m guessing the posts were notched like that to hide the ends of the beams? They should have just cut the posts down set the beams on top and out to the edge so the rim joist was sitting fully on it and attached the beam with post to beam brackets or notched the posts parallel with the beam leaving a 2 1/2” thickness of the post remaining and through bolted the beam to the posts.

The deck I removed from my house when I bought it was a disaster waiting to happen, luckily it wasn’t very high off the ground. Like D Hazelton s experience my ledger was also pulling away from the house because they only put about 8 nails in it over 12 feet instead of lag bolts and all of the joists and deck boards were rotting because they didn’t use cedar or pressure treated lumber. I also discovered they were supporting part of the deck that had rotted away with cinder blocks that was also propping the bottom of the 4X4 post for the porch roof that the bottom had rotted off. I flagged the deck on my first walk through, the home inspector flagged it and the appraiser flagged it, unfortunately it was a short sale and neither the sellers bank or seller were motivated to fix or demolish it.

View tmasondarnell's profile


134 posts in 2597 days

#8 posted 05-16-2018 11:24 PM


I’ll jump into the pool.

I have built quite a few pergolas and a couple of decks.

I few questions:

When you say “Place 4×4×10’ posts at the same locations as the deck posts. These would pass thru the deck floor and rest on the beams below”. Rest on the notchs in the posts below or on beams on the posts?

Assuming: 1. The exisiting deck structure (especially the posts) is sound, 2. The footings are appropriate & 3. You have full bearing between the 4×4x10 new posts and the existing posts (i.e. the new posts are in line and in full contact with the posts below), you should have no issues with using 4×4 for the new pergola.

What I would be concerned about is:

1. The new structure will want to rack, you need to make sure that there is sufficent structure at the bottom to keep it from tipping.

2. 2×6s for 17’ and 12’ spans are way undersized even with the minmual load (open cross boards) that will be on the roof. They will start bend under their own weight over time. You will need to up size your beams or split the spans.

View DJR83's profile


3 posts in 820 days

#9 posted 05-17-2018 02:04 AM


I do believe the deck is sound and that the footings are good.

I was thinking that the new posts would rest on top of the beams on the posts below. So the new posts would be 8 inches below the deck floor, and lag bolted to the 8” joists. Open to suggestions though, if there’s a better way.

I’d rather not put additional posts on either the 17’ or the 12’ spans, since they would end up in the middle of the seating area. Would 2×8 or 2×10 boards solve the potential sagging problem? And is it better to set them on top of the 4×4s or hang them on the sides (joist hangers or lag bolts)?


View Knockonit's profile


685 posts in 1010 days

#10 posted 05-17-2018 01:46 PM

only issue with setting posts on top of beams or other posts, without appropriate metal bases, is the hinge effect, and one must be prudent to tighten bolts occassionally till wood dries out as the shake, rattle and roll will get ugly.

the hinge effect is at the base connection, it is very difficult to get a good solid attachment in this type of build, without having post caps with post bases combined to attache new post, up high enough to keep upper post from moving laterally. Essentially the steel becomes the valid connection at that point, and becomes mechanical connection. about the only way i see it, unless you run posts completely to ground level, using the deck to diaphram the entire system. or pull down and begin afresh
good luck, might consider having a designer look at it, before going forth.

Rj in az.

-- Living the dream

View tomsteve's profile


1046 posts in 2027 days

#11 posted 05-17-2018 04:50 PM

it would be wise to check with your homeowners insurance company,too. many wont insure unapproved/unpermitted work.
other than that, i would want any posts holding up the pergola to be set in footings in the ground.

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