Building Outdoor Sofa, need help with framing, etc.

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Forum topic by WoodyHarrelson posted 12-28-2017 02:22 PM 646 views 0 times favorited 6 replies Add to Favorites Watch
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3 posts in 927 days

12-28-2017 02:22 PM

Hello, First post here.

Making an outdoor sofa (my first piece of seating). First question: Is this a good way to frame it?

Frame will be 2×4s with a 3/4” plywood seat. Entire frame will be covered with 1/4” plywood.

Sofa will be placed on 2×3 frame cantilevered so it appears floating. 6” offset in front, 2” in back.

Planning on using wood screws for everything except the 1/4” plywood which I’ll use finishing nails. Do I need glue for something like this, or will the screws be OK?

It will be under a patio so water isn’t much of an issue. Planning to coat it in polyurethane.

All cuts are 90 degrees. I will have the plywood cut at the store, boards will be cut at home. I just moved into a house so I don’t have much.

Anything else I need to consider/improve?
Any tips, etc appreciated.

6 replies so far

View tmasondarnell's profile


130 posts in 2566 days

#1 posted 12-28-2017 04:47 PM


Looking at your framing, you should be okay structurally. There are no dimensions for the spacing for the “joists” (horizontal members going front to back) but I am guessing they are every 24”?

How is the back attached? There will be a lot of stress on the back as people lean back. Ideally, you would want one of the vertical members overlapping a horizontal member.

You are okay with just screws, but glue would add additional strength.

A couple of comments/suggestions (take they for what they are worth)

1. It is going to be heavy. Real heavy. I hope you either like it where you build it or have really some big friends.

2. You are showing the back at a 90 degree angle to the base…will the cushions for the back be angled? A 90 degree will not be very comfortable for sitting.

3. The bench is going to be low. Real low. Adding up the height of the lumber (remember a 2×4 is 1.75×3.5), I put the deck at 8.75 inches. Couch benches are typically ~12 (without the cushion).

4. For the base, I would doing something special to protect it. Every though it is on a patio, you will have lumber in contact with concrete (or tile) in an non-controlled environment. It till wick moisture and cause problems.

5. I know you say this will be on the patio, not sure of your location, but you may have problems over time with the plywood delaminating due to moisture (dew and condensation).

View tywalt's profile


104 posts in 941 days

#2 posted 12-28-2017 07:00 PM

+1 for everything tmasondarnell said. Looks fine structurally to me. Seeing as it is ply and 2 by’s you likely aren’t going for a “fancy” natural wood look, so you may want to give the whole structure a good coat of exterior grade paint/stain for a layer of moisture proofing.

What kind of wood are you planning to use? Pressure treated or cedar 2×4’s and exterior grade ply are more costly but will hold up longer in the elements.

-- Tyler - Central TX

View WoodyHarrelson's profile


3 posts in 927 days

#3 posted 12-28-2017 07:54 PM

Thanks for your replies. I finished this plan really late last night so I couldn’t dimension it, etc. The picture below is the design I was inspired by.

To answer your questions:

The back is attached to the seat at the framing (9 points of contact) and it will also be attached to the bottom frame along the back perimeter. I know it’s not ideal structurally but I think it will hold.

I know it’s going to be heavy but I couldn’t think of a way to get it much lighter. I don’t forsee myself moving it much, if it all.

The sofa in the picture above has similar proportions. I sat in it, and I’m OK with it. The seat is 10.5” off the ground, not counting the cushion.

all framing is 24” O.C. except in a few places where the framing matches the bottom frame, in order to have contact.

I live in Los Angeles (not near the ocean) so it’s very dry here, but I’ll probably look at doing something extra to the base frame in order to keep moisture away. Do they sell moisture proof 2×3s or should I coat it in polyurethane or something?

Definitely will use exterior grade plywood.

Thanks for your help so far.

View tmasondarnell's profile


130 posts in 2566 days

#4 posted 12-29-2017 01:40 AM

Finding pressure treated 2×3 is going to be tough and they will all be a pain to work with.

The problem with using a poly on the base is that as people get on and off of the couch, the base will rub against the concrete and abrade the finish. I would still seal the base and then stable some sill insulation to the bottom of the 2×3 . You can find sill insulation in the local big box building supply store in the lumber/insulation section—it is a foam gasket material that will stop the wicking action between the concrete and the wood. A roll of it should cost you ~$5.

If I was building the sofa, I would break it up into 4 modular pieces. Maybe the 2 ends sections (without the backs), the corner section and then then 2 couch sections. That would make it easier to move and give you come flexibility in your seating arrangements.

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3 posts in 927 days

#5 posted 12-29-2017 01:53 AM

I’ve thought about making it modular. It’s probably the smarter choice but I don’t want to finish that many sides, and I’m worried about the seams since I’m not covering the back with cushions. If I want to make it modular in the future I can do so without tons of trouble, but it will take a while.

How is the sill insulation attached? staples, tape, glue, or does it come with its own adhesive backing?

Thanks for the input.

View jerkylips's profile


495 posts in 3347 days

#6 posted 12-29-2017 10:04 PM

I think if I were doing it, rather than doubling up 2×4’s, I’d go with a single 2×6. For the base, as mentioned before, I’d go with a 2×4 – anything smaller is going to be challenging – finding it pressure treated, as well as finding any that aren’t warped, twisted, split, etc. Those are always in really bad shape. I’d also consider adding some feet – lagged to the base. You could put them on the inside, so they’re not visible, but it would serve to keep the entire base off the ground.

Looking at the back, you may want to consider making the base deeper to do the back differently – if you put one vertical piece on the inside of the seat base and one on the outside, you could put a bolt all the way through. This would likely be stronger. I tried to draw something up, but all I have is Word right now…hopefully this explains it.

Also, as others mentioned, defintely wood glue & clamp, in addition to screws/bolts – it will add a ton of rigidity to the project. Since it’s going to be outside I’d go with Titebond 3.

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