Joints for small greenhouse foundation

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Forum topic by NerdWithASaw posted 10-10-2017 01:21 PM 1380 views 0 times favorited 7 replies Add to Favorites Watch
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6 posts in 1147 days

10-10-2017 01:21 PM

Topic tags/keywords: greenhouse foundation timber joinery

Hi all-

I’m in the process of digging out a foundation for a Palram 6’ x 8 aluminum-framed greenhouse. I picked it up ultra cheap (someone else gave up midway through assembly!).

Originally I was going to do a concrete block foundation, but I’ve decided to go with wood. I think I’ll probably use 4×6’s, laid on their 6” side. I plan on 3 courses of these, giving me a total height of roughly 12”.

Most foundations of this style just use big long lag bolts and a butt joint. I’m thinking I’d like to do something a bit ‘fancier’, and I’m looking for suggestions. I’m definitely a beginner/intermediate wood-worker, and I have a reasonable selection of tools at my disposal—none of them amazing quality. I’ve got a 1/2 HP bandsaw, a 14” miter saw, table saw, circular saw, chisels, a plane or two, handsaw, even a semi-functional planer. Probably have other relevant tools that I forgot to mention.

My first thought was to do a sort of hybrid of a finger joint and a mortice/tenon. Basically a single finger in the end of one board, and a corresponding slot in the end of the matching board. Slide them into eachother with plenty of glue. Perhaps a couple of screws in through the top ‘for good measure’. This would look slick from the outside, and should be pretty rigid with all of that glued surface. I am a bit nervous about getting the fingers to fit snugly.

I also thought it might look cool to do (sorry if there is a term for this!) a sort of “Lincoln log” style, where each timber has a 2”x6” notch cut into it, about 2-3” from the end, and the two notches will face eachother, resulting in a flat surface. Downside to this would be that I’d need to buy 10’ timbers, since I’d like the ends to extend a few inches beyond the joint. That would add a substantial cost.

Other than being straight and square, one really important aspect of this foundation is that it be as rigid as possible. As the ground freezes/thaws, I’m hoping the entire greenhouse lifts and falls as one unit.

Would love to hear your thoughts.

7 replies so far

View Lazyman's profile


5680 posts in 2198 days

#1 posted 10-10-2017 01:32 PM

How about an open tenon joint? Basically like finger joints with one finger on one side and 2 on the other. You can drill a hole vertically through the fingers to lock it together with a peg or spike or you could just drive a lag bolt through. You can use a steel spike or rebar to sort of nail it to the ground. I would probably use 4×4 instead of 2×6.

Edit: If you really want to get fancy, you could dovetail the “fingers” to lock it together.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View NerdWithASaw's profile


6 posts in 1147 days

#2 posted 10-10-2017 01:48 PM

Yep, I think that an open tenon was actually what I was trying to describe as a “hybrid finger mortice/tenon”.

I was planning to use 4×6’s,—did you mean youd’ recommend 4×4’s vs. 4×6?

View Lazyman's profile


5680 posts in 2198 days

#3 posted 10-10-2017 02:07 PM

4×6 would be fine too. I saw the 2×6 in reference to the notch and I guess had it in my mind that’s what you were planning to use.

The more I think about it, the more I like the idea of making it a corner through dovetail joint instead of just an open mortise and tenon joint. Not much harder to make than the mortise and tenon and it will hold together better, though I would probably still put a spike or dowel through it.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View NerdWithASaw's profile


6 posts in 1147 days

#4 posted 10-10-2017 02:40 PM

Nice. I like the looks of the corner through dovetail. Definitely intimidating to cut, but I think it would be a good step in my skills.

Any recommendations on what tool(s) you’d use for the cuts?

View PPK's profile


1770 posts in 1620 days

#5 posted 10-10-2017 02:47 PM

I built a green house similar to the size you are referring to when I was in High school. I dug out the sod, laid some gravel (about 5”), then laid down some 8’ by I think 2”x6” precast concrete things on the gravel. The greenhouse sits on these. To this day, the green house hasn’t moved, and is level. I have no idea where my dad procured those concrete “boards”. Anyway, sounds like a cool idea. I guess I’d just lap the courses of 4×6’s if it was me… but the way you are proposing sure would look cool!

-- Pete

View Lazyman's profile


5680 posts in 2198 days

#6 posted 10-10-2017 03:08 PM

If you think about it, the dovetail is not really any different to cut than the straight mortise and tenon. The cuts are pretty much the same but might take a little more work to sneak up on a good tight but not too tight fit. The ideal tool is a nice back saw but finding one large enough to cut 4-6 inches deep could be difficult so you may just have to use the best quality hand saw you can afford.

The key is to make sure that you keep the saw perpendicular to the end of the timber as you cut. Mark and cut the tail first and then use the finished tail to mark the pins on the end of the other timber. Use a knife to mark the line to get a nice clean line. Cut on the waste side (inside) of the lines. Err on the side of a too tight fit and clean out the waste with a sharp chisel. Test the fit and and use the chisel to sneak up on a snug fit but not too tight as you will get some shrinking and swelling across the grain, since it will be exposed to the elements and you don’t want it to swell and split the pins off. It needs to slide together without using a mallet. Make sure that you label the ends that go together because small variations in each finished joint might not fit another end.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Loren's profile


10477 posts in 4458 days

#7 posted 10-10-2017 04:12 PM

You can cut dovetails in large timbers on
a band saw but it may be more hassle to
set up than you want to go with.

One thing you’ll want to watch out for is
twist in the timbers. Even a little can
throw your joined frame out of flat,
though gravity should work in your favor
some. I would use winding sticks to make
sure the ends of the beams are in the same
plane. If not, a jack plane can fix it.

You may want to consider doing a haunched
dovetail so the tail is only 2” or less thick on
the outside with a rabbet behind. It’s done
that way often in timber framing and makes
excavating the recess for the tail a quicker
job. You can use a router for that or do it
the traditional way with a big chisel.

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