How we survived the Harbor Freight assembly manual, and modified our greenhouse to withstand New Mexico winds (so far)...

Saturday, August 4, 2007

Part Two: The Foundation

The Harbor Freight 10 x 12 greenhouse kit supplies a steel base to be placed directly in the ground. It's a rectangular C-shaped piece, the idea being that the hollow inside of the base gets filled with the earth and gravel of your greenhouse floor, and that anchors the greenhouse. The instructions don't mention using metal stakes, concrete piers, or anything driven into the ground for extra anchoring. Most folks believe this greenhouse needs much better anchoring, and would rather not bury a steel foundation in wet ground.

The popular solution is to build a wooden foundation, anchor it into the ground somehow, and mount the steel base on top. Everyone finds their own way to do this, but most use at least 4 x 4 sized timbers for the base.

Tip: Lay out the steel base pieces on the ground and measure them before you assemble your wooden base. The measurements listed in the manual are for a trench in the ground, not the actual size of the steel base.

We live in a warm desert climate, and our area was flat, so we kept our foundation simple. We used pressure-treated 4 x 6 timbers on edge, set into a shallow trench, levelled and squared. Temporaried with long screws, then drilled and bolted at corners. General Omar Bradley (large hairy dog in center) holds down the dirt.

24" long rebar stakes, 5/8" diameter, are driven into ground inside the foundation. Two on each side, and three on the west side (where we get the most wind.)

After driving into ground, stakes were anchored to timbers with strapping and bolts. Not very fancy, but we think the aluminum frame of the greenhouse will fail before this foundation goes anywhere.

Pressure treated 4 x 4 timbers are cut to fit diagonally inside the corners of the 4 x 6 foundation. This should help keep the foundation frame square (especially as the wood dries.) We set these a bit lower than the 4 x 6 timbers, and they should be covered up by the gravel floor later.

Four corner braces in place. Bradley holds down the interior dirt.

The steel base that comes with the Harbor Freight 10x12 kit looks to be powder-coated, in a silver color. It's not as flimsy as we thought it would be, but the shape of it does allow flex, which we'll deal with later.

Before we bolted the base pieces together, we drilled holes about 18" apart in the bottom of each piece (closer in the corners) with a cordless drill. Then we bolted them together as per the kit, sitting the base on top of our wooden 4 x 6 foundation.

As a barrier between the steel base and the pressure treated wood, we used polyethylene foam sill insulation (Lowes, 50' roll, 3 1/2" wide, for under $5.00.) I cut it in half since I didn't need the full width, and slid it under the metal base. We checked for squareness and screwed the base to the wood. Later, I trimmed off the extra insulation with a box knife, inside and outside.
You can check squareness by simply measuring diagonally from corner to corner, and hopefully the two measurements will be the same. If not, slide the pieces around until they are, and then screw them down. Squareness is important throughout this kit!
Update: I had hopes the sill insulation would compress with the weight of the structure and form a seal to keep out water, but no luck. I had to go back later and caulk the seam between the wood and the steel base, inside and out, with clear silicone caulk. That stopped rain water from seeping in under the steel base.

Now it's time to start using the manual. We'll assemble the greenhouse as per the manual, and then, before we attach any of the polycarbonate panels, we'll add some important bracing to modify the frame for strength.

Click here to go to Part Three: Assembling the Greenhouse Walls.


dblsmiles said...

THANXIVE READ ALL THE INSTRUCTIONA AND READY TO START--please why not use clear caulk/sealant to close the end of the panels as well as and especially to secure them in place? I have a teenager/laborer to the time isnt a bother? We could do it after a trual fir using just 2 clips for set-up?Bruce

mudhouse said...

Hi Bruce, some folks do use clear caulk to secure the panels in place, but I've read mixed reports about how well this stands up to wind. In my case, I didn't want to seal my panels in place with caulk, because I like to be able to remove them (mainly to replace them with screens in hot weather.) Even if I did caulk some panels in place, I'd still secure them with a screw or two in the cross braces, because I feel like that's the best insurance against our strong winds here. I'd be worried about depending only on caulk and clips, but that's just my own bias.

Closing the top and bottom of the panel with caulk might work (instead of using aluminum tape or special polycarbonate tape available from greenhouse supply stores.) However, some say it's best to have small airholes in the bottom tape, so any moisture that accidentally gets trapped in the small poly panel chambers has a way out. I'm honestly not sure how critical this is, and it may depend on your climate.

One note...I don't have any experience with caulking polycarbonate, but some folks have posted on GardenWeb's greenhouse forum that some clear caulks will not adhere well to polycarbonate in the long run. You might want to different types on a small area, and see how they adhere over time, before depending on one to secure your panels. Best of luck on your greenhouse (put that teenage labor to good use!)

dblsmiles said...

you are one of the best experiences ive had on www--
they always said niche-view

Ed said...

Thank you for posting this. I am intereted in buying two of these and do you think that I can add them together to make it 24 feet long? I was going to build another hoop house at our new home but then I saw this. The cost won't be that much more than the hoophouse if I was going to install twin wall glazing.

mudhouse said...

Hi Ed, some folks have indeed joined two of these greenhouses together, end to end. Here's a link to a fairly recent thread in the greenhouse forum at GardenWeb that has more info. If you follow the links posted in the thread, you'll find older threads with photos of how others have joined two Harbor Freight greenhouses together. Connecting Two Harbor Freight Greenhouses

I also recall reading a comment that there might be merit to leaving an internal wall in place between the two structures, if you have plants needing different winter temps (some being more hardy than others.) Just a thought, and hope this helps!

Jon from Olympia said...

I built this greenhouse last summer and your site was a giant help. You have to understand translated Chinese to use the instructions that came with it.
I gave up on the clips that are intended to hold the plastic panels in place, they are a cruel joke. I used self tapping screws and since the aluminum uprights are butter grade, it was simple, fast, and they hold up against the wind. I have never had any luck with sliding doors so I just converted them to regular. Simple to do.
Overall, with your help this kit is a real bargain. Heck, I grew watermelons in Olympia Washington.

Unknown said...

I am sure it would be possible to join two of these kits together, or as many as you would like for that matter. The foundation would get a little tricky and you might have to add an upright support in the middle if you live in snow country. I think you could even put the door on the side with a little work.


I am thinking about buying one of these greenhouses. I love your threads on the modifications you made. Just curious how your HFGH has stood the test of time? Are you still happy with it?

mudhouse said...

Hello Brian,
Yes, we are closing in on 6 1/2 years, and I'm still happy with it. I keep plants inside mine 365 days a year, and in the winter I really pack it full with plants that summer on our patios. Of course, to use it year round I had to invest in heaters and fans, but it was worth it to have a greenhouse I can use all year.

The main unfortunate development has been the problem with the Harbor Freight panels deteriorating in my sunny climate, requiring me to replace the panels. However, even with the cost of new panels from Harbor Freight, it's still a very inexpensive way to see if you enjoy having a greenhouse (compared to other kits on the market.)

Be sure to read my October 31 2013 comment on Part One of this blog, as I mention a product made by Top Secret Coatings. This clear roll-on protective coating may have promise for improving the life of the Harbor Freight panels. One Colorado poster to the GardenWeb greenhouse forum recently posted that he used this product on his new HF panels, three years ago, as he built. He states he has no deterioration, even on panels that have never been protected by shadecloth, so it might be a promising solution. I used it two years ago, on my new roof panels (which are under shadecloth, all year round.) So far, no visible deterioration on my panels either.

This kit is not for everyone, because it does require modifications to withstand winds. You must screw the panels down, in my experience, and the other modifications for strength (originally posted by others, on GardenWeb) have served us well.

However, if you're willing to do a little extra engineering, and willing to deal with the panel problem (either by replacing in time, or possibly by trying the protective product I mentioned) it's still a very good value.

I have never had a greenhouse before and didn't know anyone with a greenhouse. Without the low entry price of the Harbor Freight kit, I would have been too afraid to invest the money in a hobby I didn't even know much about. It has been a super way for me to learn. And so far, I'm not planning to replace it with anything else.


Thank you so much! It is very encouraging to hear that you are still pleased with the unit. Thanks again for all of the great advice!!!

dj amara said...

thank you so much for all the helpful info. I plan to install this on an existing cement slab. Do I still need to put a 4x4 pressure treated wood base or can I omit that and just screw in the HFGH aluminum base directly on the cement slab. thank you.

mudhouse said...

dj amara, as far as the greenhouse structure itself, it shouldn't make a difference (except of course your door height will be a few inches lower than if you set the whole structure on a wooden foundation.)

Kicking it around, we think the only question is how tricky it will be to keep the steel base perfectly lined up and square as you secure the masonry anchors/bolts into the concrete.

It might be a little less fussy to secure a treated 2x4 flat first, muscling those anchors into the concrete, and then finesse the squareness of the steel base on top of the wood, making adjustments if necessary. If the base isn't quite square, it will make lots of problems for you later in the build.

It would depend on how comfortable and experienced you are with getting the anchors into the concrete. As long as you can secure it well and make sure the base is perfectly square, it should work fine.

Some folks think it's important to add some kind of thin barrier material in between the treated wood and the steel base. To be honest, in our dry climate, corrosion of steel is a rare occurrence, and we don't worry much about it. You may want to do some checking to see if there's any need for concern about contact between the concrete and the painted steel base, without any barrier material, in your climate. (Just a thought.)

dogknowsgodnose said...

Thanks for documenting your build. I am having an issue I can't find answer for and I'm hoping you can provide insight. The instructions say to attach the floor plates to the base plates using the clips that go under the lip of the base. The illustrations show the floor plates being fastened to one another with the nut end of the bolts down. There are no holes in the base plate so the net result is a crown to the floor plate. Should there be holes in the bases plate for the bolts to go through or is the crown a necessary part of the construction? I am baffled.

dogknowsgodnose said...

....disregard my last question, I see the solution in part 3. Thanks again.