Building and Improving the Harbor Freight 6x8 Greenhouse

The 6x8 Harbor Freight greenhouse kit can be a true bargain with its $299 price tag and an always-available 20% off coupon making it the least expensive aluminum framed/polycarbonate paneled unit on the market.   But the entry level cost brings with it some challenges which are well documented on a number of gardening sites.   These challenges include:

1) Owners report premature deterioration of the polycarbonate clear plastic panels in hot/sunny climates such as Zone 9a where I live.   Harbor Freight claims the panels have UV protection but many owners have questioned this assertion based on rapid discoloration, brittleness and pitting of the panels.

2) The aluminum frame pieces are thin and the unit is flimsy and unstable during construction making it very difficult to assemble by just one person.   Even when finished, the unit is somewhat flexible and susceptible to damage in light and moderate wind or rain.   The manufacturer even stipulates the unit should be erected only in areas “sheltered from the wind”.

3) The 6x8 unit has minimal headroom inside (approximately 6' at the peak) and the doorway is just 5'4" tall making it difficult or uncomfortable to enter and exit.

4) The unit comes with a sliding door rather than a hinged door. Owners complain the slide mechanism often jambs and that the door can be blown off the tracks in light to moderate winds.

This Instructable demonstrates how these problems can be reduced or eliminated by:

1) Covering the questionable polycarbonate panels with high quality UV protected 6 mil plastic film. (Cost approximately $60)

2) Building a rigid 2x4 framework of benches and shelving which doubles as a solid interior structure for anchoring the aluminum framework during construction and for providing ongoing stability. (No additional cost assuming benches and shelves are already planned.)

3) “Raising the Roof” of the HF unit nearly a foot by using 2x12's set on edge for the greenhouse’s base.   (Little or no additional cost compared with alternative “foundation” systems described in the construction manual.)

4) Reconfiguring the stock door so that it is solidly hinged to the framework rather than operated as a flimsy slider. (Cost approximately $20 for hinges, latches and weatherstripping.)

The tools and materials needed for these improvements will be identified in the individual steps which follow.   One cautionary note, the assembly instructions which come with the HF greenhouse are notoriously difficult to follow. The good news is there are a number of owners on the web who have documented their builds and provided helpful tips for deciphering the directions.   With just a bit of searching around you will be able to locate these references and eliminate a lot of frustration during the assembly.

One other consideration to keep in mind. In addition to the additional $80-$90 in cost for upgrading your HF unit, these improvements are somewhat time consuming, particularly the application of UV resistant film to each polycarbonate panel. I would estimate that at least 16 to 20 additional hours are needed to build the unit as described below versus erecting the unit in stock fashion.   For some people it may be worth it to step up to a more expensive greenhouse kit that includes panels with established UV protection.

Step 1: Constructing the Base

A) Lay out 4 foundation piers.

The HF greenhouse can be built on any number of “foundations”.   I chose to erect mine using the 4-pier technique but other foundations are just as effective and directions for laying out various foundations are provided in the assembly instructions.   Whatever foundation you use, just make sure it is level and will allow you to lay out the base sills perfectly square. Concrete piers are available at almost any building center such as Lowes, Home Depot, or Menards. I prefer the type with the wood block attached to the top rather that those with metal straps but either will work.

B) Cut 2x12 base sills.

The four sides of the base are made of 2x12 treated lumber set on edge so they stand 12" high. These pieces must be cut to exact length based on the four aluminum base pieces (marked #s 16,17,34 and 34 in my particular kit) that come with the HF unit.   The HF assembly instructions say the outside dimensions of the wood sill framework should be 75" x 98 ½" but after many measurements I determined it needed to be 74 ½" by 98 5/8".   To achieve these dimensions I cut my front and back sill pieces at 74 ½" and my two side sill pieces at 95 ½". Do not use these dimensions, however, until you have laid out and measured your own aluminum frame.

Before assembling the wood sills, make preliminary door cuts in the front 2x12 sill (the one on the entry door side of the greenhouse).    Later on in the construction you will be cutting the entire doorway opening out of this 2x12.   But for now make a 3"-4" deep cut on each side of where the door opening will be at both the top and bottom of the 2x12.   The will keep the 2x12 intact during fabrication but will make it much easier to saw out the opening once everything is together.    The doorway opening can be located by temporarily bolting the aluminum doorway supports (pieces identified as #6 and #7 in my particular kit) to the aluminum base piece (# 17 in my kit).   Mark the aluminum base piece (# 17) on the inside edge of the doorway support.   This location can then be transferred to your 2x12 sill piece for cutting.    For added strength and to make sure things stay square and level, a treated 2x4 is screwed to the bottom edge of the front (door side) 2x12 sill piece.

C) Assemble the base sill pieces.

Reinforced angle brackets (I got mine at Home Depot) are used to attach the 2x12 sill pieces to the 4 piers. Make sure the sill pieces are absolutely level with one another and square. If these precautions are not taken, the aluminum framework will not go together properly and the pre-drilled holes in the framework will not line up...making for MUCH frustration. So take your time and get it square and level. The 2x12 sill pieces can be fastened together at the corners with nails although I prefer to use 3" decking screws. The four aluminum base pieces from the kit (#s 16, 17, 34 and 34) can then be temporarily assembled and clamped or screwed on the four 2x12 sill pieces.

Step 2: Support Structure (Benches and Shelving)

A) A 48" long 2x4 is screwed to the inside of the base at each corner. Use a level to make sure the 2x4's are perfectly vertical in each plane. I used treated lumber throughout the interior construction but common lumber would suffice since it will have minimal contact with the elements.

B) Add a 75" vertical door post and horizontal bench support. Then, using 2x4 material, construct the center bench/shelf support and similar support at the opposite end of the greenhouse to the height and width you want for your shelves. My top shelf is 48" above the floor and 24" wide but you can build to suit your own needs.

Step 3: Aluminum Framework

A) You can now begin to assemble the aluminum framing pieces following the directions in the greenhouse kit. The aluminum pieces are fastened at various points to the 2x4 bench/shelving structure with 1" round head screws in order to stabilize the framework . To keep the aluminum pieces square and vertical, wood spacers are used between the 2x4 structure and the aluminum framework. For the most part, quarter inch redwood slats (also known as bender board) can be used for this task. You may have to cut spacers to make a more exact fit in some situations. Once the aluminum framework is completed and all the components are square and plumb, you can permanently attach the four aluminum base pieces (#s 16,17,34 and 34) to the 2x12 sills using 1" screws.

B) This step is not required but I installed a 4x8 sheet of foil faced 1" insulation on the north wall of the greenhouse. I am also making my “floor” out of washed stone over weed-resistant landscape fabric. Owners can choose any sort of flooring they desire.

C) There are a number of points where the bolts for the aluminum framework will protrude into the 2x4 framework. To keep everything lined up properly either drill out the area behind the bolt or use a chisel to remove enough wood so the bolts are not impeded.

Step 4: Opening Up the Doorway

When the aluminum framework is complete, use a saber saw or reciprocating saw to cut through the aluminum base plate and finish the cuts you made earlier in the front 2x12 sill to open up the doorway section. You can also use an angle grinder, as I did, with a metal cutting blade to cut the aluminum base piece. The final photo shows what your structure should look like at this juncture.

Step 5: Benches and Shelves

I used wire shelving (available at hardware and building centers) for the benches and shelves. You can use wood but the wire shelving creates fewer shadowed areas which reduce sunlight to the plants. The top shelf is two side-by-side units each 12" wide. They rest on 2x2's which are screwed to my 2x4 framework for support. To allow the shelving to sit level, a 5/8" wide groove must be cut down the center support for the main wire rods to fit into. I cut this slot with a dado blade on my table saw. If you do not have that equipment, you can use two 2x2 supports instead, one on each side of the metal rods.

Step 6: Apply the UV Resistant Skin.

Each clear polycarbonate panel in the HF greenhouse kit will be covered on the exterior side with 6 mil UV resistant plastic. (Note: I did not cover the 4 panels on the north side of greenhouse which are protected from the sun by the foil faced insulation.) This is a greenhouse grade material guaranteed to hold up under intense sunlight for at least four years. It is available from a number of different web sources. I purchased mine from the Greenhouse Megastore because they sell it in smaller quantities.   My 12' wide x 25' long roll was more than enough to cover the HF unit and cost approximately $55 including shipping.

I cut each piece of plastic film to the exact same size as the polycarbonate panel. The film is secured to the panel with Nashua 322 HVAC foil tape which can be found in or near the plumbing/vent pipe section of most building centers such as Home Depot. A roll of 150' is about $8 and should be enough for one 6x8 greenhouse.    The tape is 2" wide and we will be cutting each piece it in half lengthwise during the application process.

To attach the UV film to the panel, first peel off the protective plastic coating on each side of the HF panels.   Lay the UV film on top of the panel and clamp it in place to keep it in position.   Draw a line (see arrow in photo) across the end of the UV film approximately 3/8" from the end of the panel. A Sharpie Ultra Fine Point permanent marker works fairly well for this purpose.

Cut a piece of foil tape to the same width as the panel and then cut the tape in half lengthwise. This will give you two pieces of foil tape each about 1" wide. Mark the tape to keep track of which is the cut edge and which is the original straight edge.   Peel back about the first 6 inches of protective paper on the back of the foil tape and carefully align the uncut original straight edge of the foil tape with the line you have drawn on the UV film. Press the foil tape to the film and then continue to apply the film, pulling back the paper backing about 3-4 inches at a time and then pressing it to the UV film using your line as a guide. As you do this, you should have about ½" of tape extending out over the end of the panel. Take your time applying the tape but note that no matter how hard you try, the tape will not lay perfectly flat without some wrinkles and imperfections. So no need to shoot for perfection.

Starting at the center of the panel and moving outward, fold the foil tape over the end of the panel and press it firmly against the back side of the panel. Do not try to stretch or pull the UV film tight. Unfortunately, there will be wrinkles and fold marks in the 6 mil film as it comes from the supplier and they are virtually impossible to remove. If you pull the film too tight it can cause problems later on because the UV film contracts much more rapidly than the polycarbonate panels once they are under direct sunlight. If the film is too tight it can actually cause the polycarbonate panels to bow up or result in stretching and deterioration of the film and foil tape.   Granted, it doesn’t look the greatest, but at least some of the wrinkling will be reduced once the film has been in the sun for awhile.

Continue applying foil tape on all four sides of the polycarbonate panel. Once all the tapes are applied, select the bottom edge of each panel (some panel shapes have a natural bottom edge and some you just choose the bottom edge) and using any sharp instrument, poke holes through the bottom edge of the tape every 2-3 inches so that any moisture which might accumulate in the panel channels can “weep” out the bottom.

Install the completed panels in the greenhouse framing using the clips as shown in the HF instruction manual. Although I haven’t done it yet, many owners recommend putting screws directly through the polycarbonate panels and running the screws into the aluminum framework.   With the 2x4 interior framework shown above, the polycarbonate panels can be screwed to this woodwork for additional strength and to prevent the panels from being blown out in heavy winds. If you do this, use screws with plastic or rubber washers (typically used for metal pole buildings or corrugated fiberglass panels) which seal on the outside against rain and moisture.

Step 7: Attach Hinges to Door.

A) Put together the aluminum door frame following the instructions in the HF manual. Do NOT install the door hanging hardware (pieces labeled 14 & 19 in my kit). Remove the rubber seal which will most likely be pre-installed on each side of the door frame. The seal can be easily pulled out of the channel from either end.

B) Attach three hinges to the left hand side of the door frame.   I used Everbilt “Narrow Utility Hinges” (shown in photo) which are about 3/4" wide and 2 ½" tall.   The door frame itself has a small channel milled into the aluminum and a spacer is required so that the hinge will lay flat. I discovered that 3" mending plates (available at building centers or hardware stores and shown in photo with hinge) are the exact right depth for this purpose.   Unfortunately, the hole pattern on the mending plate does not match the hinge so the plate must be re-drilled to match the hinge. This can be done by clamping the plate to the back side of the hinge and then using a drill press or just a common electric drill to create matching hole patterns.

The mending plate can then be clamped to the door frame and used as a guide for drilling matching holes in the door. I put one hinge half way up the door and the other two hinges about 3 inches from the top and the bottom of the door.   The hinges are bolted to the door frame using ½" long #8 bolts and nuts.

Before hanging the door, cut off the small plastic tabs which protrude from the bottom of the door.   These are guides used to keep the sliding type door in the bottom track and will interfere with the door being opened when using a hinged configuration.

Step 8: Attach Door to Greenhouse.

If you have already installed them, temporarily remove the polycarbonate panels which are to the left and right of the door opening to give yourself some working room. Position the door square on the aluminum door framing and clamp it in place. Make sure the bottom of the door is even with the bottom of the framing side pieces (#s 6 and 7) to insure it will open and close properly. Swing the hinges so they are against the outside of the aluminum framework around the door opening. Using the hinge holes as guides, drill mounting holes through the frame. On my installation, the hinges did not lay quite flat against the framing. And if you install a spacer behind the hinge it can interfere with installation of the polycarbonate panel. I simply left this slight angle in the hinge installation and the door works fine. Attach the hinges with #8 bolts and nuts. After all the hinges are bolted up, test the door to insure it swings freely and remains lined up with the surrounding framework.

Step 9: Install the Latch.

To make the door latchable, the center panel of the door needs to be reinforced a bit. I used 1/4" Marlite and bolted it to the inside surface of the aluminum cross braces. The UV covered polycarbonate panel is then installed in its normal position on the outside of the Marlite.

To latch the door you can use almost any screen/storm door hardware available at home centers (Lowes, Home Depot etc.). I happened to use Wright Products model #V670WH which costs around $10. The HF greenhouse door is much thinner than normal screens or storms so a 3/4" redwood spacer board was cut, drilled and mounted behind the doorknob. The inside handle is mounted as the directions provide. The latch is bolted to the aluminum framework using #8 bolts and nuts. The completed door is shown in the photos from the outside and inside.

Step 10: Fill the Void.

Since our new door sits about 12" higher off the ground than the original, we have a rather large gap left open at the bottom of the door. We want ventilation anyhow, so this gap is covered with two layers of 6 mil UV film. The film is slit approximately every 3 inches and the slits are offset to reduce air flow.