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.