Introduction: Octagonal Greenhouse

I have been dreaming about a greenhouse for a couple of years now and was not satisfied with any of the kits out there. I wanted a place to over-winter a few favorite plants and start new ones in late winter. This was just an experiment, so I wanted to keep cost to a minimum, since gardening is just a budding hobby of mine. It needed to be pleasing to look at, as well as look like it belongs on my property. I began by first finding the best location that has maximum sun exposure to the south, and will get full sun from east to west, especially with low sun angles in the winter.

I appreciate everyone who takes the time to post their ideas, plans and pictures out there for all of us Do-it-yourselfers. I have benefited from many. I wanted to document the phases of this project to hopefully give someone ideas out there.

Step 1: Foundation

After I drew up the plans, I put a spike in the ground and attached some twine to a spray paint can and the spike and marked the circle to dig the foundation. A Bobcat would have made quick work of the granite and clay, but I chose the hard way with a pick and shovel.

Next, I built the base so I could construct a concrete form. I used scrap 1/2" BCX to build the form directly around the base. It was a little flimsy, so I braced the corners and from side to side so I could move it to the location.

Step 2: Setting the Form

Before placing stakes to secure the form, I determined true north/south so one wall would face perpendicular to the north/south line. The string in the first picture is the north/south line. I pounded the stakes in partially, then placed the level across in multiple positions and drove the high spots down with a mallet. I had previously flooded the hole to soften the clay to do a final leveling. The high spots that were sticking out of the water were easily leveled with the back side of a rake.

In hindsight, I should have built the form deeper. I was not satisfied with the size or weight, because I live in a high-wind area. I see that most people just build theirs on pressure treated 4x4's. I wanted my base to outlast pressure treated wood and be heavier. So, I built a form the shape of each side of the octagon and made eight trapezoids which I later fixed in place with mortar. This allowed me to backfill the original foundation and leave the blocks above grade so I wouldn't have wood to ground contact.

Step 3: Framing the Structure, Base and Walls

The first picture is a load of recycled Redwood with old red paint on it that used to be my bosses front deck. I trashed out a set of planer blades removing most of the paint and some of the raised grain from weathering. Most of the knot holes had rotted, but the wood was still in great shape and structurally sound beneath the paint and the superficial weathering.

The second picture is the same base depicted above after a few minutes with a belt sander. I ripped the 2x6 redwood down to 2x4's. The cut-offs come in handy later for the cleats and trim work. The King studs are 72", the base pieces are 28 1/2", and the sills are 27". I used Simpson Strong Ties to fasten the king studs to the base on a 22.5 degree angle. Once all eight were in place, I cut seven sills. Each side of the sills needed to be mitered to 22.5 degrees. The sills were set 23.5" from the top of the base plate. I wanted to maximize the use of a commom 4' x 8' exterior siding panel. Since you lose some of the width or length from the kerf on a saw blade, I figured I would cut the dimensions down to have a perfect fit.

One side of the octagon was left open for a rough opening for a door. I then cut out eight more pieces, the same as the base, and fastened them to the top of the studs for wall caps using the same stud ties used at the base. Next is the trim/cleats for mounting windows and lower wall panels.

Step 4: Cleats and Trim

I didn't state it earlier, but I did some research on materials used for greenhouses. I liked the corrugated polycarbonate for windows or glazing. The problem is that it only comes in 26" wide panels, which turns out that they are only 25 3/4". So, I reverse designed the whole structure around the 26" width for the windows and decided on the height based upon the common lengths. I wanted minimal waste.

The cleats for the window sills, base and wall caps are 26" wide x 1 3/8" x 1 3/8". I planed the paint off of all the 2"x2" deck rail spindles which gave a finished dimension of 1 3/8" x 1 3/8". They are set back 1" from the edge of the sills to account for the horizontal closure strips for the polycarbonate and the depth of the panels. I used a small carpenter's square to lay out pencil lines for the location of the cleats. Again, the edges were mitered to 22.5 degrees.

For the vertical cleats, I used the cut-offs from ripping 2x4's from the 2x6's. I then set my table saw blade to 22.5 degrees and ripped that angle so the cleat would sit flush with the studs and remain parallel with each of the outer walls for easy mounting of the the window glazing and the lower panels.

Step 5: Framing the Rafters

At this point my son and I moved the structure out to the foundation. I secured the structure to the concrete with Tapcon screws. The screws are great, because they come with the masonry bit to punch the pilot hole for the screws.

Picture number two is a top view of the ridge block/compression ring that joins the rafters. I wasn't sure how tall I needed it, because I actually made it before drawing out any plans. I went with 7" in case my glue-up wasn't the greatest and I needed to trim the ends. I used double rows of biscuits and Titebond III. The glue-up was good and I left the entire length. I cut a reverse chamfer in the ends of the rafters that join to the corners of the ridge block.

The ends were joined by driving a screw from the lower inside and the upper outside. The rafter tails were joined with Simpson Strong Ties. I wound up buying new Green Douglas Fir for the rafters.

Step 6: Rafter Blocking and Framing Windows

I need to seal the greenhouse to the outside air. To do this, I cut 2x4 lumber. First I measured the distance between the rafters. I set the table saw to rip a 30 degree chamfered edge so when the roofing substrate was installed, it would sit flush with the rafters and blocking with minimal air gaps. Next I mitered the edges to 22.5 degrees to fit in between the rafters.

The last pictures are views from the outside and two inside. The windows are constructed from trim pieces that were ripped to 22.5 degrees to fit in the openings. The vertical pieces, or stiles, are the pieces of 2x2 ripped to 22.5 degrees. The rails are just plain 90 degree pieces of 2x2 cut to length. The second to last picture shows a small 45 degree chamfer that I cut so the window wouldn't hang up on the sill when opening. I discovered this was happening after I installed them. The last picture is a view from the inside with the window closed. I wanted extra ventilation for the summer and wanted it to be completely closed on very cold days.

Step 7: Framing the Door

At this point, I needed a door. I started by removing the four stud ties in the first picture. I measured the rough opening and subtracted 1/4" from the height and width. The rail ends and sill ends were mitered to 22.5 degrees. The length was the rough opening minus 1/4" minus the thickness of two 2x4's. I measured for the location of the sill so it would match the surrounding walls and be visually on plane. I trimmed the window opening and the lower panel opening with the same trim used on the walls. The still needed to accommodate the clear polycarbonate panels and lower wall panels.

I held the door in its opening with 1/8" thick shims to suspend and position, and used clamps to hold the door firmly in place while I marked the hinge locations and used a chisel to cut hinge mortises. I also cut a channel above the hinge pins so I could knock them out at a later date in the event I needed to remove the door. A traditional hinge set-up would have negated this step. At this point in the construction, I was winging it. In hindsight, I have had some better ideas.

Step 8: Ridgeblock Cap/Cupola

My ridge block is open to serve as a vent. I needed to keep out the rain, but be able to vent hot air. The picture on the left shows the cupola that I created. The walls tilt out 10 degrees. I drilled two 1" holes on top of each other on each wall and finished off the ovals with the scrollsaw. I then used a small sanding drum in the drill press to smooth out the cuts. I used this site to figure out the angles.

The top was created by gluing up a 9" x 9" panel out of 2x4's. I made it slightly larger so I could rip it clean when the glue was dry and cut to final dimensions. Once this was done, I cut all the corners off at 45 degrees. I then made a holding jig to guide the panel through the table saw on edge, similar to the idea of a tenoning jig. I set the table saw blade to 10 degrees and the maximum height, and made eight passes through the blade, one for each edge. To detach the pieces left intact from the 10 degree cuts, I set the table saw blade height to 1/2" and set the fence 7" from the blade. I made eight passes with the panel on its face to detach the pieces left from the vertical passes to create the little top hat portion on the cupola roof as seen in the first picture.

I was so excited to get this piece done that I didn't take pictures of the steps. The last two pictures are something I came up with to open and close the vents. I took a scrap piece of oak that was about 1" x 1". I drilled a 3/8" hole in the center. Then I place a Hex nut on top, marked the edges of the nut and cut a recess in the top with a chisel to embed the nut about half its depth. I cut a 6 1/2" octagon from plywood and turned a handle on the lathe. I drilled the handle for the 3/8" threaded rod and recessed a nut in the upper end. Next I cut the threaded rod to length and epoxied the rod into the handle. Then I installed some weather stripping on the underside of the ridge block and threaded the whole assembly into the upper block shown in picture three. Picture four shows the vent closed. Again, I wanted this feature for winter time.

The upper vent openings were screened with pieces of lath and stapled in place.

Step 9: Paint

Of course, paint is self explanatory. I removed the door, windows and hinges for this step. Paint on hinges just looks sloppy.

Step 10: The Roof

The roof was a puzzle. I used LP Tech Shield Radiant Barrier for the roofing deck. I liked the idea of the reflective surface to bounce around more light inside the structure and hoped that it would create more heat on the inside. Now that the structure is completed, it works a little too well. It is now October and with all the vents, windows and door open, it got very warm when I was painting the shelving.

The eight panels that I cut were trapezoid shaped like the picture of the asphalt shingle above in the second picture. There were slight variations in dimensions so each one was measured and cut individually. Once screwed in place, I installed a flashing drip edge to cover the ply and tops of the rafter tails.

If you build this structure, 2 bundles of 3-tab asphalt shingles are enough, especially if you plan out your cuts to use smaller pieces as the courses of shingles get narrower. I used asphalt felt as an underlayment. I only used about 38 feet of the roll, so if you can get a partial roll, do it or you will have lots of leftover. Refer to a site that provides instruction on roofing if you have never done this before. I didn't document this step, because it was very tedious. If you have a helper to make cuts for you, you can stay on the ladder and continue nailing. The roof took me all day to finish, because every shingle needed to be cut, including cutting the tabs off of the starter course. These cut-offs came in handy for use as ridge caps at the eight ridges formed.

Step 11: Lower Panels and Windows

The windows were easily cut to length with tin snips and installed with 1 1/4" screws designed for use with polycarbonate panels. I watched a video on the panel manufactures' website for cutting and installation. I could have gotten away with using 1" screws for this step. The lower panels were cut from 2 sheets of exterior siding and painted with two generous coats of exterior paint. They were screwed into place into the cleats that were installed in the beginning.

Four of the panels were just cut to size. On the other four, I cut rough openings for the vents, allowing a 1/2" border to screw into. On the inside of the opening, I constructed a vent box with dados cut in the long sides to accommodate panels for closure on the inside. If you wish to see these details, I can take pictures and upload to this document. I was winging it without a plan at this point. Again, on the coldest, snowy days, I want to seal off the structure.

Step 12: Interior Shelving

At this stage, I still had leftover decking rail spindle pieces made from 2x2's. I used the leftover 2x6 material and ripped into 2x2's for the remainder that I needed. There were some 2x3 rail pieces that I used for ledger boards and rim joists to support the 2x2 shelving. In hindsight, painting these parts ahead of time would have been ten times faster. Using a sprayer, even better.

Step 13: Original Drawings

My drawing skills are limited without a drafting arm and I have no training with my 2D CAD program, so I usually stick with graph paper, ruler, and front/side/top elevations. Unfortunately, I just visualize most of my projects and I usually scribble down my materials list on a piece of scratch paper.

Step 14: Updates

It is now April of 2016 and the greenhouse is serving its purpose well. Doing some research, I read that water holds four times more heat than earth and concrete. I added six five-gallon buckets with lids filled almost all the way with water. They are placed below the shelving. This has served to keep the overnight lows to 45 degrees Fahrenheit, when outside temperatures have dipped into the teens and low twenties back in March and a few times in April.

Once February hit, I couldn't wait any longer for the growing season, and when the end of the month hit, I planted all my seeds for this year's garden. I started everything in seed propagation trays, and after about three weeks, I had to replant almost everything. These photos are a month old now, taken at the beginning of April. I had one or two days where I was concerned that my plants would die from the cold, so I ran an extension cord and a heat lamp inside the structure so my plants could weather a two-day snow storm.

Over the Winter, I built some new raised planter beds with removable cold frames. My Zucchini had gotten so large in their cups, I was forced to plant them and they are thriving under the cold frames in the raised beds. My wife asked for some steps to get up to the structure, so I pushed my wheelbarrow around the property to collect flat slabs of granite and basalt to serve as steps. All the plants in the picture above are ten times larger and competing for sunlight. I am in Zone 6 and I am patiently waiting for the last frost date to put the bulk of my vegetables in the ground. My Tomatoes and Bush Beans are already the size of the ones you buy from the garden center.

In hindsight, I wish I would have at least doubled the size of the structure. I had a 72 cell and a 50 cell propagation tray and once I started replanting my seedlings, I was running out of sunny shelf space. I did go a little crazy with the hot pepper varieties this year.

Step 15: Window Friction Brace

April 2017: Last season, I was propping the windows open with sticks. I wanted something a little more finished to hold the windows open so I started looking around for ideas on the internet. I can't recall where I got my ideas from, but I have drawn plans for the wooden hardware. All I had to do was cut everything out, drill the window jambs and install threaded inserts to accommodate 1/4" carriage bolts locked into hand wheels with a washer, nut, a second nut to act as a jamb nut and a second washer for the window prop to slide along. The hand wheels were cut from 3/4" Red Oak. The window prop was cut from 3/4" plywood. Use a good plywood with minimal core voids. Refer to my plans and compare to the pictures.