Build A Lean-To Greenhouse To Expand Existing Buildings

You can build a lean-to greenhouse onto an existing structure to add to your growing space using these simple-to-follow plans.

by Amy Mings
PHOTO: images by Amy Mings

Whether gardening is a hobby or a profession, a lean-to greenhouse is a great addition to any property. By building onto an existing structure, you can save on materials and provide an ideal environment for more efficient gardening and extended growing options. With a basic design, it is an affordable, popular and eco-friendly greenhouse option.

When we purchased our property, it came with a large portable shed. Over the years, we added a three-sided structure to the west of the building as a shelter for our hobby farm animals and a chicken coop. We also renovated the interior of the building, which serves as our garden shed and tack room. Over the winter, we added a wood stove and knew a lean-to greenhouse would be the perfect addition to the east side of the structure to complete the overall building design. 

We briefly considered using a premade aluminum frame kit, but we wanted to be able to work with wood to fully customize the size, shape and function of our lean-to greenhouse. Our main structure was already multipurpose, and by going the DIY route, we knew we could customize the lean-to greenhouse to fit our specific needs and space requirements.

Our project gave us all the advantages of a hobby greenhouse in a familiar, yet custom walk-in design. A lean-to greenhouse is perfect for smaller backyards and limited spaces such as a balcony or patio. 

Because we were able to customize our lean-to, we could have added an exterior door, but we opted to access the greenhouse through the main part of our garden shed. We wanted to incorporate electricity, and this option allowed us to maximize all three walls and roof panels. Our situation is a bit unusual with the greenhouse opening into our garden shed, but we love the fact that the structures are connected and that the greenhouse has access to our new heat source.

During the cooler months, we monitor the humidity of our greenhouse regularly and ventilate as needed. 

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From the garden shed, we have a step down into the greenhouse. This allowed us to maximize the vertical space yet incorporate the roof line with the existing metal roof of the main structure. To do this, we measured carefully and factored in the rise in the metal roof seam (1 1⁄2 inches) before adding the top beam. With the lean-to design, we wanted it to complement the roof-line pitch of the other side of our barn yet allow us to stand up straight inside and have plenty of space for shelves. To do this, we carefully measured the height and factored in the rise in the metal roof seam (1 1⁄2 inches) before adding the top beam for the barn’s interior wall and the exterior.

We decided to have two side windows that open for venting. The windows were designed so that they were overhung slightly versus flush with the outside wall of the greenhouse. (This was to ensure that rain or moisture never finds its way behind the polycarbonate plastic panels or the greenhouse’s interior). Both windows were framed with hinges on top so the windows could be opened from the bottom.

Our design allows the windows to be opened outward, not inward. Likewise, our door opens outward as well to maximize the built-in shelving and the interior surface area of the lean-to greenhouse. 

When we drew up the rough plans for the greenhouse, we confirmed we would be able to source all materials locally. With a treated wood frame and polycarbonate panels, we knew we would have a structure that would offer an ideal growing environment and protection from inclement weather. Hopefully, by walking you through our DIY process, you’ll be inspired to build your own lean-to greenhouse! 

Building Plans

When designing our custom lean-to greenhouse, we wanted the structure to match the pitch of the other side, but it wasn’t necessary to use as much space for the greenhouse. (The other side is for our farm animals.)

We settled on a simple 8-by-8-foot structure with optimized shelving to suit our winter growing needs and limit heating costs.

Materials & Tools

  • (3) 8-foot 2-by-12s
  • (14) 8-foot 2-by-6s
  • (3) 8-foot 4-by-4s
  • (12) 8-foot 2-by-4s
  • (5) 8-foot 1-by-4s
  • 8 clear polycarbonate panels
  • joist hangers 
  • ring shank nails 
  • pole barn screws
  • deck screws
  • roof screws with rubber washers
  • 4-inch wood screws
  • lag bolts
  • table saw
  • miter saw
  • reciprocating saw 
  • electric drill
  • level
  • hammer
  • tape measure
  • carpenter’s pencil
  • steps 

1. Level Ground Surface 

Ensure the ground you are working on is level or nearly level. This is most important when framing out the base (next step). But you’ll want to level off any humps in the floor as well (lest you’ll find your roof much lower than you anticipated).

lean-to greenhouse

2. Build Base 

We attached an all-weather ground contact 2-by-12-by-8-foot boards to the foundation of our barn using lag bolts. We next attached the sides and front board using 4-inch wood screws, then reinforced the corners on the barn side with galvanized corner supports. 

We added treated 4-by-4s next to further support the base and provide the corner wall supports. Then, we added treated 2-by-8s to complete the base. We chose to go four boards total in height, but you can go shorter or taller depending on your needs.

3. Framing & Door Installation

After removing the exterior window on the garden shed wall, we cut out the door and framed it using 2-by-4s. We built this opening strong on both sides, so the door frame was very solid, and added cross supports to the existing barn frame for additional stability.

The roof header was added using a 2-by-4 lag bolted into the barn’s frame boards, then the sides were framed to meet the pitch requirements. If you are unfamiliar with cutting angles, measure the distance from the outer wall to where the top board will meet the wall, then repeat the process 3 1⁄2 inches down. (A 2-by-4 is smaller than its stated size.)

Measure the distance on your outer frame board, draw a line, then cut the angle. For easy installation on the outer wall, simply screw the board in and cut it with a reciprocating saw.

4. Frame Windows 

We built the window frames using treated 1-by-4s. The windows themselves were 1-by-4s we ripped down to 1-by-2 inches, and then reinforced them at the corners. The hinges were added on top and attached to the header board.

lean-to greenhouse

5. Attach the Exterior

We attached the clear paneling with regular 1-inch galvanized pole barn metal screws. (They have a rubber washer already attached.) 

It was important to factor in the height of the panels when planning the greenhouse. We also double-checked that the overhang from the gambrel roof on the main barn had a good overhang to allow rainwater to flow freely onto the downward slope of the new structure.

6. Building the Interior 

The interior was built with treated 1-by-6s for the top shelving and then framed out for better aesthetics using treated plywood. The top shelves were narrower by design to accommodate smaller planters and jars.

The bottom shelves were framed with 2-by-4s and topped with treated plywood and scrap trim for a more finished look. We wanted to keep the area beneath the shelves free, so we built angular supports cut at a 45-degree angle on both ends and attached to the 4-by-4s for additional stability. 

Key Features

  • Clear polycarbonate plastic panels block up to 99.9 percent of UV rays, are virtually unbreakable and provide 90 percent light transmission (resists moisture and rot).
  • Custom windows provide airflow and regulation of heat and humidity levels.
  • A treated wood base and structural beams provide high stability.

Once the lean-to greenhouse was complete, we added a vintage chandelier to enhance the overall aesthetic of the structure and a digital thermostat with a humidity meter. We love to garden and, being in southern Illinois, our greenhouse addition has helped us achieve our goal of sustaining our plants and flowers throughout the year.

We have used our greenhouse to start vegetables and flowers and to shelter frost-tender plants during the winter months. It’s truly one of our favorite places to be! 

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