“There is no gardening without humility. Nature is constantly sending even its oldest scholars to the bottom of the class for some egregious blunder.”
— Alfred Austin, English poet
The amount of time it takes to build a greenhouse depends on three things — the type of greenhouse (attached or freestanding), how sophisticated you want it to be, and your budget. For instance, a simple 5 x 5-foot pop-up freestanding greenhouse from your local gardening center can be assembled in a matter of hours. On the other hand, if you plan on a large greenhouse with a concrete foundation, that can take up to a week. Likewise, an attached greenhouse, much like an added den or bedroom can take days. The first step, however, is choosing a level surface on which to build.
Choosing a Level Surface
The next stage in building a greenhouse is making sure the site for the building is fairly level. Earlier in this book, you became familiar with how to choose a site for the greenhouse so that it will receive the optimum exposure to sunlight, but that is not the only thing you need to think about when choosing a location. Another important consideration is whether the greenhouse will be constructed on ground that is level or that can be made level with a minimal difficulty. For example, a location that slopes down slightly to the south will work well for drainage; however, if the ground is really uneven, or is located below a hill or steep-slope, then water will have to be directed away from the greenhouse. Use a level, a glass tube that contains liquid with an air bubble inside it, to determine if the site is basically even. It is placed on a surface and measures whether or not the surface is truly even.
The level will indicate when a surface is both level and plumb, meaning a vertical surface, such as a wall or door, is perpendicular to a horizontal surface, like a floor. If measurements are uneven, the entire structure will be crooked and could possibly collapse.
1. Mark out the area where you are going to building the greenhouse.
2. At each corner where you are going to place the corners of the walls, put a small two-by-four down.
3. At each two-by-four, put a level down on the board to determine whether or not it is level.
4. If one corner is not level, build it up with dirt to make sure it is level and then check it with the two-by-four and level again.
Testing For Appropriate Sunlight
To determine if you have enough sunlight, on a day in the summer, either watch from your home or go out for the day and determine how much sunlight hits the ground during a given day.
• If on a sunny day no sunshine hits the ground, then the area is completely shaded.
• If on a sunny day three hours of sunshine hits the ground, then it is partially shaded.
• If on a sunny day four to six hours of sunlight hits the ground, then it is partly sunny.
• If on a sunny day at least six hours of sunlight hits the ground, then it is full sun and perfect for your greenhouse.
Preparing the Site
Once you have chosen the actual location, and tested it for appropriate sunlight and level, the first step in preparing the site for greenhouse construction is marking the location of the building’s foundation. On the site that you select, measure the dimensions and mark the corners with standard 2-foot high wooden two-by-two stakes. To ensure the perimeter is square, measure the diagonals — they should be equal. Attach string to the top of each stake and pull each piece taut so that the entire perimeter is now marked.
If you plan to install a concrete foundation for the greenhouse, set up batter boards at each corner of the perimeter to determine if the ground is level and where the greenhouse will be. A batter board is a horizontal board that is attached to posts located at the corners of the building site that shows the accurate layout of a foundation.
1. Drive three two-by-four stakes 2 feet outside the corner stake.
2. One stake should sit directly on the diagonal across from the opposite corner.
3. The other two stakes should be driven 4 feet from the stake, and positioned down each side of the perimeter.
4. There should now be a 90-degree angle formed.
5. Connect the stakes with one-by-four boards nailed level near the top of the stakes.
6. Make a small cut in each of the batter boards so they correspond with those original lines.
7. Use a level to mark an even elevation and attach the strings at each batter board.
8. Next, remove the original stakes and strings. You now have a square and level frame that matches the greenhouse site. The intersections of the remaining strings mark the corners of the greenhouse.
Constructing the Foundation
Regardless of whether a greenhouse is temporary or permanent, it should not be built on a dirt floor. The earth will eventually become muddy and attract diseases, weeds, and other pests. Again, your budget and how much time you intend to spend gardening inside your greenhouse will help you determine whether you construct a simple, small hobby greenhouse, or a larger, permanent structure with a concrete foundation. A freestanding greenhouse should be built on some type of solid surface. You do not want wet soil as a floor in a greenhouse, because it will help breed parasites that could spread disease to your plants, as well as affect the temperature. There are four common types of foundations for freestanding greenhouses: wood frame, concrete slab, concrete footing and walls, and anchor stakes, which will be discussed later in this chapter.
A firm foundation is a critical component for a greenhouse. If the base of the building is not stable, the whole building could shift, which can damage the frame and windows and could cause damage. If this happens, nothing will fit properly, including doors, windows, and glass. It must be located on a relatively level patch of ground, regardless of the type of foundation it has.
The climate and the materials that will be used to construct the greenhouse play an important part in choosing the most appropriate type of foundation for building. For example, if you live in an area that regularly experiences a very cold, snowy winter, a poured-concrete foundation will be the most durable.
In geographical areas where the winters are not bitterly cold and filled with snow, a portable, freestanding greenhouse, covered with polyethylene, would only require a base and stakes to hold it in place. Without an adequate anchoring system, a storm or strong wind could severely damage or destroy a greenhouse.
A temporary greenhouse can rest on a foundation made of treated lumber. Another low-cost choice for this type of greenhouse is a wooden post foundation. This type of base is made with pressure-treated wooden posts and is a good choice for a greenhouse constructed from wooden frames.
Many greenhouse owners use a poured-concrete footing that is about 4 to 6 inches wide. This is a relatively easy foundation to build and will act as a base for a concrete greenhouse knee wall. The knee wall, typically 2 to 3 feet in height, extends from it. In colder climates, a knee wall that extends below the frost line will prevent the concrete slab from cracking or shifting.
The footing can be set below the frost line to support a foundation wall erected on it. Some greenhouses are constructed with 2- or 3-foot high concrete, stone, or brick erected on a footing. Before concrete is poured, however, the electrical conduit, the water line, and the drain line should be installed. These need to be installed first because they will often be coming from under the ground and you do not want to break through your concrete to bring in the water line.
Even if you choose to hire a professional to build your greenhouse, it will help you to understand more about the products that will be used in the construction. Concrete is a common material, especially for freestanding greenhouses. It is made of sand, Portland cement, and a mixture of limestone and clay or marl. Most permanent foundations for greenhouses are made with sand or gravel. A concrete slab is typically 4 to 6 inches thick and poured on a base of gravel to provide drainage. Water will move through gravel and drain. Standard blocks measure 7-5/8 x 15-5/8 x 7-5/8 inches. When a block is set in mortar, it measures 8 x 8 x 16 inches. The blocks can be laid out in a wall in multiples of 8 inches.
A foundation can also be comprised of brick. A brick is made of a mixture of shale and clay, which comes in many styles and colors. Other types of brick can be found in fireplaces and walls. A greenhouse foundation can also incorporate pavers, which are typically used to build walkways. Like bricks, they are durable and weatherproof. Pavers are small stone tiles that are used to make cobble streets in some cities. They are often used in backyards as a walkway towards a fire pit.
The first step in laying out a foundation is to mark the dimensions of the foundation and make sure it is perfectly square. Remember the term plumb? It is the measurement that indicates when a vertical surface is exactly perpendicular to a horizontal surface. Think of a wall where it meets the floor — if the foundation is even slightly out of plumb, the greenhouse will be off-balance, meaning that doors and windows will not fit properly. Use the level to ensure the foundation is squared.
The next step is to determine where the four corners of the foundation will be. To do this you will:
1. Drive a stake to mark the location of one corner.
2. Set one 3-foot length of wood in one direction from the corner.
3. Set down a 4-foot length of wood so that it is perpendicular to the first length of wood.
4. Place a 5-foot length of wood between the other two lengths of wood so that it lies on a diagonal. This will form a triangle that will be square.
5. This gives you one full corner and two half corners.
6. At this point, do the same diagonally across from the full corner to create all four corners. At this point you would have four boards making up where the walls will be, and one 5-foot length of wood running between the two. This will make it look like there are two triangles making up the floor.
Wooden Foundation
One of the most common greenhouse foundations you will find is a wooden one. It is inexpensive and is often the preferred choice for many greenhouse gardeners. A wood frame foundation is one of the easiest to make. It can consist of treated lumber or landscaping timbers, and it is recommended that you use a naturally resistant wood. In Chapter 3, you learned about different types of wood and their characteristics. Woods like cedar and redwood naturally resist damage caused by rot. Pressure-treated wood is another popular choice for greenhouse construction. If you are a do-it-yourself type of person, then you will be able cut the wood to size with a standard hand or power saw. Do not forget to wear your PPE (personal protective equipment), including protective eyewear, which will prevent pieces of wood or other debris from hitting your eyes.
Once you have cut the wood to fit the greenhouse, lay the boards together on the ground the way you are going to set the greenhouse on them. To ensure you build on a solid foundation and have adequate drainage, make sure the site for the foundation is on level ground. Measure, square up, and mark the perimeter.
1. Make sure you remove any rocks, sticks, or dirt clogs that could keep the board from laying flat on the ground.
2. Set your level on top of one of the boards. The horizontal bubble in the level should be floating in between the two lines marked on the level.
3. Perform this measurement for each board to make sure that the greenhouse base is level.
Now that the foundation has been leveled, it is time to secure the timbers together with lag screws. A lag screw is heavy with a square head and is specifically designed to secure wood.
1. The lag screw should be 3 inches or longer than the first board it is screwed through to ensure it is properly secured. In the initial stage of your building process, lag screws should be attached at each corner in the same way.
2. When you have completed this task, lay a protective ground cover under the base and cut away any extra fabric from around the edges. The ground cover should be designed for landscaping, as it will permit water to drain out through it, and, at the same time, prevent weeds from growing up and into the greenhouse.
3. Dig a trench 4 to 6 inches below grade around the perimeter of the base. The width of the trench should be a few inches wider than the lumber.
4. Now, fill the trench with 2 inches of gravel.
5. Stack four-by-fours or timbers on top of each other and attach them with large galvanized deck screws.
6. Double check the timbers to make sure they are level and square.
You can anchor the base to the ground using a few different methods:
1. Drive one-half-inch reinforcing bar, or No. 4 rebar stake 3 feet into the ground at each of the corners, as well as placing one every 4 feet along the inside of the foundation. Rebar is a reinforcing bar with ridges for better anchoring.
2. Use Conduit J nails to attach the rebar. Another attachment method is to drill holes through the timbers and drive L-shaped rebar through them into the ground.
3. One more option is to cut two-by-four stakes and secure them with wood screws.
A pressure-treated wooden post foundation can provide a strong, simple base for a wooden frame greenhouse. You may require a posthole digger to dig the holes. Another tool for the job is a power auger. An auger is a powered tool used to bore holes in the ground.
Holes should extend down past the frost line. To avoid potential hazards, make sure you contact the local utility companies to confirm the locations of cables and pipes before any digging or construction begins.
Once the holes are dug:
1. Place 2 to 3 inches of gravel in the bottom of each hole, leaving a few extra inches on the height of the post.
2. Cut the post to the right length after it is set in the hole.
3. Backfill the hole and post with dirt, ensuring the post is vertical and squared up with the foundation lines. Again, a solid structure must have a properly constructed foundation to support it.
4. If everything is squared and even, then you have finished building the foundation of your greenhouse, and you are ready to begin the next step: installing the floor and securing the actual greenhouse structure to the base.
There are numerous materials for flooring a greenhouse. Again, the choice of material for building the base really depends on your level of skill, construction experience, and confidence. One of the most popular types of flooring is gravel. Many greenhouse gardeners prefer it, because it is simple to install, relatively inexpensive, and provides reliable drainage. It is also easy to maintain. Follow these steps to install your gravel floor:
1. Clear the area where you intend to place the gravel.
2. Cover it with weed barrier cloth — a thin cloth that you lay down over the ground to keep weeds from coming up. You cut holes in it to allow your plants to grow, but weeds are kept to a minimum. For the foundation, you are putting down the weed barrier cloth without holes so weeds do not start popping up in your greenhouse.
3. Shovel gravel onto the barrier cloth.
Besides gravel, other flooring options include bricks, pavers, and concrete. A concrete floor takes more work to install, but its surface is smooth and easy to clean. A concrete floor also has the advantage of reflecting light and holding in some heat.
The next task is to ensure the base is square. To do this properly, get your tape measure and take two diagonal measurements of the base. One measurement should be from the front left corner to the back right corner. The other measurement should be from the front right corner to the back left corner. The base will need to be adjusted until the two measurements are the same. You are now ready to anchor your foundation to the ground.
Now it is time to secure your greenhouse to the foundation base. If you are building an aluminum frame greenhouse, it is recommended that you use 2-inch galvanized lag screws and washers for this task. A washer is a thin plate with a hole in the middle that distributes the load of a threaded fastener. To secure your foundation base:
1. First, drill a small hole in the aluminum ridge at the bottom of the greenhouse frame to begin the attachment.
2. Insert one screw for each panel in the greenhouse.
3. After all the holes have been drilled, place a washer over each hole and secure the screws into the base of the greenhouse.
4. If you want to add extra insulation, you can caulk the bottom of the aluminum ridge sealant at the point where it joins the base. This method will help seal the greenhouse, and during the winter months, it will keep cold air from entering the greenhouse and warm air from escaping the building.
Concrete Slab Foundation
If you are very skilled in construction and comfortable working with cement, then using a concrete slab to build the base of your greenhouse will work for you. A concrete slab makes a solid base for any greenhouse. A concrete slab is the best type of foundation because it is solid, it keeps cold air from the ground rising up, and it prevents weeds from growing. However, a concrete slab is rather permanent and it can be difficult to make if you are not experienced, so sometimes a wood foundation or gravel foundation is a better option. For an attached structure, the finished floor is typically set level one or two steps below the floor of the main house.
For installation in a freestanding greenhouse, it is recommended that the floor be set several inches above the finished outside grade of the greenhouse. It also is recommended that when preparing the concrete foundation, the size should be made one inch longer and wider than the actual greenhouse’s outside dimensions. Also, the outside edges of the concrete slab should be thicker to provide greater support and to better withstand damage from frost on a freestanding greenhouse. A drain should be located in the center of the greenhouse’s concrete slab so that water will drain either into a gravel pit or into a pipe that flows into a drainage area beyond the perimeter of the greenhouse. The gravel pit helps to filter toxins out of the water that may have accumulated on the concrete.
It also is recommended that at least 4 inches of compacted gravel or stone be placed over the top of the subsoil to provide drainage. In addition, placing a 0.24 inches (6 millimeters) polyethylene moisture barrier on top of the gravel or stone will help to keep the concrete slab dry.
1. Build a form out of lumber around the perimeter. The top of the form should be at the height of the finished floor. Adding some reinforcing wire or fiber will help to increase the strength of the concrete slab. Typically, it takes about 24 hours for concrete to set.
2. When the concrete slab has set, remove the lumber forms from around the slab’s perimeter.
3. Install insulation board that is 1 to 1 1/2 inches thick vertically around the outside of the foundation. The insulation board should be installed to a depth of 2 feet. This will help to insulate the floor and will also help to keep it warmer in the winter.
4. It also is recommended that whether you are building a concrete slab or wall, you should fasten a two-by-four sill plate. The sill plate is the bottom horizontal boards of a wall where the vertical boards are attached. The sill plate will be anchored to the foundation wall, with the bottom of the sill plate 6 inches above the finished grade.
In the winter, the sill will act as an insulated buffer between the concrete and the greenhouse frame, which will help reduce heat loss just as it does in a residential building. The most common materials used for sills include naturally resistant woods, such as cedar and redwood, and plastic composite timber. The greenhouse base can be attached using concrete anchor bolts that are set within 1-foot of each corner. Additional concrete anchor bolts should be spaced about 4 feet apart. You can purchase these bolts at most hardware stores and home supply outlets. Prices vary but are in the $12 range for a box of 100.
When you have finished securing your concrete base, you can start to consider the advantages of installing a concrete, or permanent, floor in your greenhouse. Permanent floors provide a stable surface that will easily support benches, storage containers, and sinks. Once the floor is installed, there is nothing more to be done.
Of course, the simplest way to install a floor in a greenhouse is to leave the ground bare, and prepare the soil so sections of it can be used for planting. If you leave your ground bare, you will want to take precautions against having a damp or muddy floor in your greenhouse. You do not want diseases and insects to invade the greenhouse and infect and harm the plants growing there. If you plan to have a dirt floor in the greenhouse, it is recommended that you cover it with pavers, flagstones, or raised slats and create a path. That way, it will be more convenient to use a wheelbarrow to move materials inside the greenhouse. Also, because weeds can be a challenge, first lay down a use weed barrier cloth to keep them from growing into the greenhouse.
Gravel is another popular choice for a greenhouse floor. Gravel is very easy to install and it is inexpensive. Applying a layer of gravel that is 3 to 4 inches deep will keep mud and weeds at bay. A gravel floor surface lets you water plants, but allows the water to drain out through the gravel. This process also helps increase the humidity level in the greenhouse.
If gravel is your choice, then you will need to prepare the floor before installation, following these three simple steps:
1. Clear the area completely.
2. Cover it with weed barrier cloth.
3. Shovel gravel or bark on the barrier cloth.
There are advantages to this type of flooring surface. For example, it is adaptable to your gardening needs. The floor can be changed with a shovel, to remove rocks or to add soil for new a bed of plants. There is some maintenance required, such as raking and occasionally replacing the materials.
Slope and Drainage for Your Foundation
Do not permit standing water in your greenhouse. It can breed disease, as well make the greenhouse cooler in winter. During the summer months, standing water can also breed mosquitoes. Wet patches could cause you to slip and fall.
Your solution to these problems is in the slope of the greenhouse floor. The slope should be planned so that water will drain out and away from the building. If the greenhouse has a hard surface such as a concrete slab, the slope should have a 1/8 to 1/4-inch per linear foot drop.
Building with a Greenhouse Kit
If you are uncomfortable working on do-it-yourself construction projects, then a greenhouse kit could be the right choice for you. Most kits are easy to build and come with detailed instructions that will guide you through each step of the process. A greenhouse kit can be purchased online or at a garden center. If the greenhouse you intend to build comes from a kit, carefully read the manufacturer’s instructions. Never cut anything unless the instructions indicate that you should.
Before you begin assembling the greenhouse, lay out the parts on a flat surface according to their location in the completed structure. Check the part numbers as you lay them out. All pieces should be identified and properly organized before you fasten them together. If there is a mistake, it will be more difficult to rectify later.
Typically, a kit greenhouse will require a special polyfoam sill moisture-barrier/insulation to be laid on the foundation. Then, the sill or base-plate cover must be installed over the polyfoam and bolted to the concrete foundation. The walls would then be assembled and stood up on the sill or base plate, all of these steps following the instructions that came with the kit for proper, safe, and easy installation.
When you build a greenhouse from a kit, it may be necessary to set up the support posts. The support posts are fastened to the foundation plate and it is recommended that you determine the location for these posts before the foundation is poured. That way, the bolts and tubing for the posts can be inserted into the concrete while it is wet. When the concrete dries, the bolts and tubing will be set. Position the posts in the proper locations where the supports will be installed.
Some greenhouse kits may require that you assemble the aluminum struts and braces for the frame and bolt together the pieces of the pre-drilled frame. Other kits provide pre-assembled walls, roofs, and end walls, in which case all that needs to be done is bolting the top and sides together. Some greenhouse kits only require the glazing to be installed.
Pre-hung doors are a feature of many greenhouse kits. Other kits require you to hang the doors yourself. One option is to use sliding doors set on tracks. They are a good choice because they are durable.
It is recommended that as you assemble the greenhouse kit’s different parts, you remember to check that the structure is square. Also, it is a good idea to install any necessary bracing as you work. Spread out the construction over a period of two days. Tighten the bolts on the first day. Then, on the morning of the second day, you will typically find that the bolts can each be turned another half of an inch.
Building Frames
For the purposes of building a greenhouse, or most other buildings, framing provides a stable skeleton on which construction occurs. Studs, which are vertical posts that support a wall, provide a stable frame on which interior and exterior wall coverings are attached.
Frames can be made of wood or metal, and PVC, which is a durable synthetic resin made of polymerizing vinyl chloride, sometimes also used to make flooring.
Many greenhouse kits provide instructions for framing. Typically, you just follow directions to bolt the walls and roof to the sill in the proper order. Most kits include pre-cut components, so the spaces for vents and doors are already made for you.
A do-it-yourself greenhouse construction is obviously more challenging, but it does offer an important advantage: flexibility to customize the design according to the builder’s preferences. For a do-it-yourself greenhouse, sections of frame are assembled separately. Then they are attached to a sill, wall, or ledger board, which is a two-by-six piece of lumber attached to the studs with lag screws used in an attached greenhouse. The studs run along an existing wall and, like a sill, provide a surface to nail, typically for the roof rafters. Then plates, which are pieces of two-by-four horizontal supports for attaching the sidewall studs, are attached at the top and bottom.
The lower plate is secured to the sill. Rafters, which form the roof, are laid out across the plate at the top of the wall. Both plates and ledgers run the length of the house. Rafters typically run the width of the building. In a freestanding greenhouse, the rafters are also attached to a ridge board to form the roof’s peak.
To help keep the frame square as it is nailed together, it is recommended that you build each section of the frame with the pieces laid out on a level surface. A garage floor or sheets of plywood will work. If necessary, ask for help from family or friends to raise the frames. It will help you avoid dropping and damaging them, or worse, injuring yourself.
Building a Conventional Greenhouse
If you are a competent woodworker, and a do-it-yourself type of person, you may feel comfortable enough to build a conventional greenhouse. There are a number of benefits to constructing a conventional greenhouse yourself. To begin with, there are more features that can be customized according to your own taste and needs. A do-it-yourself greenhouse allows the builder to modify the plans while the building is being constructed.
When building a conventional greenhouse, however, it is important to determine which type of glazing will be used before the work begins. It is the glazing that will determine the type of framing the greenhouse will require. For example, if glass will be used, it can be placed in the greenhouse frame and securely fastened. However, a material such as polycarbonate can be attached over the framework.
A tool you will need to use is a framing square. One side of this tool is the same width as a two-by-four. This tool provides a useful measure of width as well as a square. The framing square is used to ensure that the studs are at perfect right angles from the base board.
Obviously, a do-it-yourself greenhouse will require more tools and equipment, because it will take more time to build than if you use a greenhouse kit. Constructing a greenhouse from wood, such as cedar or redwood, may require power woodworking tools, such as a planer or table saw to properly cut the lumber. Although many greenhouse owners who are handy carpenters have these tools, you can also rent them or pay a commercial woodworking company a fee to fashion them in their woodworking shops.
Lumber comes in standard lengths of 8, 10, 12, 14, and 16 feet. When you are estimating how much lumber you will need, add in some extra. The additional wood can be used to fashion braces and attach the coverings. When you plan to construct a wooden building, it helps if you are familiar with standard lumber sizes. These standard sizes will need to be incorporated into the design and construction of the greenhouse. Suppose a design requires a 4-foot, 9-inch wooden structural wall, then the studs would be cut to 4 feet, 4-1/4 inches and would use a sill plate and double cap to get 4 feet, 9 inches. Remember, a stud is a vertical post that is one of the supports of a wooden wall. The problem is that this wastes 3 feet, 7-1/2 inches on each stud. If the walls are modified to measure 4 feet, 4 1/2 inches, then the stud could be cut in half and would use a sill plate and double cap plate to measure up to 4 feet, 4-1/2 inches.
For walls that are lower than 4 feet, 4 1/2 inches in height, you can get two pieces of support wood from each stud. If you size the structure yourself for standard framing lumber, you can minimize the amount of waste and reduce the cost of construction.
Another thing to keep in mind when building with wood — use treated lumber for your greenhouse. It better resists water damage and insect infestation. To increase the wood’s toughness, and boost its light reflectivity, paint wood parts with white latex paint. Also, you always want to be sure that you keep the greenhouse frame steady. You can do this by standing on one stud as you nail in the plate to help keep the frame from moving under your feet.
You are now familiar with how to choose a level surface and prepare it for your greenhouse construction. You also learned how to install both a concrete slab for a floor, as well as a gravel surface. These construction basics have set the stage for you to move ahead and begin building the other components of your greenhouse such as the sidewalls, end walls, and roof covering.
Summary
When you are building a greenhouse, the first thing you always build is the foundation. If you do not build a foundation, unless you are making a cold-frame/hoop greenhouse, you are going to have a very unstable structure. The foundation is where the walls secure, and the foundation helps provide you with a solid surface that you can work with. The type of foundation depends on you and what you can afford and build. If you want something simple, then a gravel foundation will work. If you want to be up off the ground to prevent flooding when it rains, then a wooden foundation will be your cheapest option. However, if you want to go the extra mile, the concrete may be the best foundation for your greenhouse.