The Practical Guide To a Greenhouse Foundation
In Mulberry Greenhouses, one of our most frequently asked questions is this: Do all greenhouses require a foundation? Yes, they do. It’s important to think about greenhouses as typical buildings and plan for them as you would a home including plumbing, power and water. There are times when that is more than a gardener has bargained for and stops them in their tracks. There is no doubt that planning for a greenhouse takes time and consideration but the reward of a controlled growing environment is worth the sweat equity.
Companies that offer a built in foundation or claim not to need one. At this point, it’s important to say that we think there is great value in doing it right the first time. There have been many times when we nod and shake our heads when we hear customers complain about their big box greenhouse falling over. The truth is, we’re a little old school and we like it. We have been developing our system for over 64 years and although we are not every greenhouse enthusiast’s first stop, we are the last stop for our Cross Country Greenhouse owners and proud of it. This foundation guide will cover:
🌱 Types of Greenhouse Foundations
🌱 Things to consider BEFORE you build
🌱 Building a Wood Foundation
🌱 Building a Concrete Foundation
🌱 Details on Brick and Rock Façades
🌱 Ideas on Plumbing, Electrical and Heating
🌱 Some Handy Greenhouse Flooring Options
🌱 A List of Questions to Ask Yourself Prior to Purchasing a Greenhouse
🌱 Some Thoughts on the Costs of a Greenhouse Foundation
ONE: TYPES OF GREENHOUSE FOUNDATIONS:
Your greenhouse foundation MUST be level and square to properly anchor the greenhouse. The greenhouse is manufactured true and square and must be installed this way. What if it is not level, you might be asking yourself? If your greenhouse foundation is not level, you might experience:
🌱 The frame and panels will not align and you will think you have to cut into something and you don’t, it’s the foundation
🌱 The framing for the door will be very tough to install and then the greenhouse door will not hang properly Okay moving along to the types of foundations…
The most common foundations for hobby greenhouses are wood or concrete. Wood is the most popular choice because it is easy to work with and easy on the pocketbook. The other benefit is that wood can be easily adjusted down the line if you decide to move or extend your greenhouse.
A concrete foundation is more permanent, of course, and depending on the greenhouse you select, will become a part of your landscape for generations. Other common foundation materials include cinder blocks or bricks. Believe it or not, we have also seen a terra-cotta foundation that looked pretty great!
🌳 PRESSURE TREATED OR WOOD FOUNDATION
A hobby greenhouse (under 120 sq. ft.) can be fastened to 4" x 4" or 4" x 6" pressure treated wood foundation. For a larger greenhouse, 6" x 6" beam or 8" x 8" wood timber is your best choice. Wood timbers can be stacked to increase height inside the greenhouse and create a more permanent look. Keep in mind the higher the greenhouse, the more air inside that you will need to heat. Another option is to use wood that is rot resistant and readily available in your area. Cedar is a great choice for a greenhouse foundation.
🌳 CONCRETE FOUNDATION:
A concrete foundation is the most durable option and if properly sealed, will protect you from the elements for years. And as you can guess, the heavier the greenhouse, the more important it is to build a strong base to support it. Concrete is often necessary by your city planning office if job specific engineered drawings are required.
🌳 BRICKS OR CINDER BLOCKS
Bricks or masonry blocks are also an option, but you must have a flat surface to fasten the greenhouse to. This means that the gaps and holes are closed off. Settling and cracking is common with bricks or cinder blocks so care must be taken to prevent holes or cracks as pests will find any way to get into the greenhouse. Often a treated timber top plate covered with a metal flashing is an easy way to provide a smooth, stable surface to anchor the greenhouse and you can buy regular sealant to fix any cracks or gaps.
TWO: THINGS TO CONSIDER BEFORE YOU BUILD
🌳 SELECT YOUR SITE
Determine the best place in your yard based on sun exposure (six hours of winter sun), traffic, proximity to utilities and easy access. A flat, level surface is ideal but many customers deal with a sloped yard with a bit more digging. A slope in your yard
requires building up the area to square off the greenhouse which will require more concrete or footing showing on parts of the foundation. In many cases, the concrete wall or wood foundation acts as a retaining wall depending on how steep the slope is. If it is a substantial slope, most people will drop their foundation with steps in their stem wall. The good news is that you do not need a perfect spot with our system to make it work as you can see above. This is a custom Traditional 20 x 24 Double Glass greenhouse with an offset ridge beam, extra sidewall height and two jogs in the foundation. Anything is possible = happy customer!
🌳 FREE STANDING GREENHOUSE OR HOME ATTACHED?
There are certainly benefits to each type of greenhouse and should you decide on a home-attached model, it’s important to consider that a concrete foundation may be a better option due to settling or shifting. The next step is deciding what size of greenhouse you need based on available space, gardening needs and budget. What will your garden grow? If you are thinking citrus trees, a Cape Cod gives you the most height in the greenhouse but a Traditional is most common for gardeners who want to control the temperatures in the winter months — less air volume means less chance for cold air pockets. It is often a combination of function and aesthetics.
THREE: BUILDING A WOOD FOUNDATION:
🌱 PREPPING YOUR SITE:
🌳 OPTION 1
Dig out the top three or four inches of topsoil for the perimeter of the greenhouse (at least 12 inches wide). Now dig out all four corners to inset concrete pier blocks to ensure that the greenhouse remains level and does not shift or settle due to soft ground or frost heaves. Place inset pier blocks every 6-8 feet.
🌳 OPTION 2
If you live in a colder climate where the frost level is deeper than 12 inches, you should consider using concrete sona tube supports below grade as you would do with typical building structures in your area. Again, it’s critical that the pier blocks or sona tube supports are level. Sorry to sound like a broken record, but you will be thrilled when you install your greenhouse that it goes up according to plan!
Suggested Guidelines for Wood Foundation
* 4 x 4 timbers Up to 100 square feet or any greenhouse no wider than 10 feet.
* 6 x 6 timbers Necessary with any greenhouse with a truss (a truss typically requires a 5 inch sill to attach); any greenhouse 12" x 16" and over.
For additional strength, we recommend that the 4 x 4’s are connected with galvanized or deck screws or galvanized connectors with alternating stackable joints. For larger greenhouses, two rows of alternating stackable joints will be required. If you live in a high wind area, you should consider tying down your wood foundation. One option would be to incorporate a concrete pier block that has a galvanized bracket to anchor the wood. Others will simply drive a steel rebar into the ground and pin it to the wood.
As you can guess, when you create a perfect growing environment, you will have unwanted guests. Stop them before they start by placing a weed screen or landscape fabric over the entire perimeter and floor of the greenhouse to prevent weeds from becoming those guests who just never leave. Next step is to fill the area with gravel. We recommend 3/4” crush gravel for the ease of use, you can find it anywhere and it has the best drainage. Another good idea is to place paving stones down the center of the greenhouse. It provides an even walking surface and prevents excessive dirt from tracking in and out of the greenhouse. One thing to think about is whether or not you want to step into your greenhouse. If you plan on using a wheelbarrow, decide how high your paving stones will be and match up to the timber threshold.
FOUR: BUILDING A CONCRETE FOUNDATION:
Our recommendation is that any double glass greenhouse or any greenhouse over 12x20 should rest on a concrete footing. Even so, many customers will choose a concrete foundation for a smaller greenhouse for longevity and easy cleaning. If you are currently building a new home, adding a concrete pad or footings for a greenhouse is a great idea. It’s cheaper in the long run and hey, the builders are already there! Just a note to be sure that you have the exact outside dimensions of the greenhouse before you dig. You have a few choices when it comes to concrete foundations and oftentimes this decision should be in conjunction with your local city planning department.
FIVE: GREENHOUSE PLUMBING, ELECTRICAL AND HEATING
When planning for your concrete foundation, the best idea is to rough in ahead of time and have the ability to come through the foundation if you plan on a permanent structure. Many customers choose to at least bring the PVC pipe through ahead of time so that you can plumb or wire it after the fact. Consult your plumber and electrician to double check before pouring your foundation. For wood foundations, it is fairly common to come through under the wood after it’s constructed.
🌳 HEATING AND ELECTRICAL
A 4KW heater requires a 220 Volt outlet which usually means a new electrical subpanel. If you are thinking about a gas heater, it’s a good thing to bring in a gas fitter before the concrete gets poured. Many people ask where power should be brought into the greenhouse. We believe the best location is to put the electrical panel on either side of the door. The other benefit is that it’s right inside the door with easy access to control and switches.
SIX: GREENHOUSE FLOORING OPTIONS
Choosing between greenhouse flooring options includes considering many different factors. Drainage, decomposition, price, heat retention, comfort, and traction vary between flooring types and allow for a customization of your greenhouse that extends beyond visual appearance. Read on for some considerations when choosing your greenhouse flooring:
As mentioned, drainage is crucial in a wet environment like a greenhouse. If you’re leaning toward a solid flooring material, like concrete, keep in mind that you’ll need to install drains to help keep your greenhouse flooring dry and your plants healthy and free from mold. You should consider the location of the drains carefully to ensure the drain is not in the middle of a proposed walkway or working area. Other materials, like gravel, concrete pavers, and flagstones also drain well and are most common.
Organic materials such as bark mulch, sawdust, and wood will need regular replacing. These tend to decompose and becoming a breeding ground for algae and pests and the cost savings will become prohibitive in the long term when you need to remove and replace the rotting floor
The cost of flooring materials is by far one of the most important factors to consider when selecting between greenhouse flooring options. A full concrete slab is up to your discretion. You should plan to power wash the concrete with each annual cleaning to ensure that algae are not forming on the surface. For most areas, gravel is the most cost effective floor covering that has the most benefits in the greenhouse.
🌳 HEAT RETENTION
Heat retention is an important factor in the greenhouse if you live in a colder climate. A concrete slab and most materials such as brick and flagstone will help trap and slowly release the day’s heat during the cooler nighttime hours. This can help decrease your heating costs during colder weather.
Standing for many hours while caring for your plants can be hard on your body if you choose an unforgiving flooring type like concrete. Adding rubber mats where you stand most often may help your back and are very easy to rinse clean.
Your greenhouse floor will often be wet or covered in spilled soil. This can lead to slips and falls if you’re not careful. If you opt for a gravel floor which is the most common, we recommend to lay down a weed screen, and then fill with 3/4” crushed gravel for the best traction and drainage. You can then place concrete pavers in the aisle for walkways and work areas. For greenhouses over 14 feet wide, two aisles are common with a center bench.
Remember that your greenhouse does a wonderful job at creating an environment that plants love — and that includes weeds! Before you lay down a floor base of gravel, pavers, dirt, or mulch, be sure to put landscape fabric underneath. This will save you time and hassle of pulling unwanted weeds on your greenhouse floor. If you are plagued with pests and/or disease, removing and replacing the soil can be a back breaking job.