Adding a Softcover Structure to Another Building

There are two ways of adding a structure to the side of a building. It can be done as a lean-to (half structure) that goes parallel to the building and up to the eave, or it can be a complete building at 90 degrees to the existing building. This article applies to the latter option.

When considering attaching an MSS structure at 90 degrees to another building, there are some important considerations to be mindful of before the purchase.

The first is that these buildings are almost always considered high humidity (especially when it is a greenhouse). This means that you will be subjecting that wall to a higher level of moisture. Extra waterproofing should be considered. This high level of humidity should be an extra concern if the intent is to use the warm air as a source of heat for the solid building.

The other thing to bear in mind is the potential snow shedding patterns from the bigger building roof. If the height difference is more than 2’, measures should be implemented to slow the process of shedding snow. Without slowing the rate that the snow comes off the taller building, the force of the impact could be triple or quadruple the weight of the actual snow.

If there is a likelihood of significant snow levels being shed, we recommend reducing the rib spacing of the first 12’ of the building. Going from 4’ to 3’ spacing will increase the strength by 1/3. Going from 3’ to 2’ is a 50% increase in strength. This will give your building the added strength for the impact of shedding snow and the volume that would potentially be on the roof.

The third thing that needs to be considered is how the cover will be fastened to your shelter at the wall. For a stand-alone building, you would be on a ladder or platform off the end but this is not possible if the end hoop is right against the wall.

One option is to have the first hoop about 2’ from the wall and then cover that section with something solid (plywood, sheet metal, Lexan, etc.). This will give you a place to crawl up and secure the cover into the wirelock.

Another option would have you put the first hoop about ½” to 1” from the wall. The wirelock channel would be installed on the underside of that hoop. During the cover installation, you would slide the cover through the gap and then wrap the cover around to the bottom. The wire inserts would be installed from the underside. This option is a little more tricky when doing the double plastic cover. After the cover is installed, the gap can be filled with square foam strips which are available at the building centers. Extra care must be exercised to protect the cover from bolt heads and nuts.

The third option would be to install the structure as per normal but about 1’ from the wall. The covering would be done as usual and once this is complete, the building would be slid up against the wall. This process is a bit risky since the building is not secured to the anchors for a short period. The longer the building is, the more challenging this option is.

The last challenge which needs some attention has to do with the method of ventilation which will be used. Typical ventilation flows through the building. In this scenario, ventilating though the building would also mean that you have to go through the attached building. It can be done, but you would be best to get some additional advice on the process.

If roll-up sides are going to be used, it must be noted that the attached building will interfere with proper airflow.

The challenge with using forced ventilation is “where does the air get into the building. It would be best to create a sketch of the building with thoughts as to what you intend to do. We will use our experience to advise you.

It is important to understand and work through these challenges before you purchase. We are here to advise.

Aerodynamics Considerations on Greenhouses

Aerodynamics is a very complex concept in how it applies to greenhouses and similarly constructed buildings. The better that you understand these dynamics and how to deal with them, the better your chances of having long-term success with the structural integrity of your building.

In many of Norm’s presentations, he has compared the similar shape of a greenhouse to that of an airplane wing. The basic challenge is that you want the opposite result. An airplane needs to efficiently get off the ground while a greenhouse needs to stay where it is.

The greater the distance over a surface is (wing or building), the greater that the lift is. When height is combined with distance, there is a multiplier effect. That is why a pilot will extend the flaps on a wing when taking off and landing.

It is the vacuum on the backside of a structure or a wing that causes the lift. The upward force of the vacuum on the backside is actually double what the leading edge force is.

This effect happens exponentially even when a small change is made. Many people have suggested, “I am only lifting the building two feet (or some other relatively small amount)”. They do not realize that doing that small lift is sometimes adding 50% or even doubling the wind load on the building.

These forces must especially be considered when a structure is oriented so that the prevailing winds hit the sides of the building. Structurally, it is preferable to have the structure facing into the prevailing winds. If the lay of the land or the logistics of how you work in the structure do not allow this, there are ways that the stress on the building can be minimized.

The least expensive way to stiffen a building is to add crossties (also known as collar ties). Crossties tie the left and right sides of the structure together so that the load is more balanced in how it affects the structure. Reducing the hoop spacing will automatically make the structure stronger with a greater ability to resist the effects of external forces.

Ultimately, however, a structure’s ability to deal with the aerodynamics of a situation is only as good as the anchoring provided. Anchors provide stability for resisting up, down and lateral forces. There is no such thing as too many anchors.

These things add to the structural ability to handle things but it does not help the cover. Extra steps must be implemented to secure the sides of the cover. This includes but is not limited to adding extra and longer screws for the cover fastening system.

A discussion on aerodynamics is not complete without discussing windbreaks. The only effective windbreak is trees. Trees restrict the speed of the wind coming through and therefore minimize aerodynamic stresses on a building. There is an optimal distance from a windbreak. This is determined by the size, density and texture of the trees involved. Since this is something that changes with age and season, there is not a perfect formula.

One last note on windbreaks. Another building, especially a bigger building is not an effective barrier. If anything, this building will significantly increase the aerodynamic stresses on your building. Wind will be swirling in one direction and then a slight shift in wind direction will reverse the direction of the swirling.

Please note: We are not engineers and as such can not give structural opinions. The above points are simply items that should be considered and come from 40+ years of experience of watching what wind does to buildings.

Mitigating Climate Change with a Greenhouse

In a tongue-in-cheek manner, Norm has often declared that if it weren’t for crappy weather we would not have a business.

The plain reality of the fact is, people, use greenhouses to mitigate the negative impact of weather, and nature in general, on their food and plant production.

Greenhouses allow a person to control the local environment. The more efficient they are at that process, the more profitable they can potentially be.

The main areas of this control are heating, cooling, irrigation, disease and insect and weed control. When all of these areas are exercised efficiently, production can be significantly increased and the timing of production can be better controlled.

Heating is typically the number 1 expense in operating a greenhouse in colder climates in the wintertime. Numerous covers will reduce heat loss but it is often at the expense of light transmission. The cost of certain covers may not be justified in light of the heat savings they would create. There are creative ways that the area which needs to be heated can be reduced. Growers will often choose crops with lower heat requirements to reduce the expense. Even though the heating is usually associated with cold weather, heating can also be used when dew is present to reduce the risk of fungus diseases.

Cooling a greenhouse to acceptable levels is critical during warmer seasons. During warm times, this requires a minimum of one air change per minute. If the temperature goes about a plant’s high threshold, it will stop growing for up to two weeks. There is a wide range of options for cooling a greenhouse. Many of the passive methods are more economical to install but harder to control and fine-tune. Items like roof vents are extremely effective, and have a low operating cost but are costly to install and even more costly to automate. In addition to creating uniform air changes, it is equally important to have uniformly even moving air. Stagnant air fosters diseases.

Having an area covered with a greenhouse, allows the grower to control the water to the plants. Irrigation requires the proper timing and volume of water to the plants and also proper location. Many plants perform much better when the foliage remains dry.

Greenhouses can also protect plants from excessive rain during wet seasons. One often overlooked area of irrigation is existing groundwater. When there is a high water table, care must be taken not to over water plants.

Greenhouses provide management opportunities for disease and pest control. Especially with a proper irrigation protocol, the risk of diseases can be minimized. If insecticides or fungicides are being used, it is much more controlled in the controlled environment of the greenhouse. Systems of netting can be installed over the ventilation openings to prevent the entry of airborne pests. It is important to bear in mind that this netting does restrict airflow as well.

A greenhouse also allows for more control of the weed population. Weed seeds can be allowed to germinate in the beds and then rototill them under. In the protected environment of a greenhouse, it is easy to put down black plastic for a short period of time to kill off weeds and weed seeds. It is also important to remember that if netting is used over the roll-up side openings, this will restrict airflow. Proper airflow takes priority over weed control.

When all of the above points are worked together efficiently, the grower will be able to significantly increase his or her yields. There will also be the opportunity to control the timing of the harvest to take advantage of increased pricing. i.e having a crop ready 2-3 weeks ahead of field-grown crops will easily cover the added expenses of growing in a greenhouse.

Disclaimer: we are not professional growers. The above information should only be used to draw attention to the importance of certain concepts. It is critical to seek out the advice of a professional grower to work through issues.

If you’d like to read more about these topics, let us know. We hope to have a book about this put together in time for Christmas and Winter Planning!

Retightenting a Loose Cover

A question that we are often asked in the fall is “Do I put the cover on now or in the spring?”

As with most everything in life, there are two ways of looking at things and both sides have pros and cons. The answer is always the same though, “Put the cover on now since, in our opinion, the pros significantly outweigh the cons”

The pros of putting the cover on a new building in the fall include less frost penetration and a chance for ground moisture to start evaporating sooner than later. Less frost penetration means you will require much fewer heat units to get the structure operational in the spring (significant savings). Ground moisture is a challenge every new structure owner faces and allowing that extra time will significantly enhance the growing environment (much healthier).

The main con of putting the cover on in the fall is that with colder temperatures during installation, you will most likely be dealing with an excessively loose cover in the spring. This will be a task that needs to be dealt with to prevent premature cover wear. This will NOT be an ongoing issue. Once you deal with the loose cover, it will be good to go and add life to the cover.

Since cover tightening should not be undertaken until the temperature is consistently warm and warm enough It should therefore not be done until May and until the minimum temperature is over 20C. This requirement also means that you should use a temporary fix to carry you over until you can do a permanent fix.

Do not use rope (especially nylon) over a loose cover to temporarily tighten a cover unless you are facing an emergency. The abrasion factor will create new problems while you are dealing with the other problem. Seat belt material or ratchet straps make ideal fasteners for a loose cover.

If you do not have access to this and you know you will be dealing with a loose cover, you can call our office for some tarp scraps. These can be cut into 2” or 3” strips and then put over the cover at 12’ – 15’ intervals. It is important that the straps are lying flat on the cover before tightening.

When you have a warm calm day to tighten the cover, the job can be done in two stages. Since you must pull lengthwise, you can do one end on one day and the other end the next day. If you simply loosen one side and pull tight, you will have uneven tightening which still will have premature wear.

Once the two sides and one end are loose, pull toward the end and start refastening from the peak and work down. You are always working from the middle to the corner. If any wrinkles develop as you do this, always pull 90 degrees to the wrinkle. If the wind is still calm when you are finished with one end, you can then do the other end. It is important to remember that once you have started loosening the cover, you are committed to finishing the job. Having loose cover overnight is an invitation for trouble.

If anything in this article is not clear, please do not hesitate to contact us for clarification before you start.

Air Circulation & Humidity Control

Even though many people would consider air circulation and humidity control as totally separate functions, they are closely intertwined.

You may have the proper sized openings to create the proper amount of air changes.

You could still have hot or cold spots in your greenhouse if you do not have proper circulation.

The same can be said about removing humidity.

For a ventilation system to have optimum efficiency and benefit, there must be balance. Having proper air circulation allows you to achieve that balance.

Horizontal air flow (HAF) fans typically come with a cage around the blades, a hanger bracket and a cord with plug. This allows them to be attached or suspended from the frame at the proper location. The motors are rarely more than 1/3 hp.

HAF fans always are installed in pairs and blow in opposite directions. A short greenhouse will have one in the front right corner and in the back left corner. A longer greenhouse will still have one in the front right and back left but also two half way down the length. The one on the right will blow in the same direction as the front right. The one on the left will be blowing in the same direction as the back left.

HAF fans should never be mounted in such a way that allows them to be blowing directly at plants. This would create an uneven drying. Some people will aim the fans slightly in the direction of the cover to ensure maximum air flow along the cover to maintain dry covers.

These fans run continuously to ensure that the temperature and humidity are spread evenly throughout. It also ensures that your thermostat or humidistat are reacting to air or moisture that is representative of what is going on in the greenhouse.

Ventilation is a difficult area of greenhouse production to get perfect. The more attention you pay to the details and modify what you are doing, the greater your production. The tricky part is that with all the variables, no two years will be the same. Carefully consider all your options and the situation you’re dealing with and you will have success!

The Variations of Passive Venting

In a nutshell, passive venting is creating an opening and letting the warm / hot air escape. The simplest form of this is opening a door or window. Just because it is simple does not mean that this method will be effective in cooling your greenhouse.

If your ends face into the prevailing winds, if you can make your doors big enough and if your greenhouse is short enough. That is a lot of “if’s” and you will still be doing a lot of running back and forth to have the correct amount of opening for the amount of ventilating you need to do.

We have already reviewed the most common form of passive ventilation in the roll up sides. It was noted that roll up sides work much better when paired with a high opening to create a chimney effect. It is a fact that hot air rises so the higher you can create an opening, the more effective it will be.

If you are relying solely on gable end windows, either motorized or manual, they will need to be quite large. You will also need to have the benefit of a regularly strong prevailing wind.

The most effective form of passive venting is a continuous roof vent. This will provide a continuous opening in the precise area where the air is the hottest. A roof vent should always be mounted down wind of the prevailing wind. The benefit of being down wind is that the wind creates a vacuum as it goes over the greenhouse and sucks the warm air out.  The air intake for a roof vent is often a roll up side.

A significant down side of a roof vent is the up-front cost. The cost is the same for a narrow and a wider structure. This is the reason they are almost exclusively put on wider buildings. The cost is simply spread out over a bigger area. The effectiveness of a roof vent still makes it appealing in spite of the cost.

Roof vents can be controlled manually with a chain fall opener or with a motorized gearbox. A motorized system can be a simple open / close controller that you set the limit switches or it can have a proportional controller that allows for pre-set stages. It is with an automatic controller that the benefit of the roof vent will really become apparent. Every couple of minutes it will react and adjust to the inside condition of the greenhouse.

All of the expenditures involved with your greenhouse need to be weighed as a cost versus benefit or cost versus return. This process is especially challenging when considering the options of ventilation. Many of the expenses are subtle and hard to measure. The returns are equally hard to quantify since weather is an additional variable. This makes it even more important to keep accurate records and be aware of your options for improvements.