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.

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.

Wind braces for structures

There is no dispute on the need for wind braces on any structure. There is often confusion on how and when those braces need to be installed.

The sooner that some sort of bracing is installed on a building, the easier it is to maintain plumb or vertical. For this reason, we stress the importance of tying off a building, both ways, as soon as the first section has been erected.

One detail that many overlooks are the total surface area of the hoops. It does not take very long to have the combined surface of a 4’x8’ sheet of plywood. For this reason, it is important to not rely for an extended period on your initial tie-down ropes as your “bracing”. There have been instances where the combined surface area of the hoops is more than the entire gable end.

If you are doing a long building with long hoops, i.e. 30’ wide, it would be wise to install the bracing before the whole structure is assembled.

Wind bracing can be installed in two directions, one goes away from the end wall and one goes toward the end wall. Both ways are acceptable, provided that you do both. If you install them toward the end at one end, you must do the same at the other end. By doing the same concept at each end, you are essentially holding things in opposite directions.

When bracing goes away from the end, the load is referred to as “tensile load”. Cable is often used for this. The cable and the clamps used must be rated against stretching and breaking. This is a convenient system since you are not limited by a precise measurement. When using this system, it is important to end up at the ground, in the 4 corners.

When bracing goes toward the end, the load is referred to as “compression load”. Round tubing with flattened ends is most often used. It is important to make sure that the tubing is strong enough as to not bend as it is being compressed. This is the method which has been used the longest since the instinctive way to brace something it to “prop something against it”.

With either method of bracing, where you start and end is very important. For bracing to be effective, you must start at a point that is connected to the whole structure. This would be either the ridge or a row of purlins. Starting at a mid point of a hoop will give little reinforcement since the hoop can flex from side to side. The closer that bracing goes at 45 degree angle, the stronger it will be.

In the case of longer hoops with multiple rows of purlins, it is advisable to have a series of shorter braces than one long one. This means you would start at a certain hoop/purlin connection and go down and over for 3 or 4 hoops and anchor at that hoop/purlin connection. Go over to the same hoop you did the first brace on and repeat the process from the lower purlin. You would then be going down to the base or the next row of purlins. Remember that if at one end, if you are going left to right, on the other end you go right to left.

Where ever you end up, it is wise to have extra anchoring at that point.

For extra clarification, please watch our “installing wind braces” video on the website.

Hanley Installation – from our customers

We found a very helpful video from our customers we thought we would share about building a Hanley, check it out!

Where is the best place to put the greenhouse?

Considerations for determining optimum location for growing,
air flow, snow shedding and ground moisture

The first bit of advice we share when asked that question is “Do not put a greenhouse in a place that is not good for anything else”.

You need to pick the best spot to accomplish the goals for your greenhouse. Any of the reasons why this spot would be less than desirable, will come back to haunt you.

This is particularly true when it comes to ground water. If a particular piece of land is always moist or has a very high water table, moisture will be an ongoing problem. A greenhouse will trap and exaggerate excess moisture and become a very unhealthy spot for plants.

If the soil is sub-standard or undesirable where you put the greenhouse, you will be putting extra effort into bringing the soil up to standard.

Proper consideration needs to given to shading on the property. Plants grow in direct proportion to the amount of daily light they receive. Really analyze where and when shade happens. Vegetables and flowers need sun.

A greenhouse needs to be level from side to side so shed snow uniformly. With uneven snow load on a building, it will surprise you how little snow can be a problem. 

Excavation to level a site is usually expensive and messes with the make up of the soil. There are ways to build up the low side if that becomes necessary for the location of your choice.  Some end to end slope is not a problem structurally.

It is generally acknowledged that a north/south orientation will pick up the sunlight better but this is not a serious consideration unless you are into production crops.

If your prevailing winds are constantly hitting the side of the structure, there will be a sideways push which is good to avoid. This orientation will also encourage uneven snow loading. Ventilation is typically easier if the prevailing wind hits the end of the building.

Since the chances of having the “perfect” location, is not likely, it is important to evaluate all of the angles.  Some of the undesirable points can be worked around better than others.

By doing a thorough assessment and then prioritizing, you will be able to come up with the closest to perfect that is practical. You will also have an idea beforehand on what needs to be done sooner than later.

Here is to happy planning and planting.