This is an update of an article I wrote a few years ago and has proved very popular. There are literally dozens of manufacturers who have come and gone over the years but luckily most of the greenhouses we come across are put together more or less in the same way and therefore come apart using the same techniques.
PLEASE NOTE: The following information is provided free of charge to help anyone who is contemplating dismantling a used greenhouse. We can not be held liable for any injuries sustained by anyone following these guides as it is your own responsibility to carry out your own safety risk assesements based on the individual greenhouse concerned. Used glass is dangerous and will cut you easliy, aluminium can also present hazards which must be be accounted for. If you are in any way unsure or are not totally confident in your own ability to dismantle the greenhouse safely may we strongly advise that you do not and leave the job to someone who is.
This is a typical greenhouse that has been unused for a while. This one was made by Crittal who were quite popular a while back but like many manufacturers stopped building greenhouses many years ago.This one luckily had a fully working door and all the glass was more or less complete, many do not.
Essential tools you will need are surprisingly few, (until you come across a problem!). 10 or 11mm open ended spanner, long thin bladed screwdriver, a large bucket or trug (for the clips etc. you remove) and gloves when handling glass. Small set of steps.
You will also ideally need some willing help to pass things to and hold sections of the greenhouse upright while you are dismantling. If at all possible you should beg steal or borrow a vehicle with a roof rack or a large trailer to allow you to keep the ends assembled when you move the greenhouse. This is by far the easiest way you will find to rebuild the greenhouse. No matter how many notes or photographs you take when dismantling the ends will probably catch you out if you have to dismantle completely.
I start by taking all the glass out of the roof. There are 2 main types of glazing clips, and many variations on them, but the technique for removing them all is similar.
Starting with the top pane I place my long bladed screwdriver under the clip and gently lever away. Note how my finger holds the clip once it is released, otherwise you will end up with a garden full of flying clips and end up buying new ones, (from Elloughton Greenhouses obviously). Crittall had the good sense to use stainless steel band or "G" glazing clips which do not rust so taking these out will not be a problem.
Please also note that for the purposes of this article and clarity of the photos I am not wearing gloves. Safety gloves are essential when you dismantle an old greenhouse, the glass is virtually guaranteed to be slimy and will slip as you carry it. Cuts are inevitable without gloves. Be told!
This is a good view of the moss that soon grows in the moist areas of the roof glazing bars. It also means that if galvanized wire "W" glazing clips have been used they will almost certainally be rusted in a roof such as this.
Before you can lift the glass out you will need to remove the moss. Simply running the blade of a long handled flat screwdriver down the edge will remove this quite easily. The top sheet should be prevented from slipping down by overlap or "Z" clips. You will need to lift the sheet upwards by about 5mm to clear the clip and then will be able to remove the sheet.
Once the top sheet is out remove the lower panes. Depending on the size of the greenhouse you may find various sizes of glass in the roof. Most standard greenhouses use 24" x 24" and 18" x24" glass for most areas of the greenhouse. It is a very good idea to do a rough plan of what size glass goes where before you dismantle. This will make re-assembly a much less stressful process.
Once the roof is clear start on the sides. I have shown this clip to illustrate a common problem which I consider to be dangerous. These "G" clips are designed to hold the glass in place against the frane only. They have been used here to space the glass as well as the sheets were not the correct size.
The whole weight of the upper sheets of glass are resting on this clip and slippage is almost inevitable. The overlap of the glass is designed to be around 10mm and special overlap "Z" clips are used to set this gap. They also ensure the weight of the glass is transfered to the floor through the lower sheets.
If you look at the picture on the right this shows the correct way glass should be held in place. If you are unlucky and find the greenhouse does not have any overlap clips do not try and cut corners, make sure you put them at the top of your shopping list. There is an alternative clip available which we use when building new Elite Greenhouses, these are an aluminium part formed clip. The top is formed and sits over the lower sheet of glas, once the top sheet is in the correct position the leg of the clip is bent around the glass and holds it securely in place. This also allows for a little adjustment if the frame is not perfectly square.
Here's a common problem you may encounter. Brambles had been allowed to grow against the back wall making the glass difficult to remove. Glass always has to come out from the outside so hedges and brambles are not a welcome sight.
In this instance I managed to take one section out by working from one side and then moving to the middle section from inside the house. It meant I was working from the blind side and had to feel for the clips, it did however save me from cutting the brambles down. It did not stop me getting my arms scratched though.
Once you have all the glass out it's a good idea at this point to strap it securely into your vehicle. We use dedicated stillages that we make to transport glass but you can make do with a few sheets of timber. Glass should never be laid flat as the bottom few will break due to the weight of the others on top, no matter how flat the floor. Stand the glass on its edge against timber placed against the bulkhead in your boot or trailer, if you have to break hard it will be supported. Strap it into place to prevent it moving about.
The unglazed greenhouse starting to be disassembled. The first job was to get the door off. There is usually a bolt or bracket stopping the door from sliding completely off the track. Remove the bracket and slide the door off the track.
The roof vents need to come off next. You will find that the roof vent slides into the ridge bar from one end and the vent and ridge bar form the hinge themselves. Some form of stop is often employed to keep the vent from moving side to side. Elite use a rubber tube with a screw to lock this in place, others use a bar with a screw inside.
To remove the vent you simply lift it up as shown and slide it the shortest way to the end of the ridge bar. Dirt often accumilates inside the slot in the ridge bar making this slightly difficult but they will come out with a bit of gentle persuasion.
This greenhouse had a standard bar and peg manual window opener which did not require removing. If the greenhouse had an automatic opener then this will need removing before the vent will slide off.
The slam bars for the vents come off next. Two bolts are used to hold these in place, once they are loosened slide one side down until it comes away from the roof glazing bars. Some models may need the nut removing completely but most have a slot which allows for loosening only.
It is at this point you may find out how badly corroded your bolts are. If these two shear off rather than untighten the chances are that most of the bolts you need to remove will do the same.
It is actually possible to leave these in place and remove the two glazing bars they are bolted to as a "H" section but I prefer to remove the bar as it is very easy to twist this section in transit and damage the bar, spares will in all probability not be easy to find. There are many greenhouse spares that are universal, sadly roof vents and slam bars are dedicated to the greenhouse they were made for and the chances of another manufacturers fitting are slight.
The roof bars are the next item to be removed. You may be very unlucky and find thet steel bolts have been used in places, as have been used here. These are a nightmare and I have never sucessfully undone a steel bolt in a greenhouse. (Google "Galvanic Corrosion") Even with copious WD40 applications the rust that will inevitably be there will tend to hold the nut on so tightly that the bolt head will spin in the glazing bar channel. 
This will damage the bar and generally mean a bodge at best to refit, at worst a new bar to add to your budget, (glazing bars are fairy interchangeable within reason). I carry a small pair of bolt croppers to cut the nut off, you will be unlucky if you come across this though, most people have enough sense to use the proper aluminium bolts.
The roof bars are simply bolted top and bottom and removal is very straightforward.
In a perfect world all bolts would undo easliy. You can save a little time by using a Nut Spinner with a 10mm head to remove the nuts. Most greenhouses use 10mm nut heads, (Crittall just happen to use 11mm heads). I loosen all the roof glazing bars top and bottom and remove them as I go along. If the majority of bolts are shearing it is easier to actually tighten the bolts which will shear the bolts off with less effort.
We stock the nut spinners in our tools section of our webshop, they are not expensive and are a boon especially when you rebuild your greenhouse. They make tightening the bolts in the corners of the greenhouse a very simple procedure, using an open ended spanner will test your patience!
Once all the roof glazing bars are out the ridge bar is the next. This is a simple job, two bolts only hold each end of the bar, these are untightened and then usually slide down the end extrusions allowing the bar to come away from the end sections. This is one operation where help is a bonus.
Once the roof is off you will be left with four walls bolted together. Every greenhouse is fitted to its base in its own way, this is the time to remove the greenhouse from the base. Once free from the base each corner of the house is generally held with three bolts, two at the top and one at the base. 
The photo to the right shows a top corner section where on this particular greenhouse the bottom nut will untighten and the bolt can slide down away from the junction. The bar that the nut tightens over is the top bar of the side section. The top nut will require complete removal to allow the roof corner support bar to be removed before the bolt can be slid upwards and free of the junction. Once both bolts are away from the junction the side bar will simply slide out and the top of this section of the greenhouse will be free.
The bottom junction is dismantled exactly the same but there is only one bolt holding the bottom of the end section to the bottom of the side. At this point help is more than a bonus, you will now have a very unstable greenhouse flapping around. Carry on around the other three corners and you wll be left with four greenhouse sections looking a little like this.
This is as far as I go when I'm dismantling. I transport the greenhouse in four sections which makes reassembly a breeze. If you need to completely dismantle then I strongly recommend you make detailed notes of where each part goes, especially the ends. Keeping each section's parts separate will also help.
Rebuilding the greenhouse is basically the reverse of the dismantling procedure. The main point to remember though is that the glass is square, and will only fit into a square frame. Two things are critical when you rebuild, all four corners need to be 90 degrees, most people understand that. Where many fall down though is by not having a flat level base.
Your gutters will follow the base exactly, if you stand back and look side on at your gutters they need to be perfectly parallel. If they are not your greenhouse is twisted and I can guarantee that somewhere your glass will not fit properly. The only way this can be corrected is to lift the corner that is low, ideally by making the base true. If this is not possible you can pack the low corner and make good with concrete. Spend time and care on your base though and everything else will follow.
If you have found this useful please let me know, I love to hear from you and if you have found anything less easy to follow then I will try and rewrite it. I deal with greenhouses every day and I appreciate I may be assuming people know more than is the case. Finally if you need any nuts and bolts, glazing clips or glazing seal to rebuild your greenhouse I would really appreciate your business. If you're anywhere near East Yorkshire we are also cheaper than anyone else for glass, sadly we cannot post this though.
Good luck with your rebuild and happy growing, Ian isitt.