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    • CommentAuthorShepherd
    • CommentTimeJan 21st 2009
    We're putting a lean-to greenhouse on the back of an outbuilding. Done the levelling of the ground, now working out the details of the structure before we buy the raw materials. Basically going for a DIY timber frame, and currently working out whether the roof will be corrugated uPVC or glass. Concern we have with glass is the impact strength - as in when our cat which can get up onto the outbuilding roof, jumps down onto the greenhouse roof, we don't want him to go crashing through. (Drop of a couple of feet, cat is a large ex-tom who weighs 12lbs - muscle not fat BTW)

    Greenhouse glass is usually 2mm - can we safely use that or would we need 4mm? Would we need to go to the point of toughened 4mm?
    • CommentAuthorCWatters
    • CommentTimeJan 21st 2009 edited
    I have polycarbonate sheets as "temporary" protection over some windows in my delapidated croft house. They were installed in 1999, clamped in place with a frame of timber and sealed with mastic. Due to circumstances beyond my control (!) the sheets are still in place and have put up with everything the highlands can throw at them. During a blow the windows flex in and out quite a lot (understatement) but, touch wood, show no signs of cracking after almost 9 years. Purchased mine from Wickes.
    • CommentAuthorShepherd
    • CommentTimeJan 21st 2009
    Thanks for the polycarbonate recommendations, will look into it. Crofter - your climate sounds an awful lot like where we are down at the other end of the country.

    Was hoping to use glass, following comments from folks in a thread on our conservatory plans, that we should be using glass not triple wall polycarbonate as our conservatory roof as it would last our lifetime and that was the greener option - so thought I'd ask questions about practicalities of glass. (Weight being a bit less of a problem with the greenhouse than it is with the conservatory.)
    • CommentAuthorCWatters
    • CommentTimeJan 21st 2009
    I'm pretty sure 4mm glass would also be fine. Not sure about 2mm. Sounds very thin.
    • CommentAuthorbiffvernon
    • CommentTimeJan 21st 2009
    I've never seen 2mm in a greenhouse. What's sold as 'horticultural glass' is either 3 or 4mm. 4mm would certainly support a cat but cats usually jump onto things that look solid - the bits of wood between the glass panes, so I wouldn't be too worried.
    • CommentAuthorShepherd
    • CommentTimeJan 25th 2009
    Thought I'd continue on this thread with another greenhouse roof question, rather than start a new thread - see how it goes.

    Lean-to greenhouse, timber frame, glass roof. (Basic tanellised rough sawn timber, not nice joinery.)
    The question: is it possible - as in will the glass last - if I were to build a timber frame say with the sloping timbers going up the roof at 1m apart, with maybe a couple of cross bracers at intervals in between, then to put some sort of padding onto the timber, then lay a 1m wide by roughly 2m long sheet of glass on top the padding on the timber, and use another piece of timber over top of the glass to clamp it down. Basically a clamping strip of timber that runs the length of the roof beam, screwed down through the middle (the middle being a glass free gap between two adjacent pieces of glass). I've seen various designs of greenhouse over the years (described below) and was wondering whether smaller panes of glass are essential for flexibility, or just the way it happened to be done.

    Traditional greenhouses - the roofs are usually timber frames with runs of panes of glass laid up between the timber frames, overlapping like tiles. Fancy ones have the bottom end of the piece of glass shaped into a curve to shed the water better.
    Modern greenhouses - aluminium frame and bigger sheets of glass with less overlap, held in place by pieces of bent aluminium or steel, that hook over the lower piece of glass, then down and out in a hook to hold the next piece of glass up the roof. As they get older, these tend to loose sheets of glass in gales - clips fail, glass "bounces" out of the clips etc.
    DIY Plastic greenhouses - you can buy big sheets of plastic and screw them down onto the timber frame. So what I'm really proposing is trying to use sheets of glass in the way I've previously used sheets of plastic.
    • CommentAuthorjon
    • CommentTimeJan 25th 2009
    Generally you wouldn't use glass unless the roof of the building onto which it was leaning-to had no possibility of snow build-up and a fall of snow resulting from that

    Glass is exceptionally strong in compression and in tension but has a tendency to develop miniscule fractures on the surface which can reduce its tensile (or flexural) ability to not a lot: Problem is, you can't tell.

    In short, glass is OK for green house roofs but should be used with great care in buildings that may in any way be occupied or which are adjacent to other buildings.

    If using glass, it's capacity is as much determined by the type of support at the perimeter as by the thickness.
    • CommentAuthorIslesman
    • CommentTimeJan 26th 2009
    We have used glass extensively in conservatory roofs attached to and near other buildings. Normally we use double glazed units spanning no more than 650mm i.e. in max 650mm wide panels. Length is limited usually to 3 meters. We always use toughened units inside and out or a combination of toughened and laminated. I have double glazed units of 4mm toughened in our conservatory and can walk on the glass in trainers! I therefore wouldn't fear for your cat! Polycarbonate is noisy, creaks and will have a limited life. Also you can't see the clouds or stars through polycarbonate.
    • CommentAuthorShepherd
    • CommentTimeJan 26th 2009
    Thanks Isleman. How much additional support does add with it being a double glazed unit ? I'd heard that there was a cushioning effect, as in to break out through a double glazed unit window in event of a fire you use a hammer and punch at one corner, not in the middle, because of the cushion.

    Appreciate you are making a broader point about polycarbonate sheet for people who are building conservatories, just wanted to say for this thread, what we are building really is a greenhouse - soil floor, tomatoes, cucumber and lettuce is our planned crop for this coming year.

    It so happens we are also planning a real conservatory (which will be much more insulated, though also partially for plants). So while you are here as it were, I also have a conservatory question - the last section of the thread here:


    I was explaining that I was going for triple wall polycarbonate rather than glass for the conservatory roof due to the weight and concerns about the wind load in a stormy area, and did a basic weight calculation.
    If you do have anything you could add to that thread, it would be much appreciated. ( I would prefer a glass roof on the conservatory, for all the reasons you just gave, but am not sure it is practical with the design limitations we are working to, and the stormy area.)
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