Kitchen Garden Forum

I'm keen to build my own greenhouse and i'd like to share a few problems.

I have a south facing position for the greenhouse though from past experience a glass or polycarbonate roof means the green house overheats. leaving windows open risks damage from wind effects, this is a very windy place. I'm thinking about alternatives to hinged windows though thats still in the pipeline.

Can the roof be slate ? Will that have a massive effect on light if you consider the sun isn't overhead even at summer solstice.

Do I have to have a concrete slab, i'm thinking that a strip foundation for a pygmy wall might be better. The ground is made up of six inches of scalpings and chippings over clay. Scalpings do consolidate though they are very permeable.

I would like to think I can build the perfect greenhouse though compromises are already taking place.

I'm not going to say anything about most of your questions, but I think the slate roof may well be a problem.

I have two south facing greenhouses and it was incredibly dark in there with just a thin layer of snow on the roof!

You must have a transparent/lucent roof unless you want plants to stretch and grow sideways. Think what happens on a windowsill.

So I need to deal with too-much sun by using portable shading of some type when it occurs.

Our south facing conservatory got so hot we had to use internal shades, eventually I covered the glass ceiling and we have curtains and though I have only grown the odd pot plant in there that had to be moved on sunny days even in winter sometimes.

I suggest you look at the various greenhouses on offer to see the way they are built and what they suggest. Overheating can be overcome with the autovents - indeed all my 4 roof vents have them - absolutely essential. And obviously a glass roof. A pygmy wall will be fine, presumably for the foundations. I made mine out of the remains of a coal bunker. As for the soil or lack of it, I grow my tomatoes in growbags. Not so easy with the watering, but I pierce the bottom of the growbags in the hope that they will root through.

As for shading, I bought some a few years ago and haven't needed to use it! Chance would be a fine thing.

I hadn't even considered having soil on the floor, all my growing will be in containers lifted off the floor or in tall containers.

Auto-vents ? It just occurred to me that when it's very windy the auto-vent is likely to close.

No point in me avoiding a pygmy wall, I have a Pembrokeshire bank rising about five foot within three feet of the greenhouse. I spent some time getting my head around the likely problem of the bank but even today with a low sun the bank has not cast a shadow onto the wall where the greenhouse will lean.

Yes, it's a lean-to. Always going to be a compromise.

Dear Richard,
have you considered using louvres in the sides of the greenhouse near the peak of the roof. These may be adequate for ventilation needs if the lean to is no longer than say 12'. These can also be controlled using the same system as the roof vents but they will be less likely to suffer wind damage. I would also recommend these for the general side ventilation as well.
If the lean to is bigger then you will need a couple of roof vents. These can be fitted with a piece of chain to stop them opening too far. Attach one end to the back wall and the other to the bottom of the vent. Do make sure that this is long enough to allow normal opening.
For winter I would recommend unscrewing the opening mechanism. Then fastening the window down as it will bang in the wind or undoing the mechanism and use the chain to fasten it down.
Most modern auto vents are fitted with springs which are supposed to prevent them from flapping open uncontrollably although if it is as windy as you say the extra chain would not come amiss.
Regards Sally Wright.

Thanks Sally.

I haven't yet seen any louvre auto-vents, clearly I need to do more re-search and yes I had already decided to place vents at each end, high and low.

The very most important issue here is protection from the wind, I have already used the site as a sheltered location so the amount of sunshine available is proven.

I'll produce some photo's of where this green house is to be located and eventually some details of how the whole project comes together and details of the construction methods if anyone is interested..

Evening Richard, i'm sure we would all be very interested in you keeping us updated. There's normally something in these types of projects, that other people can use when doing something along the same lines.

And apart from that, it's just nice to know what other forum members are doing.

Hi Richard,

I use auto vents, but for winter I remove them and refit the original opening bar. These lock them shut but I can still open them on a sunny winter day.

Regards Gerry.

Not the clearest of photos but it shows how auto-louvres work.
http://www.elite-greenhouses.co.uk/ac01_06.html
Elite are cheap and cheerful, I've had a 12x16 since 2002, I've lost a few panes but otherwise OK.
There are leanto polytunnels which rely on end ventilation, you could have net doors for the summer.
http://www.citadelpolytunnels.com/lean- ... unnel.aspx

Thanks to everyone for the interest and helpful comments.

To be honest, I have space for a stand-alone green house. I even have space for a poly-tunnel. Also, this thing is more about gardening than about feeding myself.

The few things I grew last year were wasted and I still have clamps with beetroot that i'm not eating but it has been fun growing them. Even fun watching my dog stealing carrot and green beans.

I'm going to incorporate more flower growing from seed and whilst my cold frame will suffice the idea of greenhouse for more tender things really appeals to me, also, a well planned and built greenhouse will be a sound investment rather than that rusty old relic that had been poorly installed at my last home.

I have a west facing wall that I could use to lean my greenhouse against, the sun wouldn't hit this option until about ten o'clock and stay on the greenhouse a little later in the evening than if I placed it on the south facing aspect, i'm still considering that as an option.

I have two choices of material, hardwood or treated softwood. In both cases the material will need to be treated after the machining so it's down to cost. Hardwood will be more stable and easier to machine because it's generally clear compared to construction softwood.

What about polycarbonate glazing compared to glass ?

Regarding ventilation on a windy site, I'm assuming that the strongest winds always come from the same direction. In which case, site the vents on the lee side. Commercial glasshouses, with computer controls, are calibrated to close their vents on the windward side when winds pick up. Louvre vents in themselves do not ventilate a great deal unless they are paired with roof vents, allowing cooler air in from the sides which then convects upwards and outwards. Shading provides only a certain degree of cooling, and only if (contrary to what many blind manufacturers will convince their customers) they are on the outside of the glass. Putting them on the outside does of course present problems with weatherproofing and durability. But, if a greenhouse has sufficient ventilation, shading is seldom necessary. Many gardeners shade their greenhouses when, in my view, they do not need to. On the other hand, not all manufactured greenhouses have sufficient ventilation area, probably to make them cheaper. At least 12% of the glass should be openable.

Cheap and cheerful flooring can be achieved by levelling and grading the ground to eliminate sharp stones and putting down a durable membrane. This gives you the option of being able to do something different with the flooring later, such as concreting it, while in the meantime it will remain weed-free and well drained.

You've all provoked a lot more thinking, thank-you.

I'll have to show you some photos of the site, on the face of it-it's fairly sheltered but only sheltered directly from the north and some shelter from winds coming from the other direction. However, vacuum induced is another matter, I have four coal bunkers and two dustbins along the south aspect. if I forget to tie the lids down on the bins or put the locking pins in on the bunker lids the lids will get ripped off.

Venturi effect ? Because it's like an ally-way. In fact right now after 30 knots all night and it's still blowing I have stuff all over the place that I didn't tidy away.

A couple of photos will help the conversation along and i'm already thing about a door at each end at the expense of planting space.

Coal bunkers are along the south facing aspect, the other photo is of a possibility on the north facing aspect. In both cases the need for a small wall is mostly because of the Pembrokeshire banks which in the case of south facing essentially controls water running of the paddock. Yes, these are compromise positions but the bank don't put shadow on the wall of that building which we will call, "Shed". For planning purposes it was called, "Garage/store".