Greenhouse dimensions

[SJW]

Not sure if this is a stupid question or not, but here goes. Let's take it as a given that you should always go for the largest size of greenhouse that garden space and budget permit. Beyond that, what are forumists' experiences with regard to greenhouse dimensions? By that I mean are there advantages in going for long and narrow rather than wide and shorter? Say a 6ft by 12ft/14ft, compared to an 8 ft by 10 ft. For example, all things being equal is the airflow/ventilation more efficient in a narrower but longer greenhouse? What set me pondering this was which width of greenhouse gives the most efficient use of internal space.  The standard raised bed for vegetables, for example, is usually 4 ft wide which gives a comfortable 2ft reach from either side. So if the same applies to bench width where you can only reach plants from one side, 2 ft deep benches on each side with a central 2 ft path gives a 6 ft wide greenhouse. If you go wider than this, to what extent is the extra 2 ft (in an 8 ft greenhouse) actually utilised efficiently? Would it make more sense to go for extra length rather than width?   

Good grief, the things you think about when staring into space...

[ArnoldT]

Steve:

I can tell you what I built.  My greenhouse is a lean to.  It's attached to the house on two sides.  Measures 7ft wide x 12ft long.

I have two benches I had  built that are 32 inches wide.  I have no trouble reaching plants at the far end of the bench.

The bench along the long side facing south is the entire length of the greenhouse.  The one against the house is shorter to allow me to enter on that side from the house.  There is no outside entry to the greenhouse.
The aisle down the middle is 24 inches wide.  Benches are 30 inches high with a shelf under for storage of dormant plants and those that have finished flowering.  The top of the benches are wire galvanized  mesh.  The shorter bench has a stainless steel trough that I have most of my miniature Narcissus plunged in. The mesh  lets water drain through and allows light to reach the lower plants.

The only addition I'm thinking of is a shelf against the house side about  two feet above the surface of the plunge to put plants on.  This wouldn't block any light for the plants on the trough plunge.

I can take some pictures if you like and send them.

[Steve Garvie]

There is more to choosing shape and dimensions than the simple pragmatism of best use of available space -as I've learned from bitter experience! 
The ratio of total volume to glass surface area is very relevant.  A long narrow greenhouse with the long axis at right angles to the prevailing sun will always heat up faster than a shorter but wider greenhouse of the same volume sited in the same position. A long narrow lean-to facing south can heat up very fast (especially if the back wall is dark) -so much so that it only becomes suitable for growing cacti and succulents (or for drying the washing!).

[brianw]

I would agree the volume is very important, from the cold growing plant, point of view particularly. My 8' wide heats up very quickly even with louvres wide open. I have 2' benches and 2' path. So 3' wide under the 2' bench, for shade lovers or dormant plants, but currently no plunge benches. If I had plunge benches then they would be closer to 2'6" if both sides or larger if only 1 side. There is little advantage in a narrow greenhouse as you need the same space to move or work regardless, and if you go even wider you then need 2 paths to access the benches, unless you have lots of money and have mobile benches like Kew, if I remember correctly.
I remember visiting someone a few years back who had benches all the way round, including the doorway. You had to go on your knees to get in under the "door wall" bench, but could the use the space all 360 deg. round you. In his other greenhouse the whole length under 1 bench was a water storage tank mostly covered by boards, but inevitably occupied by frogs too.

[Graeme]

all our buildings are east to west - with the doors on the east side - one of the alpine houses has double doors and the other because it was second hand one just has a single door - the one with double doors has vents in the far end at ground level - that one is a 8x12 with plunge benches
The other one is a 8X10 with just gravel benches
The larger greenhouse has a slot in potting bench that can be removed at the far end - it fits between the plunge benches

I would have liked x10's but we are on a big slope - I have another 8x12 and a 18x12 in crates waiting to go up hopefully this summer

6' wide ones always seem a bit to short in height to me  - but I wish now we could have had 10' or 12' wide houses

[brianw]

Although a 6' is likely to be shorter, the height above the path is up to you. Lower the path or raise the greenhouse base, and set the benches accordingly. Sunken greenhouses were very popular for growing pineapples etc. Cheaper to heat. I settled on a concrete base for mine. Lengths of concrete gravel board cut to size and bolted together, on a simple foundation. Much quicker than a brick or block base (for me).
I thought I was doing well buying a 12x8 on ebay for < £100 and extending it with an 8x8 I had, but a couple of weeks ago I saw an identical 24x8 for £200. Though not local this time. Plenty around if you know what you are getting.

[mark smyth]

As big as you can afford!

My 12x8 was filled in no time

[SJW]

As big as you can afford!

Yes, that's a given , Mark.

Thanks for all the detailed replies. A few years ago I decided that my old Alton greenhouse wouldn't last much longer (you don't need a hard hat to go in there but it's only a matter of time...) so sacrificed my veg plot for a new aluminium one. More by accident than design - it's a long story - I ended up with a 4m (13') square model which I'm very happy with given that it has to be multipurpose. I have a 4'x8' raised bed in the middle for tomatoes (and extra storage space and/or chicory in the winter), benches on both sides and the end bed I use for peppers etc. Here's a couple of photos during construction. But I have been wondering if there is an optimum width of greenhouse I should be considering to put in the space left by the old Alton when it finally does bite the dust. I'd like a new glasshouse that I could use mainly for alpines, hence my original question.

[Maggi Young]

Well that's a first for me - never seen a 4m square glasshouse before.  Very neat construction too.

[ian mcdonald]

My garden is small and the "greenhouse" is a normal 8ft. x 6ft. I converted it for alpines by installing a self opening window attachment for the roof window and an additional louvered vent in the opposite side. I have to put white shading material in the roof for summer. Not ideal but it serves well for alpines in pots in a sand plunge bed down one side. The end bed contains sandstone tiles set on end for a "crevice" garden, again in a sand bed with a mix of compost and the third side contains a mixture of sand, bark and coir for pleiones. The beds are on a framework of "handy angle" from a local scrapyard. I know many people will shudder at the thought of such a mixture but when you are short of space? My small garden is the same. A non-planned mixture of small habitats in which to grow plants that I like.