Advise on new greenhouse and foundations

[LithopsRule]

Dear Folks,

I have looked at the reference section already on greenhouses, but buying, siting, and foundation-ing a greenhouse is always worth a fresh ask, I think?

Make of Greenhouse:
Living in moist NW England (Bollington, Cheshire) in a windy-ish location, I am considering buying an 8' x 12' Rhino Basic greenhouse: http://www.greenhousesdirect.co.uk/rhin ... ouses.html. I was planning to run unheated, except for a fan-heater to run in emergencies during coldest spells of winter. Lithops and hardy-ish cacti are intended inhabitants (no Melocacti or other cold-intolerant species planned).

This Rhino Basic seems to be at the more expensive end of the market, so should I just be getting an Elite or some other cheaper make? Any thoughts welcome! Everyone quote their favorite make and wax lyrical?

FOUNDATIONS:
I was planning just to bolt it onto a hardstanding made of 4" cement based on 4" compacted hardcore on my lawn. But I see lots of talk of damp-proof membranes and insulation built into the base: am I being cavalier in neglecting these features? Speak now, or I will surely be kicking myself shortly...

Anything other tips you can share very welcomely received.

Yours, Euan

[iann]

Definitely don't insulate the base unless you plan to heat fairly regularly and to a temperature well above freezing. Even in February the ground a foot down is much warmer than a typical winter night and not frozen.

[Ali Baba]

Hi Euan
I have a Rhino greenhouse and my garden is quite windy (last year we had a summer tornado which knocked down a garage wall less than 1/4 mile away), and so far it has stood up to everything. Mine has the toughened glass which gives the advantage of no overlapping panes. Mine is the version with triple roof vents on both sides which certainly keeps the temperatures down in the summer. They are expensive , but very robust.
I put a concrete base down over hardcore, with a damp proof membrane. This keeps the floor bone dry which cuts down the humidity in the winter. My previous greenhouse which had an earth floor and I had a few problems with moulds and rot in the winter months.
Cheers
Ali

[lobman]

I think that any medium grade greenhouse would do the job, as a greenhouse this size will not be offering much wind resistance , you only tend to have problems if panes of glass come out in a gale and this can happen with any design if glass is broken by a flying object ; I think strong frames are only of much benefit on big houses , say 10 x 40 upwards . I would save the money from the cost of the Rhino and spend it on a couple of courses of block foundation as most are too low at the eves , this gives more room for shelves , tall plants ,and better air circulation ( mine has every fourth block missed out on one course for air flow in the summer)

[Peter]

I've had lots of greenhouses in my time and my advice is to get an Eden or similar reasonably priced item. A sliding door is best - I once had an expensive Hartley g/hse with an inward opening door which was a pain.
Eden's will readily cope with very high winds - I have lived in a number of places not far from you and am fully aware of local conditions.

Buy the Rhino if you really want to - I suppose there is a feel-good factor with buying a costly item, but in the end the plants won't grow any better inside it. The Hartley was a shocking price but the construction meant that it couldn't be moved when we changed address.

The base you describe is very heavy duty in that the hardcore and concrete could be reduced by an inch, but of course there's no real expense in the base. You must, though, incorporate a DPM or else you'll be plagued with rising damp and excessive condensation. Visqueen is the recognised damp proof membrane.

As I always advise, make sure you cover the new concrete with polythene or wet cloth as soon as you can walk on it and leave it there for a few days. That way, you'll have the hardest, dustfree concrete possible.

[LithopsRule]

Dear All,

Thanks to for the helpful comments and advice.

Note that Rhino Basics are on sale at the moment, making them not so outrageously expensive as usual.

Due to my inexperience, I am a bit confused about the whole damp-proof membrane bit (I am a bit confused about a lot of things in life... )

Where precisely would the mebrane go in the foundation?
It sound from elsewhere like it is simply laid on top of the hardcore before pouring the concrete?
Or is it placed between two layers of concrete?

I was planning to have the concrete slab (4" thick) so that 2" is underneath the grass and 2" protruding above.

I thought the hardcore layer was meant to facilitate drainage as well as providing firmness?
i.e. there will be no damp soil under the concrete slab, and the water will not rise through permeable coarse hardcore by capillary action. A membrane between the hardcore and the concrete would therefore seem (in my ignorance) unnecessary?!

Is it that the hardcore layer gets totally filled up with water when it rains heavily?

Sorry to be dim. Awaiting help. Yours, Euan








So if the membrane goes on top of the hardcore, then soil is still in contact with the concrete slab

[Paul D]

I have a hardcore and concrete base for my 18' x 10' greenhouse. There is no damp-proof membrane. The base is bone-dry (except when rain gets in through the roof- which is another matter!)
Re water getting in from the surrounding land- I think I avoid this happening, because I dug a deep hole to one side of the greenhouse and filled it with boulders and builder's rubble, then put a weed membrane sheet over it and more rocks on top. We have clay soil all around us so drainage is important. The concrete base of the greenhouse has a box-type drain channel down the middle, which then goes into a pipe below the sink and out into the drainage pit, where water has time to soak away gradually. So any water flowing sideways (so to speak) drains away from the greenhouse quite easily.
Before pouring the concrete I made sure that there was a fairly broad conduit pipe on either side, entering underground but coming up through the floor of the greenhouse, which allows water pipes, electricity wires, etc. to be run in and out of the greenhouse.

[DaveW]

Re damp proof membranes. These are usually thicker grades of polythene sheet obtained from a builders merchant. Ask for a damp proof membrane, most will cut you the required size off a roll, if not try elsewhere. The procedure for laying them is to lay your compacted hardcore then "blind" it with a layer of sand so no sharp edges can puncture the polythene, then cast your concrete slab on top of the membrane watching you dont poke holes in it with your feet or anything else (lay a batten or board on top to spread your weight). Fold the membrane back over your shuttering at the edge then you can fold it back up the edge of the slab and just under the base of the greenhouse when you fix that.

I have an 18ft x 12ft Elite greenhouse with the drop down door conversion on top of 3ft high concrete block walls meaning it is about 9ft to the ridge and it has not blown away yet, or even popped a glass in spite of gales in the past.

DaveW

[Peter]

Euan: Dave's specification is 100% correct and if you follow it you'll never have those 'wish I'd done it' thoughts.

With regard to the need for a DPM, bear in mind that the greenhouse will always be warmer than the ground it sits on, so any dampness in the ground will be drawn up, leading to condensation. This will be especially bad in Winter - just when the plants don't need it!

You could end up in the situation whereby you are forever going to the greenhouse to open and close windows in order to try to dry control the damp in there.

When you come to lay the concrete, ensure that the shuttering is good stout stuff,say 3" (4" if you wish) x 1", set absolutely level (take care with this), and fixed using lots of pegs or stakes and then nailed. Ensure that you and a pal tamp the concrete using as heavy a tamp as you can get hold of - if you use a light length of wood you will end up with a hump in the floor. Lightly trowel smooth - don't overdo this. Allow to set for a few hours then cover as described above.

[DaveW]

Also I should add to Peter's make sure the shuttering is level, also make sure it is square since an out of level and out of square base can mean problems when erecting. Using a small set square is useless, the method the building trade used is the 3-4-5 triangulation method devised before fancy optical instruments were invented. That is 3 units along one side and 4 units along the other, and if the corner is square the hypotenuse will measure 5 units (can be feet, meters or any multiple of these i.e. 6-8-10). We usually use a lath with the five units marked on it and measure and mark the 4 and 5 units along the shutters.

The other method is to measure diagonally corner to corner of the slab one way and then corner to corner the other way and if it is square both measurements will be the same.

DaveW