On 22/06/2013 21:06, Howzit_China wrote:
Hi,

Looking for advice on a greenhouse I am looking to build in the sunniest
(relative) spot in my London garden.

My garden is West facing, the patch is South facing and gets sun from
12:30 to 18:00 (5.5 hours). Last summer I grew cherry tomatoes and
courgettes with fair success in this patch (birds got most of the cherry
toms tbh). I got a bunch or large tomatoes which didn't ripen in time
before the plants died at the end of the season (I didn't realise you
could ripen them indoors).

I want to build a greenhouse on this spot to increase my success rate
with tomatoes and other sun-lovers like peppers, cucumbers, squash and
pumpkin.

Originally I was going to buy a 4 x 6 foot aluminium greenhouse as the
space is awkwardly shaped and this is all I could find that would fit,
but then I thought if I design and build my own I could make better use
of the space and have a bigger greenhouse for the same money.

It's going to consist of a brick foundation (neighbours old driveway),
wooden frame (scrap liberated from skips), kitchen flooring for floor
(have this lying around) and a polycarbonate surface which I will need
to source. The whole structure will be covered in polycarbonate as I
don't want any wood exposed to the elements. See CAD drawing attached
(dimensions in mm). I've got my brother visiting in Sept and he has some
carpentry skills so planning to start work when he's over.

OK. But if I were you I would design it around the standard sizes of
twinwall polycarbonate sheet. BTW not sure I would bother to use
twinwall unless I was going to heat the greenhouse during winter.

Clear UV stabilised sheet would let more light in. Don't underestimate
the amount of ventilation that it will need (at the bottom and at the
top ridge) that will determine success or failure. Back wall on north
side if up against a fence could be made of anything.

What I couldn't figure out how to get across in the drawing with my
limited CAD skills is the panels A and B, I want the poly carbonate
sheet at B to be fixed in place, and A to open upwards on hinges, like a
grow box, except a 1.6 meter high grow box = ). This should give me good
access to all the plants as the space behind B will be quite large for
me to be able to duck under.

So questions:

1. What thickness polycarbonate and how many chambers is usual for
greenhouses?

I used 1cm UV stabilised. The thinner stuff flexes too easily although
it is fine for opening vents and much less likely to break if the wind
snatches it. Expect to have to do clever anchoring down of the sheets to
prevent them springing out. Unlike glass plastic bends very easily.

2. Does the pitch of the roof look OK? Is this important?

Initial reaction is make it square rather than triangular and a single
slope - preferably with enough headroom that you can work inside it!

Have an overhang, gutter provision for rainwater storage.

3. Will leaving the door open (A) occasionally be enough for ventilation
purposes?

Which is the door? I just see a space frame too small to stand up in.

You probably want a couple of thermostatic vents in the top (or along
the entire top ridge) and opening louvre panels on the side to let cool
air in. Most kit greenhouses have inadequate ventilation as supplied.

4. Does the design look practical?

Apart from the funny roof, awkward angle and dimensions requiring tricky
cuts and joints. Polycarbonate is fairly easy to work with a decent saw
if you take your time but you can make life much easier by designing
around the easily available stock sizes!

If using twinwall don't forget to use the sealing vent tape on the open
ends or it quickly fills up with dead insects and worse!

5. Is it worth the effort considering limited access to sunlight?

What direction is it facing?

I would consider making it at least 1.9m high and perhaps out of
aluminium if you intend to grow things that like plenty of water.

--
Regards,
Martin Brown