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Author Topic: Timber sizes for the roof of a greenhouse  (Read 443 times)
Charli
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« on: August 07, 2018, 03:28:50 PM »

More of just a 'building' post.. if you don't mind...

I'm trying to determine what timber sizes I will need to build a greenhouse roof. The roof itself will be a timber, with 16mm polycarbonate sheets on. the greenhouse building is 4.8m by 2m, and the roof is peaked (peak running east to west)- with the south facing side being 1.8m long and at roughly 28 degrees, the north facing side being 0.8m long and 34 degrees. As a greenhouse I want to use as little timber as possible to let the most light in, but obviously I don't want the roof to collapse! Any advice or general rules of thumb to follow?

This beautiful image shows a rough diagram of the roof, with some things labelled to ensure I'm sticking to the correct nomenclature.


I would like to use the following:
6x2 for the roof peak (4.8m long)
4x2 for the rafters- spaced 1.2m apart (so with 1.2m wide polycarbonate sheet)
All figures plucked from the air, feel free to adjust!
I was hoping to not need purlins, but could be very wrong.
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desperate
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« Reply #1 on: August 07, 2018, 03:44:21 PM »

You can probably get away without purlins but you should put collars or joists to join the bottoms of the rafter pairs to create a triangle or the roof will spread forcing the walls apart. 4 by 2s will be ok for a 1.8m rafter but there will be quite a lot of wind loading and a spacing of 60cm might be safer TBH

Desp
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stannn
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« Reply #2 on: August 07, 2018, 04:35:16 PM »

Just looking at my 13ft x 8ft Alton cedar which has lasted 46 years and looks good for a few more. Height was always marginal on small glasshouses and so it has a span roof, ie it has a horizontal wooden brace only 600mm long near the top of the rafters. It also has a similar brace where each rafter meets an upright. Done properly this adds a lot of stiffness against asymmetric win loads but gives you ample headroom.
The sides each slope in about 200mm which reduces the span at shoulder level. I was always concerned about any failure of the roof during maintenance because it uses horticultural glass and so I never lean on it too heavily when up the ladder. The rafter section is 60mm wide by 45mm deep but they are only 1100mm long.
Your ridge looks a sensible size and will not cause much shading. Your rafters at 100mm deep will be a shading issue and I would look to using 75mm deep.
Ventilation is a huge issue in a summer like this. Mine has 4 massive vents at ground level and 2 hinged vents in the roof. More would be better. To be able to remove whole panels would be great for this weather.
Stan
« Last Edit: August 08, 2018, 08:53:40 AM by stannn » Logged

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JohnS
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« Reply #3 on: August 07, 2018, 05:21:32 PM »

What is going to support the wallplate on which the rafters sit?

Have you considered aluminium or galvanised T, U or box section for the rafters?
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Charli
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« Reply #4 on: August 08, 2018, 10:04:42 AM »

A few more details:

The north and west walls are made of 3x2 at 600 centres- covered in osb and insulation and cladding- so not a wall to let light through! The wall plate being another length of 3x2. There are fences nearby so not much light would have gotten through these walls, I will put a small window in the west wall where it pokes above the fence I think.

South wall is rescued double glazed units in pvc frames, with 6x3 vertical supports and 3x2 horizontal- with another 3x2 as wall plate again. Majority of the windows openable for vents!

East wall is mostly door, in a frame of 3x2 with polycarbonate panels for the rest of the space. Door is pvc door with the middle panel removed and replaced with a sheet of polycarb.

All sizes up to now have been based on timber I already had- 6x3 might be overkill but I happened to have a load of it and it therefore was free.

The picture I gave before was a limited view of the roof, there will be 'triangles' as well- I was going for a span roof so the horizontal braces will be part way up. The whole structure to be 2.5m tall at the peak (back north wall 1.9m tall, the front wall works out at 1.6

Another stunningly technical diagram:

I must learn to use Sketchup or something.

I could quite easily use 3by2 for the rafters, and I could add purlins, or I could make the polycarbonate sheets thinner (and have more rafters)... it all comes down to which combination of things to do!
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Charli
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« Reply #5 on: August 08, 2018, 10:07:23 AM »

Have you considered aluminium or galvanised T, U or box section for the rafters?

I did look into buying a self-supporting alu roof, but couldn't find a peaked one suitable for polycarbonate in the necessary size- so polycarbonate on timber was my other option (using purchased glazing bars). I'm a diy-er, not a builder- so timber I can work with if it isn't too complicated, but fabricating things out of alu is a bit beyond me!
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Sprinter
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« Reply #6 on: August 08, 2018, 10:53:33 AM »

I know nothing about building anything, let alone greenhouses, however we do have a 8" X 12" greenhouse that i have had running for 4 years (this is its fourth growing season) and can wholeheartedly support the fact that the ability to over ventilate is much more important that the size of the beams used, and the amount of light a 4x4 will cut out compared to a 4x2 (for example only) especially in a roasting year like this, my relatives have much less ventilation and have had a very poor year.

The first year we had a lot of mould even though the greenhouse was only just put up and cleaned with Jays Fluid before use (also cleaned with this annually), this first year we put it down to god knows what (we ignored it), the next year it was the same, so the third year we added 2 auto roof vents (years 1 & 2 we had the vents but did them manually) and we had less mold and general issues, but as we still had some, this year i replaced a 2"x 2" panel at the rear of the greenhouse with another auto Louvre opener and its made all the difference, a much healthier crop.

The front door is opened at breakfast (first smoke of the day) daily so we now have pretty good ventilation and much less issues, they are expensive though so next year i plan on putting another vent at the front.

As your at the planning stage, plan as much ventilation as possible, what we also found helped is installing a 100 ltr water butt inside of the greenhouse (takes up less space) and then having 2 X 200 ltr water butts connected in parallel outside of the greenhouse to form a reservoir and total storage of 500 ltr's, this has made out lives much easier and we have only had to top up the water once or twice this year.

i also have a 360 ltr fish tank so when i do water changes the old water goes into the house water butts and then gets pumped up to the greenhouse water butts with a water butt pump, the plants not only love the ventilation but the high in nitrates fish tank water has also done wounder's for the crop.

Enjoy your greenhouse.

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Tigger
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« Reply #7 on: August 08, 2018, 11:19:36 AM »

Have to agree with Sprinter.  Unless you're in a particularly low sunlight area, don't worry too much about losing a little bit of light by putting in stronger timbers and make sure you can cool the thing down and let some air though.

This summer I've had to 'whitewash' the glass as even with the two roof windows and the two side windows fully open (on Bayliss auto vents) it was going over 50'C and my tomatoes got stunted by the heat.

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Charli
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« Reply #8 on: August 08, 2018, 05:11:10 PM »

I will put in plenty of vents- I do have another glass (but conventional alu) greenhouse a few metres away- it has done fine this year with an auto vent and leaving the door open as we are a bit shaded by large trees to the south edge of our property. I was planning some auto-opening louvres (I find the auto opener vital when I forget things!) on the east and west walls, up near the ridge. As well most of the south wall will be openable. I had also been looking at polycarbonte sloped roof windows, but hadn't yet found a useful one.

Decision of 3x2 vs 4x2 largely made for me- timber mill having no 3x2 left, or due in for quite a few weeks! So 4x2 it is, I'll make it at 1.2m rafter distance and if I think it isn't sturdy enough can either add in purlins or change it to 1m distance (as I won't order the polycarbonate until the frame is all done)

Photo of the building site at the moment:


Insulation goes down 0.8m, with 1000 litres of water storage sunk in (that ibc above ground does have space to be below ground!). The blocks are so I can make fairly high raised beds inside. Chair is for surveying my work whilst drinking a beer at the end of a hard day!
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Tigger
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« Reply #9 on: August 09, 2018, 08:45:57 AM »

I think it's fair to say that the chair is probably the most important feature of the whole set up, closely followed by the cold beer which we can't see Smiley

I like the idea of the sunken water/heat store.
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30 tubes, south facing gable wall (Navitron Fornax Trial System).  Hunter Herald 8, integrated boiler hooked up with Oil Boiler via H2 control panel.  Scrounging fire wood wherever possible Smiley
kristen
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« Reply #10 on: August 14, 2018, 04:19:47 PM »

My 2p-worth:

Polycarb has a habit of blowing out. Siliconing-in helps, but then makes it hard to do any repairs I hear.

Are you building something to look nice? if not how about a second hand Ali one off eBay or similar? I've got a 30' x 10' that I paid a couple of £hundred for. (although from photo looks like you are already started ...

Agree ventilation is key.  I would want at least every other 2' section opening, and the same on the other side, and louvres in the walls. The louvres will create convection current (until the weather gets so hot you just have to throw door and everything open), and the convection will be much kinder to plants in lower temperatures of Spring / Autumn than forced draft

Water butt - get something with a big enough diameter that you can dunk a watering can, filling anything from a tap takes far too long. I have some IBC's connected to my dunking-tank (with 1/2" hose on a daisy-chain siphon a-la-cheapskate)
« Last Edit: August 14, 2018, 04:21:29 PM by kristen » Logged
Charli
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« Reply #11 on: August 15, 2018, 09:00:56 AM »

I'm a bit greedy in that I already have an ali greenhouse as well... (and another at the allotment!).. but for this particular greenhouse I wanted something that retained heat somewhat more (hence the insulated back wall)- for seed starting early in the year (I have no useful windows for such tasks), and that I could put a hammock in- it's more of an 'unattached conservatory for the shoulder months that I can grow plants in' than a normal greenhouse (nowhere to put an actual conservatory on my house). I also grow some specific awkward things I would like to keep in the greenhouse (mainly cactii and succulents). So.. the gist of this is- it isn't quite a standard greenhouse I am looking for!

No more building yet- timber didn't arrive. The timber company say 'they got stuck in traffic'- it is 1.4 miles from the shop to my house, so that's some good traffic. Cancelled order, got to find a new timber shop.
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kristen
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« Reply #12 on: August 17, 2018, 07:59:20 AM »

it isn't quite a standard greenhouse I am looking for!

Smiley Fair enough

Quote
for seed starting early in the year (I have no useful windows for such tasks)

I use Lights for that. Even with reasonable temperature in a greenhouse in early Spring the light levels are pants, and very variable one year to the next, so all my seed raising is under grow lamps. insulated box in the garage will do, mine are in the boiler room so toasty-warm anyway.  If interested have a read of the Cannabis forums, those guys have absolutely got it down to a fine art ... their high-value-crop is no doubt an incentive!
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Tigger
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« Reply #13 on: August 17, 2018, 02:50:01 PM »

>>If interested have a read of the Cannabis forums

Hmm, you'd just better make sure you're roof insulation is up to scratch before you start looking at that forum or the next time the police helicopter flies over with it's heat seeking 'naughty person' detector going you could find yourself getting a knock on the door from the boys in blue  hysteria
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30 tubes, south facing gable wall (Navitron Fornax Trial System).  Hunter Herald 8, integrated boiler hooked up with Oil Boiler via H2 control panel.  Scrounging fire wood wherever possible Smiley
kristen
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« Reply #14 on: August 17, 2018, 06:14:23 PM »

Smiley

I've been expecting a knock at the door since I installed my lights. Originally Metal Halide, several of them, and using a lot of Watts ... now LED so hardly any Watts, but I don't bother to try to hide the light (all night long, as i use E7 rather than daytime Leccy) ... still no knock on the door though ...
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