15 Degrees Pitched Roof: Wooden Greenhouse with Simple Plastic Cover

Hi!

We're building a greenhouse on a roof terrace, and due to some various considerations we ended up on a 15 degrees pitch of the roof.
Factoring in that this pitch is a bit low for water run off, the initial idea was to add something like a metal mesh on top of the roof to avoid any potential gathering of water between the rafters. Near the end of the construction, I received the message that we won't have any metal mesh at hand after all (our budget is extremely low), and now I'm fearing that this pitch might be too low.

We will use wiggle wire on the sides, to pull the plastic as tight as we can.

I believe that for tiles at least 22 degrees is considered the minimum pitch. Any experience or insights on a plastic cover?

Cheers!

The poly will be 1 solid sheet? It would work with even less roff pitch.  As long as it is strong enough to support the snow load.
  I have a ~20 year old green house in California with a roof very much like that.  It is about 20° though, recovered every 4 years with a new 18x20 6mil greenhouse sheet.  Works fine. The plastic fails on the ridge and each rafter. I assume because of the extra heat.  
  Where are you? I would not use 6mil poly in snow country my sister lives in South Dakota and I have seen pix of golf ball size holes in greenhouse poly from hail.   There is 10mil fiber reinforced uv stabilized gh poly available.   Tom

Thomas Michael wrote:The poly will be 1 solid sheet? It would work with even less roff pitch.  As long as it is strong enough to support the snow load.
  I have a ~20 year old green house in California with a roof very much like that.  It is about 20° though, recovered every 4 years with a new 18x20 6mil greenhouse sheet.  Works fine. The plastic fails on the ridge and each rafter. I assume because of the extra heat.  
  Where are you? I would not use 6mil poly in snow country my sister lives in South Dakota and I have seen pix of golf ball size holes in greenhouse poly from hail.   There is 10mil fiber reinforced uv stabilized gh poly available.   Tom

Hi, Tom-- thanks for the reply.
We're in Northern Greece. Snow can happen during winter, but some years none at all- Hail can happen too, but rarely.
I probably wasn't clear enough, but we're not talking about polycarbonate sheets-- just straight up plastic (from a roll, let's say), of the sort you'd usually see on a greenhouse. That's the only thing we have at hand right now. Maybe in the future we could upgrade to poly.
Cheers!

I think it will flow off nicely.  The only place it might pool is at the lower edge of the roof.  So if the plastic is tighter at that lower edge (where it goes from 15 degrees to 90 degrees) it might act like a dam to hold back a puddle.

I think you will want to seriously round off those wood corners at the edge of the roof so it doesn't wear/tear the plastic prematurely.  Rounding off the long edges of the roof rafters would likely help as well.

Mike Haasl wrote:I think it will flow off nicely.  The only place it might pool is at the lower edge of the roof.  So if the plastic is tighter at that lower edge (where it goes from 15 degrees to 90 degrees) it might act like a dam to hold back a puddle.

I think you will want to seriously round off those wood corners at the edge of the roof so it doesn't wear/tear the plastic prematurely.  Rounding off the long edges of the roof rafters would likely help as well.

Yeah exactly, that's the main spot of concern for me also. I've actually rounded the edges after this photo was taken, so that part is taken care of.
If you think the roof pitch is alright, we'll try to get the plastic tight with the wiggle wire, and we'll just have to see how it goes.
Cheers!

The framing as shown has the edge beams well below the rafter tops, so plastic stretched reasonably snug would have no chance of causing ponding there.

I think that after smoothing and rounding potential rubbing edges of the framing, wrapping a strip of plastic over the rafter tops would further minimize chafing and early failure at those points.

puerh tjoba wrote:

Hi, Tom-- thanks for the reply.
We're in Northern Greece. Snow can happen during winter, but some years none at all- Hail can happen too, but rarely.
I probably wasn't clear enough, but we're not talking about polycarbonate sheets-- just straight up plastic (from a roll, let's say), of the sort you'd usually see on a greenhouse. That's the only thing we have at hand right now. Maybe in the future we could upgrade to poly.
Cheers!

Yep same stuf, 4year uv stable 6mil greenhouse polyethylene sheet @ $0.10 Sq ft.  Polycarbonate is about $3 sq ft. Don't use construction polyethylene from a hardware store it will only last about 9 months.  

Weather sounds very similar to mine.   Snow most years melts in 48hrs.  Just enough to take pictures, then it's gone. :)

Tom

Mike Haasl wrote:I think you will want to seriously round off those wood corners at the edge of the roof so it doesn't wear/tear the plastic prematurely.  Rounding off the long edges of the roof rafters would likely help as well.

Agreed!

Another trick greenhouses use is to cover the wood support members with a strip of plastic. That way, when the wind and expansion/contraction causes the cover to move, it's plastic-on-plastic, which is low friction, extending the life of the cover.

Douglas Alpenstock wrote:

Mike Haasl wrote:I think you will want to seriously round off those wood corners at the edge of the roof so it doesn't wear/tear the plastic prematurely.  Rounding off the long edges of the roof rafters would likely help as well.

Agreed!

Another trick greenhouses use is to cover the wood support members with a strip of plastic. That way, when the wind and expansion/contraction causes the cover to move, it's plastic-on-plastic, which is low friction, extending the life of the cover.

That's a very useful tip!
Thanks everyone!

puerh tjoba wrote:

We're building a greenhouse on a roof terrace...

It looks like you're doing an awesome job!

From the photo, it looks like a concrete edge on the terrace. Is that well supported all the way to the ground? Do you know how the "floor" is built and supported and what it's over top of? You might want to do rough calculations of the weight of the wood, and the weight of the pots/dirt/planters you plan to have inside the greenhouse?  

The danger with "repurposing" is if you've done something beyond what the original architects ever imagined. If they pictured 4 people having lunch on a table, and you're picturing a greenhouse overflowing with big pots of soil, you want to know that bad things aren't going to result.