Does Twin-wall Poly sag - Not if installed correctly.

I'm a little confused on your description, a picture is worth a 1000 words.

I'm assuming your installing your roof panels with the corrugated lines running from the roof peak to the gutter.

If you build your frame on 24" centers, not roughly but exactly, and you purchased 4ft wide poly you will not need any additional support.

Can I assume you purchased roof panels that run from your roof peak all the way to the gutter in ONE section so they are like 4ft x 12ft long? (depending on the width of your house and the angle of the roof)

And yes I have a 12 x 20 wood framed poly house.

Thanks Mylu...sorry for the stumbling description. Sorry, I have no pics yet..but you have answered my question.

Yes, they are exactly on 24" centers (actually 24 1/8" as suggested by Sundance to allow for panel expansion) and yes, the twin wall will run with the ribs running from peak to gutter.

The roof sections are 6' each side of the roof - peak to gutter, not 12' peak to gutter.

I asked the question becuase I tend to over build (most any prject) and wanted to be sure I didn't go overboard here as well.

Sounds like you have it under control if you would like send me an email and I'll send you a picture. Might help you out.
wenwall@earthlink.net

Mine sag. It was kinda cold when I installed them and now that it's warm, they sag. Of course, the 10" snow that fell after I had the roof on got the sag off to a good start. Thin purlins between the rafters will help a lot. I need to get mine in before winter hits or I'll likely lose the edge of a sheet or two.

-Ed

Ah, purlins...that's what they are called huh? Vego can you tell me the span (lenght x width) that the panels cover?

gIRL, i KNOW exactly what you are talking about..

At first, I thought it would be so much easier to attach the twinwall DIRECTLY to the wood frame of the house and from the outside, it looked GREAT..the problem is the inside if you attach a shelf, it cannot go any higher beacause that is covered with the twinwall..tonite i'll send you pics..

I liked how the twinwall is all framed in on all four sides like a window, therefore leaving the wooden frame showing on the INSIDE where you can hang a shelf..

On the sides of the greenhouse, we did have to use full sheets..

Got all the twinwall up and installed and we are 2 panels, 2x6 too short-oy! at least that can be shipped ups..

I really like PEARLE at International GREENHOUSE where i purchased my twinwall..sent me samples and advice and just about matched the sundance price..It arrived well crated EXACTLY on time and she was forever helpful throughout the project.

ruth

There were no instructions with my sheets or recommended span. Mine roof span happens to be 24". The sheet's ribs run parallel to the roof joists. This stuff in 4-mil thickness is very flexible either way, so running ribs perpendicular would not have made much difference. I think they'd sag a bit even at 16" centers, maybe at 12" you would not need purlins, but think of all the joists you'd need!

I'm planning on simple 1x2 purlins, shouldn't need much structurally to help keep it flat and a 1" board may cast a thinner shadow.

I noticed the fasteners I chose stand up from the roof about 1/4." No biggie, but they act like little anchors for a snow load. If the fasteners are fairly flat, low-profile, snow would probably slide off. One half of my roof is a gentle slope, this aggravates the problems of snow collecting. The easy answer is to add the purlins. A steeper sloping roof would have helped a lot.

When it's warm, the sheets expand a lot. Since I put them on when cold, they sag more due to the heating effect. Again, not a big problem so far but some years later, screw holes may enlarge from all this expansion/contraction. The panels flex or bulge more than stress and pull on the screws, so maybe I'll be alright.

Lovin' this stuff so far! Cooler in there, not melting down any plants and it's held up to snow, heavy rains and some light hail.

-Ed

Am I missing something in thinking about the expansion factors in twin wall here? Seems to me I could simply take a panel of twin wall and lay it flate on a sheet of ply in the sun (to heat it as much (perhaps a bit more?) than it would heat it when in place and then measure it in its two deminsions.

Then slip out at zero dark thirty on one of our coolest nights and check it again. Then space it to accomodate those factors when I install it.

Or just install it at an inbetween temp. Mine's a bit floppy, I don't mind. Maybe better that than pulled too taut.

-Ed

i DON'T SEE the twinwall on our GH floppy one little bit..8mm..

ruth

otomatoo, what is the span between the rafters on your GH? Are there any supports (purlins) between rafters?

Another way to ask the same question is, what size is the largest unsupported section of the twin wall on the roof?

Thanks

clavero, I am not an expert, but am building a greenhouse out of 10 mil tripple wall poly and wood. I had an engineer do the structural design. The spacing of the rafters depends on the width of the poly you buy. You can get it in 47 1/4 47 1/2 and 48 (probably some others as well). Know what you have and space the rafter so that you have 1/4 inch gap between sheets at the center of each rafter. My roof is 10 feet from the peek to the top of the walls. The engineer has three 2x4 cross-members between the rafter. They are equally spaced from the peak to the wall. This is to add support to the poly.

Unless someone else knows why not to I trust my engineer (he's my brother and has been doing structural drawings for almost 15 years) and I would put in some cross-mebers for support.

jb

Clav,
I used 8 mil with 24 inch rafters. My does not sag at all under any conditions it has met so far. Snow and ice.

And I have crawled on the roof without any problem.

Mylu, can you post a pic?

jb

Will do so this evening....

Thanks Mylu for the specifics...also JuniorBallon, I (ever so briefly) considered triple wall but recognized that I was going to far out there for a costal CA location.

My rafter spacing is 24" and the run from peak to gutter is 6' but I think I'm going to put in 1x1 1/2" purlins mid span...it's a bit more shade but I hate the idea of those panels sagging.

I could just wait and see if they sag but I want to build this thing and then move on to the rest of the outside projects.

If they are 8mil they are not going to sag with 24" centers. This house is 3 years old and they do not sag. I'm not sure why you think they would. Are the 4 mil?

PolyCarb Panels are tough as heck. At least 8mil is. I can see sagging on 4 mil but not 8. The panels in these pics are on 24" centers allowing 1/4" between panels for stretching. If you need more pics let me know.

Just remember this is the house that was picked up off its foundation, put on a 28' flatbed trailer and moved 3 miles down the highway. Put back on a new foundation and never bent or twisted an inch.

Never could figure out the source code to post pics in a thread but please follow the link

This pics are rather big sorry if you're on dailup.

Here is a link that might be useful: Poly Carb Panels

I was out again tonight working on the GH and was considering what you had posted earlier...now with the pics (Thanks and I'm on cable so the pics came through just fine) I'm a believer.

I'm going to go without the extra support...I'm planning on 8mm so that's fourteen pieces of wood I don't have to cut, sand and fit.

I appreciate your help...thanks

You're more then welcome.

Great pics. Mylu. You can really see the detail of the poly installation. Looks like there is no problem with support. I do like the idea of more light and less cutting and treating. I'll have to ask my brother what load calculation produced the cross-memebers.

jb

Your welcome also. Let me know if either of you need more pics.

hOW IS YOUr greenhouse coming along, girlfriend?

Mine is filling up with JUNK..I have been working on the flooring, brick by brick..

ruth

The reason for the extra supports is to meet code. My greenhouse is large enough that it required a permit. Our area has a max snow load rating of 25 lbs/sqft. The original specs my engineer used showed that to get the required support for that snow load rating, three cross braces between each ratfer would be required in a ten foot run.

That being said we can't find the original spec and the compnay has been sold and can't be contacted to send out another. I have looked on the web and the spec info that I have found is sketchy to misleading. And the companies ignore my request for spec info. Some say no bracing is required others say they do. I switched to 10 mil from 8 mil and am compromising by using 2 cross braces instead of three.

Does any one have a link to strength specs for poly carbonate?

Thanks,
jb

I finally found some info. It's in a pdf so I can't post it. But it looks like I won't need any cross bracing to meet code in my area. In fact I could have a 31 inch span tween rafters and still meet the 25 lbs/sqft load requirement.

Excellent.

jb

That's good to hear JB.

After having my brother read the chart I do indeed need the cross bracing. It is mainly for code as it would be a bizzare event that put a 25 lbs/sqft pressure on the roof, but I would also hate to do this much work to have it sag and buckle. The problem is not with the poly breaking. With that much weight it can pull out from under the vertical cap system.

Oh well, it would have saved me some work and breathing the preservative vapors. Actually I'm kinda getiin' to liek the smell.

jb

Hehe I'll wait for you to change you mind again....

Oh I won't be changing my mind now. I have facts...I think.

Seriously, people should talk to the mfg of the poly they are using and find out what the specs really are. Then find out what your snow load from the County building office wher eyou live. If you know the facts at least you can make an informed decision. Or try to. ;)))

jb

Where's the pictures????

They are coming.

jb

Here's what I'm thinking. I was looking at glass but seems that twinwall might be just fine for light transmission (I will have to supplement in winter EITHER WAY and in summer could use the slight shading) and much cheaper.

10mm twinwall. 22 X 20 greenhouse with 6' sidewalls, but a high-sloping roof 14 feet high at the peak (so I can grow bananas and taller plants at the center). Anyone know of some good plans for that? The county inspector says we need 25 lbs./sq ft. snow load rating here.

I think the difference here between the sag/ no sag twin wall Poly is the manufacturer. I may be wrong (it's happened before) but my guess is that Vegomatic has Solexx. It's called twin wall poly, but I think it would barely fit under the definition. I would think of it as more like a corrugated cardboard. Obviously it is made of some type of poly, but not what most people recognize as twin wall.