Corrugated plastic roof

[Dada Joe]

That is a good looking system, would be more sturdy if the purlins we're rotated to vertical, framed between the rafters, or over the top of them. We are building as light as is possible, the 2x4s laid flat will easily span your rafters at 2 foot centers. Even a 1x4 will do that, but with a low slope, even that has limitations

 

[Dada Joe]

What’s the lifespan of the plastic? Sunlight degrades plastic, so it will get brittle over time. Even greenhouse grade plastics have a limited life.

I have installed commercial grade that lasts on roofs over 20 years. Building supply store grade? No good idea

 

[llombardo]

Not corrugated plastic, but the concept is the same unless attaching the plastic to OSB or plywood. In this pic, rafters are on 2 foot centers, purlins are on 18 inch centers.View attachment 1692957

Rafters are 2x4s and purlins are?

 

[llombardo]

I'm doing a sloped roof-about 5 inch slope on both sides--what do I put across the top on the center(I guess the main beam) across. can I just over lap the panels. Not sure if this makes sense.

 

[aart]

Rafters are 2x4s and purlins are?

Rafters are usually 2x4, purlins are usually ~1x2.

 

[llombardo]

Not corrugated plastic, but the concept is the same unless attaching the plastic to OSB or plywood. In this pic, rafters are on 2 foot centers, purlins are on 18 inch centers.View attachment 1692957

And one more question because I can't tell 100% in picture.. Is it 2 ft all the way across or at the end are the 2x4s closer together for more support?

 

[aart]

Is it 2 ft all the way across or at the end are the 2x4s closer together for more support?

Not sure what pic you are referring to, but, rafters usually remain parallel.

 

[Brahma Chicken5000]

I'm doing a sloped roof-about 5 inch slope on both sides--what do I put across the top on the center(I guess the main beam) across. can I just over lap the panels. Not sure if this makes sense.

Yes overlap the panels a bit. If you get snow consider a 1 foot slope.

 

[Ted Brown]

Is it 2 ft all the way across or at the end are the 2x4s closer together for more support?

@aart I expect that the question is "Are rafters always 2' apart?".

Rafters are typically 24" or 16" on center (apart); these spacings will generally support most loads such as snow. Greater than 24" not advised. Regular spacing makes most sense for nailing etc. Overhangs sometimes require a special "ladder" structure.

 

[Ted Brown]

I'm doing a sloped roof-about 5 inch slope on both sides--what do I put across the top on the center(I guess the main beam) across. can I just over lap the panels. Not sure if this makes sense.

Normally one uses singles turned sideways and over-lapped end to end (shingle roof) If it is a metal roof there is a "v" shaped 8" foot metal section that covers the seam. Depends on how you plan to cover the roof, a careful wood wood overlap and some roof tar can work.