About to roof a barn I hope to have panels on in the near future...

[Sunofagun]

I built a large sawmill shed last year, shanty style. (just built the 28' by 100' North side). This year I am doing the Southern slope, which will be about 20' by 100'. Rafters are about to go up right now, and I'm going to roof it with purlins and metal roofing. As I am milling the lumber, I can do any board width and spacing I want. Is there a good rule of thumb for panel screw-down spacing? I like to do close purlins anyway, as it makes it a lot easier to walk up there before the steel roofing goes on. I do 5 quarter purlins, so they can bear a lot of weight. I just don't want to end up having to shove anything between the tin and the rafters to catch a screw-down point.

[sensij]

It is all about the load calculations. You can play around with Iron Ridge's online design tool to see the consequences of different spacings in your region.

[ButchDeal]

I built a large sawmill shed last year, shanty style. (just built the 28' by 100' North side). This year I am doing the Southern slope, which will be about 20' by 100'. Rafters are about to go up right now, and I'm going to roof it with purlins and metal roofing. As I am milling the lumber, I can do any board width and spacing I want. Is there a good rule of thumb for panel screw-down spacing? I like to do close purlins anyway, as it makes it a lot easier to walk up there before the steel roofing goes on. I do 5 quarter purlins, so they can bear a lot of weight. I just don't want to end up having to shove anything between the tin and the rafters to catch a screw-down point.

is it standing seam roof? if so then use miasole flex-02N stick on shingles between the seams.

If not then you will use something like IronRidge rails with mounts. You would bolt the mounts to the rafters not the purlins.

[J.P.M.]

FWIW, assuming you're planning to put some solar PV devices on the south facing roof, and depending on other design considerations, a roof slope equal or close to your local latitude (say, +/- 10 degrees or so, maybe a bit toward the shallow side) will probably result in higher annual system output.

[Sunofagun]

is it standing seam roof? if so then use miasole flex-02N stick on shingles between the seams.

If not then you will use something like IronRidge rails with mounts. You would bolt the mounts to the rafters not the purlins.

This was exactly what I needed to know, thank you very much. I'll check them out. Unfortunately my rafter spacing is already somewhat set, I hope this will still work.

[Sunofagun]

FWIW, assuming you're planning to put some solar PV devices on the south facing roof, and depending on other design considerations, a roof slope equal or close to your local latitude (say, +/- 10 degrees or so, maybe a bit toward the shallow side) will probably result in higher annual system output.

I appreciate the input. I am already set at somewhat around a 4/12 pitch, what with the current part already built. I need to do some more studying.

[ButchDeal]

This was exactly what I needed to know, thank you very much. I'll check them out. Unfortunately my rafter spacing is already somewhat set, I hope this will still work.

Well for ironridge you can handle different spacing with different models. Put in your location and roof pitch into their web design tool, and you can see the different spans with the different models of railing

[DaveDE2]

Ironridge design tool assumes nothing about your supporting structure. For all practiple purposes it assumes it is bombproof and of no concern of theirs. Only then do they suggest rail mount type and support spacing etc. Rafter sytems are generally not as strong as pre-fab trusses. In my juristdiction, I needed to have a PE structural engineer analyze my roof and provide a letter to the AHJ verifying max snow plus panel loading. If I were you, I'd contact the AHJ and softly ask if anything is needed there. If so, contact a structural engineer to make sure your support system will work.