roofing calculations - purlin dimensions

[julianf]

Using this corrugated sheet material -

32/1000 Box Profile PVC Plastisol Coated Roof Sheet

<h2>About Our 32/1000 Box Profile PVC Plastisol Coated Roof Sheets</h2> <p>A great choice if you’re looking for a metal sheet that offers enhanced protection against scratches and UV exposure and everyday wear, particularly in agricultural, industrial or exposed environments, our 32/1000 box...

www.cladco.co.uk

With a maximum unsupported purlin distance of 1.6m, how do i know the strength of the purlins to use?

The datasheet gives the purlin spacing of 1.2m, but, so far as i can tell, gives no information as to what material should be used for the purlins themselves.

Obviously this is dependent on distance between the a-frames, so one size will not fit all, but how can i calculate the purlin requirements for my 1.6m span?

I want to keep the total weight down, so i dont just want to go bigger (heavier) than needed, otherwise id just overengineer it anyway - but, again, i dont want to increase the weight.


Thank you!

 

[mike os]

Wood or steel?

 

[julianf]

Wood or steel?

I was thinking wood, but if steel gives a significantly better strength to weight ratio then ill go steel.

(by "significantly" i mean in practical terms, rather than someone just reading off a sheet and saying one or the other)

Wood will be easier for me to work with, but i have both available on the same industrial estate, so either are equally as available.

Its only a small roof, so cost is neither here nor there.

(edit: assuming i can get locally - i dont want to have to mail order, as delivery will be half the cost of the items!)

 

[mike os]

Building regs doc a table a23

 

[dan.taylor.1]

The size and strength is based on the span between supports too, like a standard for agri sheds is 15' on a 7x3 timber purlin, you can go 20' or even 30' on steel z purlins

 

[julianf]

Obviously this is dependent on distance between the a-frames, so one size will not fit all, but how can i calculate the purlin requirements for my 1.6m span?

The size and strength is based on the span between supports too, like a standard for agri sheds is 15' on a 7x3 timber purlin, you can go 20' or even 30' on steel z purlins

This is just a case of you not reading my comment, rather than me missing somthing, right?

(im just checking)

 

[julianf]

Building regs doc a table a23

Thank you. Can you spoon feed it me, as I'm looking at this -

And nothing is springing out at me (I have not read the whole document, but have skimmed through)

 

[Cato]

Timber span tables will give the info you need, or if you don't want the faff reading through those an online calculator such as this one will give you the required size.

 

[dan.taylor.1]

Sorry yeah, missed your comment. How big is the shed?

 

[mike os]

Thank you. Can you spoon feed it me, as I'm looking at this -

View attachment 287678


And nothing is springing out at me (I have not read the whole document, but have skimmed through)

https://www.labc.co.uk/sites/default/files/approved_document_a_1992_superseded.pdf

page 67 I think

 

[mike os]

Hopefully you can work it out otherwise let me know

 

[gaz1]

Using this corrugated sheet material -

32/1000 Box Profile PVC Plastisol Coated Roof Sheet

<h2>About Our 32/1000 Box Profile PVC Plastisol Coated Roof Sheets</h2> <p>A great choice if you’re looking for a metal sheet that offers enhanced protection against scratches and UV exposure and everyday wear, particularly in agricultural, industrial or exposed environments, our 32/1000 box...

www.cladco.co.uk

With a maximum unsupported purlin distance of 1.6m, how do i know the strength of the purlins to use?

The datasheet gives the purlin spacing of 1.2m, but, so far as i can tell, gives no information as to what material should be used for the purlins themselves.

Obviously this is dependent on distance between the a-frames, so one size will not fit all, but how can i calculate the purlin requirements for my 1.6m span?

I want to keep the total weight down, so i dont just want to go bigger (heavier) than needed, otherwise id just overengineer it anyway - but, again, i dont want to increase the weight.


Thank you!

he will struggle mike

hes not taking into account rafter space x width

purlins can be inbetween rafters or on top of rafters

the datasheet only provides maximum purlin gap distance between them

therefore a 2m sheet would need 3 purlins from top to bottom

 

[mtt.tr]

Trada span tables

 

[julianf]

he will struggle mike

No, i cant work it out.

Using this corrugated sheet material -

32/1000 Box Profile PVC Plastisol Coated Roof Sheet

<h2>About Our 32/1000 Box Profile PVC Plastisol Coated Roof Sheets</h2> <p>A great choice if you’re looking for a metal sheet that offers enhanced protection against scratches and UV exposure and everyday wear, particularly in agricultural, industrial or exposed environments, our 32/1000 box...

www.cladco.co.uk

With a maximum unsupported purlin distance of 1.6m, how do i know the strength of the purlins to use?

The datasheet gives the purlin spacing of 1.2m, but, so far as i can tell, gives no information as to what material should be used for the purlins themselves.

Obviously this is dependent on distance between the a-frames, so one size will not fit all, but how can i calculate the purlin requirements for my 1.6m span?

I want to keep the total weight down, so i dont just want to go bigger (heavier) than needed, otherwise id just overengineer it anyway - but, again, i dont want to increase the weight.


Thank you!

What additional information is required?

 

[MetalMonkey]

If it isn't subject to regulations - any reason not to just close up the spacing and use a standard roofing batten - cheap, lightweight and will spread the support more evenly?

I'm not familiar with that type of roofing sheet, but the corrugated sheeting I removed on an outbuilding roof just recently had sagged between purlins. I've now switched to ply sarking to avoid the problem reoccurring - more support the better to my mind.

 

[julianf]

It's on a prefab. The distance between the a frames (the unsupported distance of 1.6m that the purlins have to span) is fixed.

I could increase the number of purlins if this would improve the strength weight trade off, but I can not (or rather do not want to, do not want to entertain the idea of) adding further a frames, so would rather that did not become a red herring here.

 

[mike os]

It's on a prefab. The distance between the a frames (the unsupported distance of 1.6m that the purlins have to span) is fixed.

I could increase the number of purlins if this would improve the strength weight trade off, but I can not (or rather do not want to, do not want to entertain the idea of) adding further a frames, so would rather that did not become a red herring here.

Remind me tomorrow and I’ll take a look

 

[gaz1]

what is the distance between the a frames then and what size is your a frame ?

 

[julianf]

1.6m maximum between the a frames.

A frames are steel, but I will likley screw a 2 by somthing on to them to act as a spacer.

Is this what you needed to know?

Thank you.

 

[gaz1]

yes now look at the posted picture ive posted

take note because of the spread of the roof you need a sitting board or spreader board

for every metre its 3"+1" thickness of wood beam needed

so wood used on the above is a 4x2 fastened to 4 a frames

so yours would be either metal I beam 4x2 or 5x2 or c channel or in 6" wood so would be a 6x2

the alternative old way is a 4x4 4x5