Timber sizes and spacing for a monopitch roof on timber outbuilding

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MrPlank

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Hi Guys

I am building a garden room in timber and wanted to ask what size timbers I need to use for the roof. The span it will need to cross is 4 metres, the actual size is 4m x 7m, the monopitch of the roof will be 4 degrees. Will using smaller spacing mean I can use slightly smaller sizes or is this irrelevant. I intended to use 400mm spacing. I had planned on using 145x 47mm but recognise that this is not going to be good big enough, the roof covering will be cladco box profile. As always any advice greatly appreciated and I hope my wording makes sense.

Thanks
 
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170 x 47 C24 is really the min.

145 x 47 would sag a bit over 4metres, putting in more joists wont help with the sag, but it would somewhat increase load capacity, say for snow loading. Maybe 145 x 47 would be ok for box profile, its not particularly heavy.

If the external building size is 4 metres, the clear span will be less, say 3.8m
 
Hi Notch7

Thank you for prompt reply yes the clear span is 3.8 metres, the roof will have an overhang at the front of the building, the length of timber I intend to use is 4.8 allowing around 50cm ish over hang at the front. will 170x47 still be sufficient or is this detail not really rellevant.

Thanks again
 
MrPlank said:
Hi Notch7

Thank you for prompt reply yes the clear span is 3.8 metres, the roof will have an overhang at the front of the building, the length of timber I intend to use is 4.8 allowing around 50cm ish over hang at the front. will 170x47 still be sufficient or is this detail not really rellevant.

Thanks again
Click to expand...
no the overhang is irrelevant -what counts is the clear span.
 
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Notch7 said:
putting in more joists wont help with the sag,
Click to expand...

More joists would reduce the deflection of the individual joists because each joist will then be supporting less load, which proportionately reduces deflection.

Additionally, deflection is only a 'serviceability' issue and not a structural problem.

If there is no plaster ceiling, and if the steel sheets are full-length, the deflection may be of little consequence, 6" x 2" @ 300-350c/s would be fine.
 
Hi guys

Thanks for the responses, the box profile sheeting will be full length and the internal ceiling will not be plastered, I intend to finish it in 10mm ish softwood cladding.
 
Hey MrPlank,

I’m planning on building a similar shed/workshop in my garden with exactly the same span as you mentioned above. Just wondered if you completed the build and what spec you went for (timber size and centres).
I’m planning on using the Cladco box profile too.

Cheers
 
Last edited:
MrPlank said:
around 50cm ish over hang at the front. will 170x47 still be sufficient or is this detail not really rellevant.
Click to expand...

Not wanting to speak against any of the more qualified people on here, but the overhang did help when our roof was being built as the engineer appears to have taken it into account for deflection under the dead loads.
Basically without the overhang there is no bending force at the support, but with the overhang the overhang will cause a bending force which reduces the deflection on the main span.
With plenty of overhang if you put loads on and off the overhang you would expect to see the main span vary in deflection.
However you do have to be sure that someone won't come along later and cut off the overhang. In our case that was true because it's one roof and the overhang is down to where the supports are below.
 
Great stuff MrPlank, that's what I'll prob opt for as it’s tried and tested by your good self!

With regards to the Cladco box profile I was gonna use 2” x 1” for purlins running across the joists at 600mm centres as only carrying weight of sheet. Would this suffice or should I reduce to 400 or 500mm centres?

Cheers
 
Hi steve
When I do those I use 3x1 rough sawn and if there is nobody likely to walk on it then 2' centres are normal.
Reduce this to 18" if you think people will climb on the roof.
 
catlad said:
Remember those sheets are prone to condensation and you can buy them fleece lined to reduce condensation.
Click to expand...
Hey catlad,

Thanks for purlin info but as for sheet condensation I never gave that a thought.

MrPlank - You experienced any condensating issues with your build?

Apologies for all the questions but as a DIYer embarking on a large costly project I want to ensure its fit for purpose.
 
Hi Steve

I put the purlins at 600mm, as you say the weight of the metal sheets is not that much, regarding condensation I have to say I didnt really consider this (maybe wrongly so I don't know) I didn't notice any condensation prior to sealing up from the inside. I put a breather membrane under the purlins, again not sure if this was a good thing or not, I tried to research on this site and I felt like it wouldn't do any harm to have it. Just want to stress I'm a diyer so really might not have done everything in the best way, I guess time will tell but Im very happy with how it turned out.
 
Anybody just starting might consider Plastisol coated insulated composite corrugated metal roof sheeting.
 
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