Roof rafter sizes

[mrgobby]

Looking to re-roof a brick rendered garage as part of garage conversion project. Existing corrugated (likely asbestos) sheet roof on minimal timber structure to be replaced with 400 centre 150x47 C24 rafters. Garage attached to next doors, a mirror version of mine and forms half the pitched roof. External wall span is 295cm (inside 275cm). Pitch 12.5-13 degrees. If neighbor wants to re-roof at same time then all good else will still go ahead as mono pitch.

Most span tables list 15 degree upwards for pitched roof timber specifications or there are flat roof tables. I think this span and spec is within the tables but considering bolting another rafter along side every third or fourth for added strength.

Just wondering if I am being a bit OTT in that idea? Any timber span table knowledgeable bods advise welcomed.

Marley do concrete interlocking concrete tile, Mendip 12.5 for this and I'll need to create a valley gutter between garage and existing house (30cm gap) for about 3.5m as plan knocking down that bit of garage wall so house wall becomes the garage inner wall with box gutter running above. Garage is 15m long in total.

Thanks

 

[mrgobby]

Birdsmouths for attention

Birdsmouth

• mrgobby
• 10 Mar 2020

 

[^woody^]

Use the 15° tables and just have a bit of extra capacity in the rafter section selected

 

[mrgobby]

Thanks Woody appreciate your input. From google I'd read @15 - 22 degrees it was 3.4m clear span for 150x47mm C24 rafters so it put them within the range but certainly not in the middle. I've added 5m on the back of the garage already and started putting them up at weekend and got nervy because when I bought them I'd planned on a lightweight roof but changed my mind having come across the Marely Mendip 12.5 degree concretes but know they will weigh in quite a bit. Left me wondering if I'd have to cut losses and up-size or bolt on a bit of personal reassurance.

 

[mrgobby]

Looking at the weight of these tiles they are 49kg m2 so approx 3x5m =15m2 that is 735kg or equivalent of about 8 of me on the roof add another for felt and batten so like the x9 14 stone chaps lying on the roof full time clear span tables I've googled are 3.4m @15-22 degree and 3.2m for flat roof with C24 150x47 rafters, I have 2.75m clear span.

 

[catlad]

16" centres ?

 

[mrgobby]

Yes those figures are for 16" centers, metric 400. I do have the option of 300 centers as well.

 

[mrgobby]

Originally thought insulated roof panels on 150x47mm rafters but went off that when I found Mendip 12.5 tiles but had already bought a bunch of 150's at that point. If I'd started out with mendips in mind I may have gone 200x47mm. Could always cut my losses but the 150's from what I can tell are up to it looking at the tables if at the top end of the scale. Considered Redland Cambrian slate as much lighter but more than twice the price and pitch min 15 degrees. Mines probably on the edge of 12.5 having now checked the actual measurements rather than relying on a phone app on a rafter. (285cm run 62cm rise calcs 12.3?)

 

[Notch7]

Looking at the weight of these tiles they are 49kg m2 so approx 3x5m =15m2 that is 735kg or equivalent of about 8 of me on the roof add another for felt and batten so like the x9 14 stone chaps lying on the roof full time clear span tables I've googled are 3.4m @15-22 degree and 3.2m for flat roof with C24 150x47 rafters, I have 2.75m clear span.

dont forget to allow for snowloading, usually 75kg per sq metre (I think, depending on region)

 

[mrgobby]

Oxfordshire appears to be quite a low ground snow area @ 0.30Kn/m2 although I've seen 0.6-0.8kn/m2 as the imposed load to use across the UK for snow with 0.75 being the tabled value. The tables I found listed the joist sizes with these taken into consideration https://www.rightsurvey.co.uk/roof-rafters-span-tables-for-surveyors-roof-construction/ what do you think Notch?

 

[catlad]

If I was doing that I would have probably bought 7x2" timber. 14" centres would add more strength.

 

[mrgobby]

Yes I'm not 100% happy with 150's but like to convince myself they are up to it tables seem to indicate it's okay just not the belts and braces like the main extension (200x47's) which was specified with 150's and upsized.

 

[mrgobby]

Couple of photos for anyone interested to see

Garage1

• mrgobby
• 14 Mar 2020

Garage 2

• mrgobby
• 14 Mar 2020

 

[martin hill]

how about some of this. https://roofin.co.uk/product/coroti...kKAXA_6S_WwCV8kRFZjLs8fOLfDl_fIxoCQJAQAvD_BwE

 

[mrgobby]

I'd looked at those as well as corrugated panels, insulated sandwich panels, metal sheet was another option but figured tiles are more robust and less noisey when underneath them.