Constructing a roof - first time

[Dilberto]

Hi,
I’ve been renovating an old outbuilding during which I removed a rotten roof and strengthened and secured walls and replaced corrugated iron gables with concrete block ends. It’s going to continue to be a shed.

I'm now ready to commence the roof construction which will be pretty-much a like for like replacement- but will be cut by hand and not trussed.

The internal wall dimension of the building is 8m x 2.8m. I plan on placing rafters at 0.8m intervals and these will be secured to a ridge-board and to 100x75mm wall plates. The main modification will be that I intend to add sarking boards, and then clad with corrugated iron roof sheets as before.

The pitch is about 40 degrees, (run of 138 with height of about (from memory) 120).

My questions are:
• is a rafter dimension of 50 x 100 sufficient (as before);
• is rafter interval of 80cm sufficient (as before)
• what nail size is most suitable for the wall plate end and at ridge end

I’ve not placed a wall plate across the gable ends as I’m not planning on adding a ridge stud support. Is this ok?

Thanks, and sorry if these questions make you cringe!!

D

 

[noseall]

• is a rafter dimension of 50 x 100 sufficient (as before);

1.8m (i worked out your rafter clear span using 40 deg pitch with 1380 run) is fairly close to the bone at 40 deg. You would need to speak to a relevant structural bod with the latest TRADA tables.

• is rafter interval of 80cm sufficient (as before)

Absolutely not. You are likely to need 400mm c/c.

• what nail size is most suitable for the wall plate end and at ridge end

Use a combination of 4" and 3" nails or use similar sized screws, i.e. 100mm 5's and 75mm 5's.

 

[Dilberto]

Noseall,
Thanks for your reply. I'll reduce the inter-rafter distance, and increase rafter cross-section from 100x50 to 125x50. I should have said that I'll be adding purlins (which will reduce the rafter free-span by 50%) and wind-bracing.

My plan was based on replacing the timber sizes, rafter spacing, purlins and bracing like-for-like. Interesting that the shed/roof having stood in deepest Aberdeenshire (snow up to 1m at times) since at least before the war if not the turn of last century - I only removed the roof as the rafters were powdering under wood-worm.

But, having said that, I'd prefer to be comfortable with the safety of the replacement and so will adopt your advice.

Thanks,
Dilberto.

 

[RedHerring2]

I should have said that I'll be adding purlins (which will reduce the rafter free-span by 50%) and wind-bracing.

Are you suggesting 8 metre long purlins? Or do you you have some support for the purlins, midway?

 

[Dilberto]

RedHerring2,
That was the plan. A purlin midway along the rafter length on each side so reducing span from ~2m to ~1m. These would run for the 8 metres of the shed length and be attached to the rafters and the gable ends. Each rafter nwill be collared.

This was what was pretty-much there before although attachment to gable ends was not possible before as these were of corrugated iron. Each rafter will be collared, but the purlins were not supported by struts previously, although I could add some.

Dilberto.

 

[RedHerring2]

According to my tables, a 275 X 63 mm purlin will span 4.3 metres using C24 timber and with a loading of .5 kN/m². That's the longest span available. What size were you thinking of making your purlins?

Hopefully others will give their opinion.

 

[ladylola]

'Tis quite a span for a single purlin. I'd form a truss about halfway and put in the purlin in two pieces or even two trusses and three purlins, that would allow for smaller timber sizes. You will also to able to keep your rafters at 100mm , noseall is correct about probably going for deeper but that was before you told him about the purlins.

 

[Dilberto]

All,
Thanks for advice.

The shed roof will be made from 5x2" C16 boards; collared; spacing 600mm; and after consulting with two local builders/roofers, no purlins required as it's being sarked [previously it had purlins but no sarking].

Thanks,
Dilberto.