Rafter Sizing for new roof..

[WelshWood]

Hey guys,

So i've been asked to remedy some work on an old shed.

It's currently clad in metal sheeting, and i'm removing those, adding rafters, then felt,batten and slating.

As it's working on the existing A Frames and Purlins, i'm not 100% sure whether 4x2" or 6x2" will be better, so i'd love some advice

The length of the shed is 6.2m long, and 5.25m wide

Thanks in advance,

~WW

 

[Bigchasbroon]

So this drawing is of the existing a frame and Purlins? The purlins are 75mm square? What's the spacing of the A frames. The section of the rafters might be irrelevant if the Purlins aren't up to it, therefore 6x2 rafters with a tie across might be safer.

I usually ask my mate who's a structural engineer if I'm in doubt. If you have someone to do a quick calc it takes the onus off of you and it's good to have a professional contact in your network. Be warned though they do charge £500 a day and up!

 

[WelshWood]

Yeah, the drawing is of the existing members,

Total length of Shed is 6200, and 4 A Frames equally spaced, gives a spacing of around 2066mm between A Frames, and Yes, the Purlins are 75mm square.

By using a tie, what do you mean exactly? Collars?

~WW

 

[Bigchasbroon]

Yes a collar so the rafters and collar would essentially form an A frame of its own. My thinking is if the original roof has been designed for metal sheeting how much heavier is slate? It's maybe overkill but I would be looking for the rafters to support a bit of weight by themselves with out all the load being taken by the purlins.
An easier and possibly cheaper option for the client might be to bring the lot down and buy engineered trusses. It would probably only cost around £600.

Hope I'm helping  :

 

[WelshWood]

Yes a collar so the rafters and collar would essentially form an A frame of its own. My thinking is if the original roof has been designed for metal sheeting how much heavier is slate? It's maybe overkill but I would be looking for the rafters to support a bit of weight by themselves with out all the load being taken by the purlins.
An easier and possibly cheaper option for the client might be to bring the lot down and buy engineered trusses. It would probably only cost around £600.

Hope I'm helping  :

Yeah, was thinking about that, putting 1 or 2 coach bolts on the collars would help the strength quite alot.

The slates will be quite heavy, but of course it's shared over the square meterage of the roof, so i'm not too worried if I strengthen the rafter by using a collar.

I probably should have mentioned, that bolted into each set of rafters will be a 9x3" Joist to put a floor ontop for storage space.

With these rafters tied into the joists, I suppose this will create an A Frame in itself?

~WW

 

[Bigchasbroon]

Exactly. I think that'd be plenty strong enough. For the difference in cost between 4x2or 6 it's better to go big.

My local Jewson rep phoned me up to say they were doing a promotion on 6x2 regularised for 1.48 pm. Dunno if it's nationwide.

 

[WelshWood]

Exactly. I think that'd be plenty strong enough. For the difference in cost between 4x2or 6 it's better to go big.

My local Jewson rep phoned me up to say they were doing a promotion on 6x2 regularised for 1.48 pm. Dunno if it's nationwide.

Sounds like a great deal! I usually avoid Jewson and Travis Perkins, not only because most of the time their wood is barely useable, but because I prefer supporting local merchants

~WW

 

[Bigchasbroon]

I normally use my local timber merchant but they're stuff isn't as good as it was and I'm paying 1.68 (ex vat) for 6x2. That Jewson price was a one off promotion running for October and November.