rafter sizing

[holboob]

Hi there

We have a clear span in our new roof of about 4.7m from wall plate to ridge. The ridge is constructed of steel as it is a vaulted ceiling and has been calculated by an engineer.

I was therefore wondering whether it is possible to have continuous rafters without the need for a purlin at the mid span as having looked at the tables in the old Part A of the Building Regs the largest span is around 3.5m or so or is it essential to have purlins as opposed to increasing the size of the rafters.

Also, in part of the roof it goes to a timber ridge with a vaulted ceiling so is the clear span calculated from the wall plate to the ridge or from wall plate to wall plate?

thanks

 

[Shytalkz]

Where you've got a ridge purlin, span from ridge to eaves is a simple calculation. Where you haven't got a ridge beam (steel or timber), but an A-frame arrangement, it's more involved.

Both fall outside the scope of span tables.