50s bungalow understated rafter? but hold the test of time.

[champagnecharly]

I've just been looking at the spans for a 50s bungalow.
according to the architects drawings
the 3x2 rafters at a
pitch of 37.5
span 2.55 between 7x1 ridge plate and purlin and
2.4 spans down to wall plate
with 400mm centres.
Obviously it works as it hasn't moved in 65 years and unfortunately trada dont go down to that size.. but is it me or is that excessively skinny?

Playing around with eurobeam using loads specified in trada. I'm getting failure of 26mm deflection or .01 the length!!
Maybe i need to check the actual dims but it's 250miles away!
Any thoughts ?

 

[champagnecharly]

Ok.. so from further reading i see the allowable deflection varies considerably between type. however its still way over the L/125.

 

[Nige F]

Any thoughts other than I best not get up on the roof?

why do you want to get on the roof

 

[champagnecharly]

Thanks for your post.. I removed the section about 'not' getting up on the roof so that nobody else gets distracted by it and because it is irrelevant.
The question is 3x2 timbers at that pitch and span. Does that seem right?

 

[RonnyRaygun]

The rafters are undersized, but the chance of the roof ever seeing the design imposed loads are minimal.
Also take into account the material factors of safety and this will go a long way to explaining why it's still standing after all these years.
They may well be deflecting more than is desirable but that is not going to cause any problems unless it is massively beyond the SLS limits.

 

[champagnecharly]

Thanks for the confirmation that it was not me imagining they were undersized and that it is not (as has proved the test of time) going to collapse at the next heavy snow.