Wrong size rafters advice

[Ian348]

Hello!
I'm a novice DIYer and building my first shed. I thought I'd follow the advice of some pros here https://youtu.be/b3JgPX5fVUo

so I used c24 2x4s at 12 inch centres to span 8ft at 12.5° pitch.
Now I've realised (from span tables) that these rafters are under the correct size. Is there anything I can do to support the roof so it doesn't bow?
I've added a 2x10 beam at 6ft to support the rafters.

I was thinking about creating a mono truss and connecting the remaining unsupported 6ft to the beam with joist ties like these https://www.screwfix.com/p/sabrefix...ICyyqQhRsJ6iPQwTGF0aAgltEALw_wcB&gclsrc=aw.ds.


Is this a good idea? Am I doomed to a bowed roof?

Any advice is greatly appreciated!!

 

[RandomGrinch]

Hi,

I'm also only a DIY'er, so may be very wrong!
But, my interpretation of these span tables:

https://www.timberbeamcalculator.co.uk/en-gb/span-table/rafters?load=0.75&class=C24&slope=15

Suggests you are spot on!

...and at 300mm rafter centres you are exceeding these specs.

These span tables are designed for use with many different building types. If the specified timber sizes are good enough for your house roof, they should be more than adequate for a shed!

 

[Ian348]

Ah thank you RandomGrinch!

I was looking at American span tables which didn't even show the span length of 2x4s, my mistake.

As you point out, it's over engineered for a shed!

 

[Dereekoo]

Unless you consider it to be a flat roof as its less than 15 degrees as per rafter tables then limiting span would be 2.1m according to TRADA table but since the spacing is at 300 and not 400 as stated its well over engineered

 

[RandomGrinch]

Unless you consider it to be a flat roof as its less than 15 degrees as per rafter tables then limiting span would be 2.1m according to TRADA table but since the spacing is at 300 and not 400 as stated its well over engineered

...but, if we assume:

I've added a 2x10 beam at 6ft to support the rafters.

Is an adequate purlin (although we don't know the length), with a clear span of 6ft, I think the rafters are probably ok!

An interesting other thread here, about 2x4's being used for a 5m span!

https://www.diynot.com/diy/threads/help-needed-please-roof-timber-bending.582025/