Dormer extension, are 75 X 195 joists necessary?

[DIYjake]

I have a dormer bungalow that was built in 1969. I've had structural drawings made up to extend the front and rear dormers out as far as possible. The current joists are 100x150 at 400 centers.

 

The drawings specify for the joists to be 75x195 and raised 25mm and to be doubled up below the bathroom. The longest span is 4584mm.

 

One of the carpenters/builders has said that doubling up the current joists will be substantial enough and has been in contact with building control who seem to agree but haven't yet visited

 

I have a gut feeling that it should be done to drawing. Any thoughts?

[tonyshouse]

Yes, if you are very clever you get away without raising them up the 25mm , humps up and secured to existing ceiling joists, wires are usually the problem 

[nod]

SE will tend to over spec 

Your joiner is probably right 

But BC will want you to follow the drawing you have 

[PeterW]

SEng is correct - the 100x150 isn’t deep enough for that span to not bend in the middle - 195mm is barely good enough but assume he’s got you bolting them together ..??

[bassanclan]

If you look at the Trada span tables 195x75 is just enough to span 4.96m

 

If you want to reuse existing joists at 100x150 I would sat doubling up will be fine, but I'd want them glued and bolted together which may take more labour than simply putting in new correctly sized timbers.

 

Also double check the joists really are 100x150. That is suprisingly beefy for the 60s.

[DIYjake]

Thanks for the input all. I've told the likely builder that I want to stick with the drawing for peice of mind more than anything, I don't fancy this biting me down the line by not going to drawing

 

Sorry, the joists are 150x50. No idea why I put x100.