Waste pipe diameter?

[UserError]

Hello, I'm installing a shower tray on a 4m run from the soil stack. Building control have specified 50mm pipe so that's what I've got.

However, the 50mm pipe means that the only way they would allow a hole of this diameter through the joist was to put a doubler on it, which is OK, but just complicates things a bit especially allowing for the fall.

The building regs state "Where the larger branch pipe sizes are used the diameter of the trap is not increased but the tail of the trap should be lengthened by 50mm before increasing the diameter". Understood.

Not quite sure what the function of the smaller tail is, other than I couldn't find any shower traps with a 50mm outlet so they may not be available?

Unfortunately, the reducer fitting is tight up against the joist, making it tricky to execute.

My question is: Is it allowable to increase the 40mm tail to say 300mm length as shown in B of my iffy sketch? This would take everything the other side of the joists and be a lot easier, not least as there'd be a bit more leeway on the hole location with the 40mm diameter, and the majority of the 4m run would still be in 50mm.

Many thanks

 

[king of pipes]

I would take 40mm through the joists then increase to 50mm . Kop

 

[UserError]

I would take 40mm through the joists then increase to 50mm . Kop

Thanks for that. Would make life a fair bit easier.
Is the larger pipe diameter on longer runs, to reduce the risk of siphoning or a different purpose? Ta.

 

[king of pipes]

Over 3 m in length then you should use 50mm and have a constant fall to drain to aid drainage and reduce the chance of blockages. Kop

 

[UserError]

Over 3 m in length then you should use 50mm and have a constant fall to drain to aid drainage and reduce the chance of blockages. Kop

Many thanks for your help.