Thanks to everyone for their input on my previous thread on rain gutter systems. I now have a new stainless steel rain gutter with painted PVC downspouts out of sight, which feed into a drainage system.

However on one site connecting to the drain is not straight forward, as there is an auxiliary building in the way. The only feasible way is to go under the auxiliary building with a small drain pipe in order to connect the downspouts to the main drain: i.e. a buried pipe under the concrete floor of this building for approximately six meters. I am envisaging a Twinwall Drainage Pipe to be installed for this part (see first picture). The vertical PVC downspouts (100mm) will than connect to the horizontal twinwall pipe (150mm?) via some final connector piece, maybe with an opening for inspection (see second picture).

I'd like to hear people's opinions as to whether there are any potential structural and maintenance problems with this design: i.e. burrying a pipe in sand underneath a concrete floor or even partially embedding it in the floor. Comments in terms of costs and cost effectiveness for twinwall pipes would also be interesting.

I would do what the Thais would do, 150mm regular PVC, or concrete pipe if on a budget.

There's zero pressure and zero danger of damage unless your building foundations are carp and things sink enough to damage the pipe, in which case you have bigger issues than a leaky pipe.

Even if it leaks no issue, it's only rainwater not toxic waste.

Thanks Crossy!

The foundations are fine. The only "pressure" would be people walking across or a spinning washing machine sitting on it.

Do you think, twinwalls are more expensive than concrete pipes or is it that my plumber would not know anything about twinwalls?

I am reluctant about PCV pipes as they will be embedded/ encased in concrete at the in and out points.

Concrete pipes are dirt cheap, TBH I wouldn't use them under a floor that couldn't be dug up for access.

Regular PVC is ok in concrete and will handle traffic (we have a run of 100mm under the lawn where vehicles regularly cross with no issues). If you can move the connection points to be accessible / install a pit, then they have to be the best locally available option.

Can you even get twinwall here? I detect Baht in large numbers

Concrete pipes are dirt cheap, TBH I wouldn't use them under a floor that couldn't be dug up for access.

Regular PVC is ok in concrete and will handle traffic (we have a run of 100mm under the lawn where vehicles regularly cross with no issues). If you can move the connection points to be accessible / install a pit, then they have to be the best locally available option.

Can you even get twinwall here? I detect Baht in large numbers

That's great. I like your pragmatic approach Crossy.

So as PCV pipes are normally only four metres, I have one access pit at the connection point and the rest will be embedded in concrete inside the building, with the next connection points outside in gravel.

Re: twinwall I think I saw them in a builder's yard here. I'll investigate and report back.

Buy a big pot of PVC glue and a joint sleeve

Buy a big pot of PVC glue and a joint sleeve

Or PVC pipes with bell-ends