112, 125 or 150mm?

[RFw3Duwrd7Yu]

It seems that the last aging length of rainwater guttering has now begun to come off the roof of our recently purchased (but very old) home. So now I've got to replace it before monsoon season properly hits us here in Wales. I'm very much hoping to keep fascia and brackets in place and simply drop in a like-for-like replacement. The roof is approx 9m by 4.5m with a 45 degree pitch, so functionally around 61m2 of roof to drain. The current drains are just a straight length with stop ends on each side and an outlet just a foot in from the L side and are in the old 5" half round gutter style. Whilst I was hoping to purchase 125mm gutters to replace them, it seems like these are a bit hard to come by, and I'm a bit concerned about whether they'll be compatible with the floplast 68mm round downpipes I've already put in. Should I simply upgrade to 150mm? Or revert to 112mm floplast drains? Am I overthinking things? Also, plastic or aluminum? I'm not really sure what the difference is between them from a functional perspective...

 

[Exedon]

Have a look at polypipe deepflow gutter extra carrying capacity but fits standard fallpipe.

 

[^woody^]

Standard guttering will do. It's the number and position of outlets which it's the important thing.

Unless cast, aluminium gutters and downpipe can be noisy.