a couple of rainwater runoff downpipes are blocked in the house we’ve just bought, by moss and dirt presumably- a garage has been built adjoining the house on one side, with the cast iron pipes concreted through the floor- is it worthwhile pouring sulphuric down, or is that not going to help?

Whats the best solution to the problem?

I got some sulphuric acid to clear our plastic waste pipe from the hand basin in the bathroom. The guy behind the counter at the builder’s merchants put the container in a plastic bag! Eh?! “You’ve seen Alien haven’t you? Someone spilt some in their car and it ate through the floor, doesn’t melt plastic” They said if you had metal pipes then use it at your own risk and properly flush/rinse it ASAP

One Shot drain clearer and don’t breath in.

HAHAHAHAHAHAHAHAHAHAHAHAHAHAHANO.

Sulphuric acid is the last thing you want near cast iron. Would probably be better just skooshing some unleaded in and igniting it to shift the blockage, probably cause less damage…

In all seriousness either rod it out or take a power washer to it.

Squirrelking speaks the truth.

If you have to use chemicals use caustic soda (sodium hydroxide, often sold as bathroom unblocker, Oven Pride shelf cleaner or available on ebay). It’ll attack organic matter, remove rust but not do much to cast iron. The only problem will be WHEN it doesn’t work you’ll be rodding a tube full of caustic soda.

Little Johnny took a drink,

alas, he is no more.

‘Coz what he thought was H2O

was H2SO4

Definitely just rod them much easier and you don’t end up with a tube full as acid when it doesn’t work as mentioned above. Worst case wish you have to remove a section or split at a joint for better access.

I borrowed one of these off a plumber mate, but would have been happy to buy one as it was so good. Once it’s clean put a wire ball in the top of the down pipe – either buy one or make a chicken wire ball.

https://www.screwfix.com/p/drain-unblocker-25ft-7-6m-/11325?tc=CT8&ds_kid=92700022888056721&ds_rl=1241687&ds_rl=1245250&ds_rl=1244066&ds_rl=1248184&ds_rl=1245250&ds_rl=1247848&gclid=CjwKCAiA5qTfBRAoEiwAwQy-6aiJeRJjTpLtX2kcyq-A0NKfCe7h2rYXNviUGiuLZrxz8gvmnujDfxoC92MQAvD_BwE&gclsrc=aw.ds

Another long term option I would be thinking about is putting an inspection/rodding hatch in – you would have to cut out a section of pipe, and then put in the new section that contains the hatch. Your local cast drainage supplier will be able to talk you through it and show you the fittings.

I cleaned my guttering a couple of months ago. Removing the tennis ball from the hopper and the other three from the guttering seemed to get the flow back!!

If you’re going to use rods turn them clockwise,  you don’t want to get them stuck down there.

I would use a combination of jet wash and wet and dry hoover . Use the jet wash to break up the gunk, then hoover   it out after its softened abit, If you must use chems , then yes caustic will  attack organic .stuff . I would start with a diluted solution , then stiffen it up if it really is stubborn

Worst case  is the cast iron has cracked and fallen in on itself , maybe you could put a pvvc tube inside as a liner although this may not work if you cant get it to seal up top

https://www.screwfix.com/p/drain-unblocker-25ft-7-6m-/11325?tc=SA3&ds_kid=92700022855719489&gclsrc=aw.ds&ds_rl=1241687&ds_rl=1245250&ds_rl=1244066&gclid=CjwKCAiA5qTfBRAoEiwAwQy-6dcQqafPYyv7z0ck9u-RnIvoGqAmZMEoWGpJrpQXQpUDKObHrZ7nvBoCT9cQAvD_BwE

Is there an inspection chamber /manhole nearby? rod it and jet it from that  no need for chemical then

I do most stiff myself.  I’d pay someone else to sort it….(along with putting up ceilings, not doing that again!)

If there’s a dog leg coming from the gutter to the downpipe that’s very often where the blockage is so if you can get that apart it’s worth checking first to clear that then as others say put some scrunched up chicken wire back in the exit off the gutter afterwards, makes it much easier to clear this every few months. Avoid acid on metal pipes.