I have spent the last few days working on this problem, and have made a bit of progress.
We dug away behind the corner where the water came through and the way the dpm has been folded up around the corner looks a bit suspect, but essentially now water has got inside it it’s kinda useless anyway. What was surprising was the amount of water in the french drain so the next step was to get a pump in there to get rid of it.
I have spent quite a while on the phone with my Dad as much of his career was spent in water treatment so he knows his stuff. He calculated what rainfall we should expect on the roof of the studio and what capacity for water the french drain has. We concluded a small pump running more often but with a lower flow rate would be needed in order that we don’t overwhelm the surface water drain that we’ll be pumping into.
For several reasons I ended up buying a surface mount pump that draws water up via a 1” hose from a strainer mounted at the bottom of a sump we made in the french drain - from a bunch of plastic manhole rings that I drilled loads of holes in. The idea is to get the pump to kick in when the water in the french drain is near the top of the concrete slab so this sump goes about 800mm down from ground level. Even so the difference between when the pump needs to switch on and where it needs to switch off (before it sucks in air) is only 80mm different so I’ve had to search out a tiny float switch.
I have been working on a control system with a float switch that I’ll hopefully be installing tomorrow. Thankfully Amazon deliver all these sorts of bits too. My Dad has also suggested I install some kind of high level alarm in the bit of the french drain behind the place the water broke through so I know if there’s a potential problem again under storm conditions - even though the tanking will stop water coming in it’s still best for it to not be at that height.
Alongside this I’ve been trying to keep the water out of the studio. I’ve had to use a wet/dry vacuum to suck it up and then empty that into a bucket with a borrowed submersible pump in (as the submersible will not pick up less than 2” deep water). It’s laborious but it works!
The tanking guy came and we agreed he’d quote for tanking the dwarf walls up to the stud wall, and then about 18” in on the concrete slab. My stud walls for the inner structure will sit on the tanking therefore.
Unfortunately the slab has to be dry to do the tanking so my priority now is keeping that water out so I can get the tanking done ASAP. British weather is not on my side so this little pump better work!
How much water was in french drain
Water pumped from studio into manhole
Contraption to make borrowed submersible pump reach manhole
Plastic manhole rings with holes in for sump
Sump being put in french drain
I also lined my intake external silencer with the new, correct duct liner today so that can be out the house and installed tomorrow. I borrowed the idea of using those insulation mounting discs to hold the duct liner in place, alongside spray glue.
Finally I finished off some annoying little jobs like adding tyvek to the back soffit area and wrapping and taping the roof vapour barrier down at the front. I dropped my second hand ryobi cordless staple gun in the mud which really helped my mood! By the way that thing is utterly rubbish - not sure if it’s this particular second hand one or they’re all like this but I can only get it to fire every third time or something. I’ll be putting it back on eBay I think.
After this all I can do is insulate the stud bays of the external wall then I’ve got to wait for the tanking to be done before I can make any more progress. Annoying!
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You can get a cheap 1gal compressor that will run a brad nailer and stapler for like <$50. It'll even run a framing nailer, at 5 nails before refill, lol (I have a much bigger compressor in the garage with a 100' hose I've been using in the basement).