Damp proofing mess up by builder

Rambosmum

We have had an extension built. As part of this work - instructed at the same time, agreed at the same time and on the same quote we got a patio built to sort out the extension-meets-garden issue. 

When the brick work was started we raised an issue with the bricklayers that the engineering brick (beneath the damp proof course) wasn't coming up high enough and that the patio would bridge the damp proof course. The brick layers advised they had been told by the building company to X layers of engineering brick so have done. We spoke with the building company who we had the contract with and he dismissed our concerns. We took and sent photos however he kept brushing us off and didn't do a site visit. The brickwork was finished and the patio guy started. Low and behold, the patio bridged the damp proof and we started to get damp on the bricks. A French drain was put down one side of the extension, which solved the problem but along the back stretch this would have meant digging up large parts of the patio which they refused to do. We've chased and chased the issue and they've kept knocking us back, claiming its "surface water" or if it is damp it doesn't matter because of the large cavity etc. We've finally driven them to do something and they've sent round some people to take a look:

1. Company 1 want to do an injectable damp proof course - I don't want this, think they aren't worth a penny.
2. Company 2 want to put in a physical membrane by removing 3 bricks at a time and putting it in. Is this a good solution? What are the concerns here?
3. Company 3 have suggested continuing the French drain. I have issue with this due to aesthetics and the fact the first bit of the french drain is hit and miss on effectiveness!

Which would you go and why? Is there an alternative solution? 

Thanks.

Doozergirl

Do you have photos, please? 

Rambosmum

This is the best I have, you can just about make out the french drainon the lefthand side. 

stuart45

Why was the DPC put in at such a low level? It's normally determined by the floor level.

Rambosmum

stuart45 said:

Why was the DPC put in at such a low level? It's normally determined by the floor level.

There's a deep void/ crawl space below the extension floor to raise it to the level of the rest of the house. The damp proof course is just over halfway.  Before the patio was put in it well above ground level. the patio raised the ground level because it was on a slope - all this was known and obvious prior to the work commencing. There were 5 steps from the old back door to the old patio, and we wanted it reducing to 3, and then 2 from patio to garden - again, confirmed and agreed as possible with the builder. 

stuart45

I guessed you had a suspended block and beam or timber floor because of the air bricks, but on sloping sites a stepped DPC would normally be used on the external skin. If the internal DPC is correctly installed it's not so much of a problem. You might find the dampness is restricted to the lower couple of courses which would be damp anyway with a DPC 150mm above the ground.
I wouldn't bother getting a chemical DPC, or knocking out a load of brickwork. Brick chainsaws are sometimes used to cut out a joint to slide in a DPC.

Rambosmum

stuart45 said:

I guessed you had a suspended block and beam or timber floor because of the air bricks, but on sloping sites a stepped DPC would normally be used on the external skin. If the internal DPC is correctly installed it's not so much of a problem. You might find the dampness is restricted to the lower couple of courses which would be damp anyway with a DPC 150mm above the ground.
I wouldn't bother getting a chemical DPC, or knocking out a load of brickwork. Brick chainsaws are sometimes used to cut out a joint to slide in a DPC.

The builder is paying if that makes a difference to your answer?

stuart45

If he is paying then I still wouldn't use a chemical DPC. I would even think twice about having a physical DPC installed. Those bricks look like pre war commons, which are a strong enough brick. My guess is that the damp will be restricted to the lower courses, but if it worries you get the physical DPC installed.

FreeBear

And if you go for a physical DPC, you can get away with removing quite a few more bricks than three at a time. Six would be my suggested limit.

And a point of order - The drainage you have installed is a "linear drain" or Atco. A french drain is a gravel filled trench next to the wall.

Aaaand.... I hope you are going to fit a handrail either side of the step by the patio door. That is quite a step that will cause injury if someone isn't watching where they go.