Should I 'bridge' my DPC to tank the exterior of my property to protect from flooding?

[boxrick]

Hello folks, 

I was flooded in Jan 2025 have recently re-built my house and I am about to get EWI. I should have taken flood protection more seriously earlier, but that ship has now sailed.

Regardless I am about to tank the external of my property, and ideally I want to take the flood protection above my DPC .

I plan to use this https://www.newtonwaterproofing.co.uk/wp-content/uploads/2021/10/Newton_HydroBond_2K-Flex_TDS_1.0.pdf below DPC and approx 30cm under ground level and then https://www.newtonwaterproofing.co.uk/products-systems/products/cementitious-tanking-slurries-renders/high-performance-cementitious-coating/ to 'bridge' my DPC and provide some level of flood protection above it. My thinking being that the cementitious coating will at least somewhat breathable.

See the below diagram for my thoughts, I would just like some input on this? My DPC is very high currently about 40cm off the ground. 

I have dug around the entire of the property and opened up the pavement at the front. 

[Roger440]

As someone who has been flooded, please stop, now.

 

Glossing over the rockwool in your cavity for now, the house as it stands can dry out post flooding.

 

Your proposal, both the bitumen and the EWI will ensure in the event of another flood, that it cannot dry. Ever. Lots of problems will stem from that that dont need explaining!

 

Yes, i know the thinking is keeping the water out means it wont be wet. This is utterly futile, it will get in, probably first through the void under the floor..

 

On my last house, if you accept the water is coming in no matter what, the question is what to do about it.

 

My house was slightly diffrent in that it had a solid concrete floor, but what i did is:

 

I created a sump in the lowest point (front room in my case) which would be in your ventilated void. In this sump i had two electric pumps which would pump water out through tubes in the wall higher than any flood would reach.

 

Additionally, as we lost power during out flood, i also added a battery powered pump with a controller that switched on if the power went out.

 

The simple theory was based on our flood experience. On the night in question, we found that 2 screwfix submersible pumps could remove water and pump it out faster than it came in . My final installed pumping capacity was 8 times that.

 

So, in the event of a flood, yes, water will come in but as it will immediately be removed, your water depth is limited to a few mm. This massively reduces the damage a flood will do.

 

Addtionally, i tiled all of the ground floor, and, as i observed during the flood, there were a few places water came in such as around waste water pipes through walls. The biggest was water coming up past the concrete base between the concrete and the walls. I sealed all this using a sika product they put in expansion joints on runways.

 

So i significantly reduced water infiltration rate as well. Stuff like the newel post on the stairs, i cut 20mm off the floor and installed a stainless spacer bar. So the wood wont soak up water.

 

During my time there, i admit, i never had to use it, but im 99% confident that it would have done as designed. My target was to be sitting on the sofa 4 hours after the last drop of water had left. 4 hours to mop of and clean up the silt etc. get a couple of dehumififiers for a few weeks and sorted.

 

It was, because of the risk of flooding that i DIDNT insulate, in particlar the old part of the house that has solid walls. Whilst there would have been significant benefit in doing so, anything i did would increase drying time. In your case, the products will simply prevent it entirely.

 

If you accept you cannot have a completely watertight house, and you cant, at least retrospectively, find a way to mitigate the inevitable.

 

Im not saying my way is the only way, and clearly yours is complicated by the void, but please dont try to waterproof your walls.

[boxrick]

Thanks for the feedback. I'm not even convinced right now that flooded rockwool will dry out, I have no weep vents on my property. Only subfloor anti flooding air bricks.
 

My entire issue is complicated by the fact that numerous decisions have been taken which shouldn't have been. Half the house is sat on ground level ring beam, it has two different foundation types. Movement joints between the two, so many things which make it difficult for flooding resiliency

Such as I already have cavity wall insulation, which is Rockwool below DPC which should have never have been the case.

 

My underfloor under my ground beam is dirt and not concrete

 

And a variety of other issues.

 

BUT if I can slow the entry of water long enough to protect the inside of the property then thats a big win and just peace of mind. If a bit of cavity wall insulation has to be removed then that's fine. I was going to remove it earlier but decided if I get flooded again then it can a problem then.

 

The main priority here is all about slowing the water ingress significantly enough.

But in respect of this I don't want to be causing issues in a none flood event doing things like messing with my DPC.

 

 

[Roger440]

29 minutes ago, boxrick said:

Thanks for the feedback. I'm not even convinced right now that flooded rockwool will dry out, I have no weep vents on my property. Only subfloor anti flooding air bricks.
 

My entire issue is complicated by the fact that numerous decisions have been taken which shouldn't have been. Half the house is sat on ground level ring beam, it has two different foundation types. Movement joints between the two, so many things which make it difficult for flooding resiliency

Such as I already have cavity wall insulation, which is Rockwool below DPC which should have never have been the case.

 

My underfloor under my ground beam is dirt and not concrete

 

And a variety of other issues.

 

BUT if I can slow the entry of water long enough to protect the inside of the property then thats a big win and just peace of mind. If a bit of cavity wall insulation has to be removed then that's fine. I was going to remove it earlier but decided if I get flooded again then it can a problem then.

 

The main priority here is all about slowing the water ingress significantly enough.

But in respect of this I don't want to be causing issues in a none flood event doing things like messing with my DPC.

 

 

 

If the underfloor area is dirt, you are literally wasting your time. In the event of a flood, the water will simply rise through the soil. Bit like one of my neighbours who had a 500 year old cottage with stone flag floor. When the floods came, the water just came up through the floor. Fortunately, she was smart and well used to it, so furniture on blocks, wait for the water to leave, light two open fires in fireplaces to dry out. All original lime plastered and rendered walls, so dryed out easily.

 

Id definitely be doing similar to what i described. And removing the rockwool.

 

Unless the flood is 30mins max, stop trying to stop it coming in. You will lose, the water will win.

 

[boxrick]

My house is absolutely airtight, when it comes to an internal quick recovery I'll be honest at this point it's basically impossible.

 

If it's such an inevitable loss, why does flood guidance say fill all holes and install flood doors?

 

Feels sensible to me to attempt to considerably slow the influx of water and then pump it out the subfloor as part of the protection?

[Roger440]

16 minutes ago, boxrick said:

My house is absolutely airtight, when it comes to an internal quick recovery I'll be honest at this point it's basically impossible.

 

If it's such an inevitable loss, why does flood guidance say fill all holes and install flood doors?

 

Feels sensible to me to attempt to considerably slow the influx of water and then pump it out the subfloor as part of the protection?

 

You didnt mention pumping it out?

 

Yes, slow it down, but you cant stop it.