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Renovation: suspended floor, slab and block or concrete slab


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I bet this has been asked some times, but I was not able to find a good response. Sorry for the repetition.

We bought this terrace house in London, with the idea of renovate all of it. It is a very extended property with a double rear extension and side extension.


The floors are as follows (see diagram below):

  • Front reception/living: suspended wood floor. Several issues:
    • Serious damp issues with very rotten wood (30-50% needs replacing)
    • Debris and rumble from botched chimney removal, heart foundations left behind,
    • no ventilation in the rear slab.
    • front ground levels elevated
  • Rear/Extensions: All concrete done at different time, but poor quality.
    • Old suspended floor is now a concrete slab
    • Kitchen has the original 1900 concrete (I see quarry tiles). At different level than the rest
    • Rear extension. Level to the old kitchen.
    • Side extension:

All the concrete work is  of dubious quality, I see DPC plastic raising from the concrete, etc.

We were advised to remove all that concrete, and we will do. Too many issues: In our project we need to add 2 big pad foundations  (1x1x1m), dig some draining, add damp solutions to the concrete, retrofit telescope vents if we keep the concrete, the ceilings are low and no easy to add insulation, ventilation, grounds ... I guess it is the right decision.


We are looking to add UFH underfloor heating to use with Air Heat Pump, if we can.

Now the question is what to do with the removed concrete and the existing suspended floor. We get different advices from builders, architect...

3 Options:


  • Fix existing rooten wood floor in the front,  and reinstante it in the rear also wood
  • Use pre-cast concrete slab and block
  • Lay and ground bearing slab (insulated)

My questions are:

  • What option is the best? and how it works with cost
  • Do we need to inject chemical Damp proof in each case? One builder recommended slab+injection.



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The floor rotted because of the elevated moisture levels. 


These were caused by the removal of ventilation and the elevated ground levels. 


You will need to. 


1. Lower the existing ground level and reduce the localised water table through drainage ( French drains work well if done right). Damp proofing ( especially the injected type) is peeing into the wind against a high water table. 


2. Remove the existing rotten/uninsulated floors. Get a mini digger + dumper in and mechanise as much of the process as possible. Dig out everywhere to 500mm below floor level. 


3. Reinstate the floors with appropriate insulation. Given the problems and distance needed to ventilate the existing timber floor I would replace all with a ground bearing slab if possible assuming suitable soil conditions.

The next part will take some planning. First fix all your ground floor services. Put ducting here for all your new electrics. Run your bathroom+kitchen extract ventilation ducts, drains and water pipes in ducts.


All Preferably elevated say 100mm by chairs.


Then pump is something like TLA insulating screed to a depth of 350mm enclosing the ducting in insulation. Then an appropriate membrane. Finally pump a 100mm concrete slab with UFH pipes clipped to mesh and a good upstand to all the walls.


There's a fair amount of disruption +muck away (maybe)  here initially but man hours and drudgery would be really reduced.  An electrician mate first fixed a 1000m2 warehouse with one colleague in a day with this method once.  You would be left with a perfect base to continue the rest of your build from. The thick slab (u value about 0.15)  would be ideal for UFH and ASHP. It could be used to build lightweight stud walls from it if appropriately specced. 







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How old is the house, does it have an existing DPC?


We had similar problems with our 1850's house without a physical DPC. All advice we received was to excavate and lay a slab. Micro digger and dumper in to excavate then 150mm MOT 1, 25mm sand, 100mm Conc, 100mm PIR and 60mm screed. We were also rewiring at the time, so dropped all our services from above to avoid ducting within the floor itself.


We did most the work ourselves and it was messy, uninhabitable during that time. It would have probably have been cheaper to replace the joists when DIYing, certainly not so when paying a contractor. 


Since we did ours, others have raised concern regarding a new slab with a DPM potentially pushing moisture to the perimeter and therefore up the walls (without a DPC). Those concerns haven't come to any realisation (yet...)

(FWIW, if I was to do it again I'd probably go 150mm MOT 1, 250mm EPS (50mm as a sand blinding), finished with a 100mm conc reinforced slab instead. Slightly deeper but better thermal properties and arguably cheaper than PIR + conc + screed.

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Thank you @Icevergeand @jayc89 .  Just for the record, I am not doing the work myself, we will have a contractor. We are trying to decide what to do before tendering.

Yes, you are right about the cause of the rot. I also see damp raising in the concrete and I am very confident that is due the concrete bridging the DPC, but it is true that the concrete there is really poorly setup.
Anyway, I am still very worried about the risk of bridging the DPC, for me and my neighbors. Also, apparently and in theory, any intervention that affect the party wall requires a PW agreement, that includes adding a slab. My neighbors if advised correctly, should object.

so after thinking a lot, I think I can do the following mix approach (BTW, I put a copy of this in reddit too):

  • in the front kept and fix all the suspended timber

  • in the back, remove the existing concrete, and rebuild a new solid ground bearing concrete slab, with proper insulation, DPM, etc.

Reasons to not put concrete in the front, but timber

  • No need to worry about soil quality

  • both my neighbors have timber in the front area.

  • installing a concrete in the current suspended timber area, will bridge the DPC. This might eventually bring damp up the party walls. I can add a chemical in my side DPC, but my neighbors might be affected.

  • in theory to add the concrete or a DPC injection I need a part wall agreement at both sides. And IMO they should be advised to object. I would object.

  • in theory, if well ventilated, timber can last. We identified the source of all the damp issues, and will remediate them.

  • we'll fully insulate it, will not be that bad.

  • it will be cheaper to keep/fix the timber here

And to not put timber in the back, but rebuild the concrete:.

  • there's already concrete, soil should be fine.

  • more efficient for underfloor heating

  • my party wall neighbour have concrete, they are bringing the DPC already, I might get damp anyway. I'll add a chemical DPC and/or a damp membrane.

  • If I am not wrong build suspended timber requires remove A LOT of material, I think we need to dig 500mm or more? We even risk get below the foundations (unknown depth).
    Note: Although we might need to dig also a lot per @Icevergecomment?

  • I won't need to lower the ground levels in the garden in the rear. Although I will do it anyway to allow for the vents.

  • when rebuilding, we can try to fix/reduce a lot of damp issues, avoid cold bridgig, add proper ventilation for the front.

  • Maybe it will be cheaper or the same? (no need to remove that much material, no need to rebuild resting walls for joists, etc)

For the underfloor heating, we will only install it in the rear concrete floor, and put big radiators in the front with the suspended timber. This way we benefit of ufh in the biggest area with concrete. We won't increase the height of the front area, making it easier to keep it level with the rear. We can keep the current solid 1.5cm oak flooring that is now. If there's issues with the timber, it can be lifted easier.

Does it make sense?

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