Replace sagging suspended floor - no ventilation

[twillmo]

I've got a 1870's house with a 1990's extension. Helpfully when they did the extension, they put down solid floors, and blocked up the air bricks at the back of the house. I have two air bricks in the living room, and one in the garage which isn't heated, so I think the living room floor is okay (I've pulled some of it up previously to repair and it was dry down there).

Our problem is the hall floor is clearly rotting, and sagging and bouncing in the back left corner (where I assume another air brick has been blocked up). I don't think adding ventilation is an option, so I'm wondering what the options are to replace this subfloor?

I guess easiest would be to brick up the gap between hallway and living room, and then convert the floor to a solid floor with aggregate, sand, insulation, screed etc. However keeping the stairs propped might be a bit of a nightmare, and ideally we want to do this in a day or two as it will make living in the house a nightmare.

Has anyone got any experience or suggestions?

Other ideas I've had that might be stupid:
- Put some kind of DPM down, and hang joists across the small span of the hallway, put a floor on that
- Some kind of block and beam solution to avoid future rot, but I know that also needs ventilation

Diagram attached showing the house. Green is extension. Red is original house with suspended floors. AB means air brick. Blue is old kitchen with solid floor.

 

[blup]

Don't assume its lack of ventilation, you may only need to replace the rotted boards and joists and keep an eye on them. That might be worth doing that before ripping out the living room floor (or kitchen floor to ventilate side to side). Check the entire kitchen floor is solid, sometimes its solid around the units and suspended elsewhere. You could add in ventilation with telescopic vents either through the kitchen side wall or the extension but its a lot of disruption.

 

[ree]

OP,
Halls are well known for lack of ventilation or even sub-area access. Back in the day they often installed venting grills or air bricks under the front door sill. Worth a try?
Under floor through ventilation is vital to prevent dry rot.
You could try using Decor Grates (Amazon) or Air vent grill covers to help provide thro ventilation. But you would need to open up the floor pockets at the back of the lounge maybe, and perhaps under the stairs.
Add a couple more air bricks from the garage.
All underfloor solid knee walls need honeycombing.
Underfloor access & honeycombing to the hall area might be needed?

If you can show pics of the present hall bare board floor, & skirting, it would help?
IMO: More rot damage is caused by lack of through ventilation than damp sub-soil - the air can become stagnant & heavy with humidity.
You should carefully inspect all floor joists esp their tails in wall pockets. Get under, as best you can, the sagging hall timbers.

 

[twillmo]

Unfortunately there are no air bricks by our front door to the hall way, so there is absolutely no air flow through there, so I'm not surprised it's gone rotten there.
Ideally we don't want to take up the living room floor as that has UFH etc so would be a real nightmare, but I guess I could add more underfloor access and air bricks from the hall/garage sides without touching the living room.

All of the knee walls do have gaps for air too.

I'm just a little reluctant to do so much work and put a timber floor back in for it to all rot again in 20 years or so.

Photo of the dipping floor attached.

 

[ree]

OP,
There's no need "to take up the living room floor" - why do you say that?
But you do need to go under the floor into the crawl space and examine, & pic if possible, below the hall floor sagging.
The grills I mentioned are merely covers like vent covers on blocked off fire places.