Replacing 1910 subfloor after damp issues

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Hi All

I’ve bought a c1910 detached house knowing it had damp issues in the subfloor, the damps been there for a while! . I’ve fixed the issues causing the damp, cleared the vents and in the procsss of drying out the walls and concrete base. I will need to address the sleeper walls and the damaged timber soon and this is where I need help.

The existing floor has four sleeper walls as shown in the images and all sleeper plates and joists are 4x2’s.

The room is roughly 6.5m long and 3.5m wide.

What I already know is:
>the mortar in the sleeper wall in the previously damp area is weakened and I think I’ll need to rebuild the wall
>the dpc is slate tiles… will need to replace that

What I need help on is:
>do I need to have any dpc on the concrete sun floor
> how far from the wall should the sleeper walls on the sides be?
> what’s the best way to determine when a sleeper wall needs to be rebuilt
> what size joists should I use to replace the 4x2’s (plenty of space for ventilation with larger joists)
> in one place in particular there is a tree root penetrating the wall, how would you tackle this (recognising that just chopping it off might not fix it).
> what if anything would you do to seal the walls preventing damp again

Plus any other tips are welcome!
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You won't prevent the damp by sealing the walls. You need to improve the ventilation to enable any moisture to escape. The existing ventilation obviously worked before it was blocked which is why the floor has lasted 100 years.

Can't help with some of the questions, but you only need a DPC where the timber is in contact with the walls, not under the sleeper walls.

Sleeper walls need rebuilding if they're not stable.
 
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What I already know is:
>the mortar in the sleeper wall in the previously damp area is weakened and I think I’ll need to rebuild the wall
>the dpc is slate tiles… will need to replace that

What I need help on is:
>do I need to have any dpc on the concrete sun floor
> how far from the wall should the sleeper walls on the sides be?
> what’s the best way to determine when a sleeper wall needs to be rebuilt
> what size joists should I use to replace the 4x2’s (plenty of space for ventilation with larger joists)
> in one place in particular there is a tree root penetrating the wall, how would you tackle this (recognising that just chopping it off might not fix it).
> what if anything would you do to seal the walls preventing damp again
The sleeper walls touching the outside walls isn't an issue. Any new timbers laid directly onto the sleeper walls should be laid onto a DPM and not touch the outside walls, but that's all, really. If your existing bearers are good that it's obviously been dry beneath that floor

How good are your sleeper walls? If they are firm and not wobbly then there's no need to do anything. If the mortar is crumbly and they move then they need attention

Was the existing floor on 4 x 2s solid and not bouncy? If so you don't need anything bigger than 4 x 2s in the replacement - that's just a waste of money. Use C16 structural graded, though

Tree roots are a pain. If you want to deal with them they will need to be cut back outside the house walls and a root barrier installed. Most tree roots grow in the top 12 to 18 inches of earth. be cautious, though, some trees can die if you cut off portions of their root systems. A couple of times in the past I've seen trees dealt with by dosing the errant roots with Paraquat (now a controlled substance) then cutting down the tree and grinding the stump. Not an easy option

Don't seal walls to stop damp - make sure that your ventilation is sufficient (as the previous poster stated)
 
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Thanks @JobAndKnock I’ll keep all of that in mind. Where the ventilation was blocked the damps crept up the sleepers and weakened the mortar so it’s wobbly, I’ll replace those bits and go from there
 
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Have you considered replacing with solid floor.?
 
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What was the problem with humidity? What have you done to solve it if I may ask please?
 
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Not necessarily a problem with humidity. The air bricks were blocked, two drains on the outside of rooms walls were blocked causing water to sit and then soak into the walls. Simply repaired the drains and the air bricks and aired it out to dry.
 

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