Joists Rotten at Both Ends - Sister or Replace?

[FergusMacdonald]

We recently noticed that some of our floor joists are very rotten at one end - the outside wall. The ground level outside is approximately level with the floor level inside and earth sits up against the sandstone wall causing moisture to filter through the wall and into the joists. Also, the void under the floor has had rubble and stones dumped into it, sometimes encasing the joists. Wherever the rubble has touched the joists they're rotten. Also, wherever the joists touch the walls they're rotten - there doesn't seem to be a damp proof layer/membrane - they seem to be sitting on wooden supports within the walls. There's also a central supporting joist which is also heavily rotten at one end which will be getting repaired. This has all been like this for 30-50+ years, maybe more. The house was built approx 1890.

We've now removed the floor and noticed that the joists at the other side are also rotten only where they're recessed into the internal wall. Until then my plan had been to sister the severely damaged end and lay a new concrete footing just inside the external wall to support them. The external sandstone wall is noticeably wet and could take months/years to dry hence the new foundation. Now that we've noticed the other end is also rotten, wondering whether to sister both ends or simply replace the joists? We could also potentially put a new footing for that end too. Advice much appreciated.

The room is approx 5m wide and the joists are 160mm x 65mm. I assume they're original.
I'm also intending to:
1. Dig out all of the rubble
2. Dig out a border around the external wall and infill with stones
3. Fit a breathable membrane, wood fibre insulation and vapour control membrane as per this article.

Left hand side is external wall.

Some joists almost entirely rotten so will need at least 1m replaced.

This is the internal wall side and it's really only the amount recessed into the wall that's rotten.

 

[Wayners]

Not my area of knowledge however.
I've seen some switch to concrete floor.
Insulation. Dpm. Pour concrete.

 

[Deluks]

It doesn't look like there's a huge amount of space between the subfloor and joists. Have you considered converting this into a solid floor?
You'd put a new damp proof membrane in, and up the wall. Insulation, screed, even some underfloor heating pipes if you like.

If you proceed with the current set up, you'd be better off replacing the joists, and adding airbricks in.

Check to to see if the floors in the other downstairs rooms have adequate ventilation too.

 

[FergusMacdonald]

It doesn't look like there's a huge amount of space between the subfloor and joists. Have you considered converting this into a solid floor?
You'd put a new damp proof membrane in, and up the wall. Insulation, screed, even some underfloor heating pipes if you like.

If you proceed with the current set up, you'd be better off replacing the joists, and adding airbricks in.

Check to to see if the floors in the other downstairs rooms have adequate ventilation too.

I'm digging it out at the moment and we'll have a decent enough space once all the rubble is removed. The ground slopes but we'll have approx 150mm at the highest point sloping to 500mm at the deepest. Will ensure we have good ventilation too.

 

[FergusMacdonald]

Not my area of knowledge however.
I've seen some switch to concrete floor.
Insulation. Dpm. Pour concrete.

I'm not super keen to do this as we've purchased new wooden floor for the whole of the downstairs and will have suspended timber floors throughout. The other rooms shouldn't be as bad.

 

[Deluks]

You could bolt a ledger board to the wall, for the joists to sit on (strip of dpc behind) or face fix to it using joist hangers.

 

[FergusMacdonald]

Just realised since I need to replace 1m at one side and 150mm at the other, I could just cut off the 150mm that's rotten and sister 1.15m at the other end . I don't need to sister both ends

 

[ree]

OP,
A pic showing the outside wall at ground level would help?
Stick with your suspended floor - installing a solid floor might give you further troubles in other rooms.
Presumably you have air bricks on the outside wall - do you have air bricks on any other elevation?

 

[FergusMacdonald]

OP,
A pic showing the outside wall at ground level would help?
Stick with your suspended floor - installing a solid floor might give you further troubles in other rooms.
Presumably you have air bricks on the outside wall - do you have air bricks on any other elevation?

The ground level outside is approx level with the top of the vent. Inside that's approx equal with the floorboards.
There seems to be 1 vent per room and the rooms are not open to each other under the floor other than random bricks here and there for pipe work. Definitely need to sort that.

Inside bay window with vent. Floorboards sit just above the top.

Outside you can see the vent to the left of the rock in the centre of the bay. Earth level on the right is about same level.

Internal wall with no gaps other than random bricks.

 

[ree]

OP,
Yes, cut off one rotten joist end & bolt on a length at the other end - treat all cut ends with, say, Sovereign Deepkill penetrating mayonnaise or similar - cap all ends that go into wall pockets with DPC material.
It will probably be best to skip - debris the floor plates but until I see a pic of what the state of play is now I cant suggest an alternative solution for giving bearing & ventilation?

You need to flash pic up behind the bay panelling to check the conditions?
While the room is disturbed why not raise and sink all outlets a min of 18" off the FF?
Be extra cautious around the door threshold - dont touch it.

 

[FergusMacdonald]

Thanks for the reply.

I've had a more detailed look now and it seems that all the joists are resting on a top of a wooden floor plate which is embedded in the wall. On the exterior and interior walls it's within the wall and the joist rests on top of it. I can't see any DPC of any sort on the floor plate or the joists.

There's also a dwarf wall which runs across the bay window and the floor plate rests on top of it, with varying depths of notches cut out to bring the joists level.

Be extra cautious around the door threshold - dont touch it.

What's your thought behind that? What are the risks? I'm going to need to replace the floor on the other side also so it will be coming up at some point too.

This is the internal wall and the nail puller is touching the embedded (rotten) wood. There's very crumbly mortar around it. This is a pocket that a rotten joist was resting in.

This shows the small dwarf wall running along the bay window opening with the rotten floor plate removed on one side and cement underneath.

This is the external wall showing the joist pocket and the gap left by removing the rotten floor plate it was sitting on. You can just see the imprint of the timber on the bottom of the pocket so presumably the wood was embedded on wet mortar.

 

[opps]

I don't know the answer... I just wanted to say that your Bakuma Japanese pry bar is my favourite hand tool.

https://www.amazon.co.uk/Bakuma-Japanese-Premium-Puller-Hardened/dp/B00DXUXYN4

resting in.

View attachment 352860

 

[martygturner]

I have used these a fair few times, simple enough and saves a lot of faffing around. https://joist-repair.co.uk/joist_repair_shop.htm#Joists

Sorry just read the other bit about damp pocket, just creosote the pocketed bit and wrap the end with membrane.

 

[FergusMacdonald]

I don't know the answer... I just wanted to say that your Bakuma Japanese pry bar is my favourite hand tool.

https://www.amazon.co.uk/Bakuma-Japanese-Premium-Puller-Hardened/dp/B00DXUXYN4

Ha ha it's a great tool right! I didn't know the brand, I got it from my dad and love it.

 

[FergusMacdonald]

I think what I'd still love help with is

1. What do I need to do about the rotten wall plates
2. Should I build a sleeper wall inside the exterior/interior walls or just wrap joist ends in damp proof membrane