Sub-Floor wall plate and joists rotted due to damp

[Mat_J]

Hi,

I have been pulling up floor boards in a property I have just purchased in the front room, as there has been evidence of damp.
After pulling up the floor boards, I have found that the wall plate was rotten due to damp, as dirt and debris have built up between the wall and the wall plate. and some of the ends of the joists have some rot as well. There is slate used as DPC under the wall plate and in the wall (but at a higher level than the wall plate)
My concern is if I just put in a new wall plate with new DPC under it, that as dirt builds up again the same problem will occur, damp will rise up through the dirt onto the wall plate and joists.
Would it be best to somehow wrap the wall plate and joist ends with new DPC to prevent this occurring again?
I have attached pictures, and a sketch to try and detail the layout of the where the wall plate and existing DPC is located, and where the dirt/debris had built up.

Any help would be greatly appreciated.

 

[Doggit]

I can't see any evidence of the rot, so I'll take your word for it. If dirt has built up, then having cleared it, it shouldn't build up again, and it was most likely dumped or left there from previous work. When you replace the joists ends and the wall plate, use tanalised (treated) timber, and put a plastic dpc membrane under it, and around the ends of the joists. The slate under the wall plate is lower, simply because the wall plate itself is lower, but the joist ends are vitually the same level as the wall dpc, and that's odd as it'd normally be a couple of inches lower than the joist ends, so you need to make sure that there is either a gap between the joist ends and the wall, or you wrap the joist end, er even paint them with butumen to make sure no moisture can jump across in the future.

How much of the joist ends is rotten. Could you paint them with wood resin to reinforce them, or do they need replacing, and if they need replacing, do you know how to do it.

 

[Mat_J]

Hi Doggit,

Thanks for the info. The wall plate I had removed as it was rotten the entire length underneath, so it's not in the picture. A couple of the joists have about 1 inch of rot at the very end, I think the rest of the joist is ok. Being at the end, would it be enough to cut off the inch of rot, and as you say paint with wood resin, as there should still be enough for the floor board to sit on?

thanks

 

[Doggit]

Unless the ends are really rotten, then you may well be able to spray them with a wet rot treatment, and then lpaint the wood resin on to harden them. Use a small screwdriver to determine how bad they are. If you cut the damaged ends off, then you'd only need to treat them, not resin them. not sure how much access you've got, but as the rot is normally on the bottom, can you cut up at an angle.

 

[Mat_J]

Hi Doggit,

Thanks for the help, I will see how bad the ends of the rest of the joists are, and take a view if I can get away with treating them or replacing them. I have been looking over the rest of the sub floor and about half of the wall plates need replacing.

I am intending to use 450mm DPC under the wall plate, and up around the end of the joists. My thinking is that this way there would be less chance of any further bridging of the slate DPC, and no possible micro climate when wrapping the ends. I would be trimming around air bricks, and the skirting - see picture attached - no wall plate yet still need to add more air bricks before completing it.

 

[Mat_J]

I had left the DPC temporarily in position see photo in last post. I have just taken I tout, and found condensation on the side facing the wall on the left corner. I was surprised as is was quite substantial. I am a bit concerned that applying the DPC in this way might actually cause problems with condensation build up, that might then track up the inside wall. should I be concerned able this?

 

[JohnD]

condensation

how is this sub-floor void ventilated?

 

[Doggit]

If it's wet where it sits against the wall, then that suggests the wall might be damp.

 

[Mat_J]

There is one air brick. I am looking at adding two more either side of what is there already. Also the ground level outside is too high bridging the DPC by about 6cm, someone had raised the level by adding concrete paving's directly out side the front wall. I am going to bring down the ground level, hopefully to the recommended 150mm below the existing external walls DPC (or close to it). Even with these changes needed, I was surprised with how much condensation had formed, it was dripping off the DPC when I took it out. But it was only on one side (the left hand side on the picture posted.)

 

[JohnD]

you need enough airbricks for the air to blow in one and out the other. But as you describe the ground is making the wall too wet, it may improve when you lower it and slope the paving away from the wall.

Are you sure the drains, gutters and downpipes are not making the ground wet?

 

[Mat_J]

there was a blocked drain on the right hand side of the wall, this has been cleared. But nothing (no drains, or downpipes etc.) on the left where the condensation was.

 

[Doggit]

Are the joist ends above or below the DPC, and was the blocked drain outside above the DPC. If it was, then the wall may just need time to dry out, and dehumidifier would help that.

 

[Mat_J]

The joist ends are above the DPC, the blocked drain I think had filled up to be over the DPC, but the drain was cleared about 4- 5 weeks ago and is on the other side of the wall (right hand side), form were the condensation had formed on the sheet of DPC I had temporarily put down (on the left hand side).

 

[Doggit]

It could take several months (longer over winter) for the wall to dry out, and you'll get condensation on the back of the DPC till it does. I'd keep the dpc away from the wall for now to give it a better chance to dry out though.

 

[Mat_J]

Sounds like a plan, I will give it some time to dry out.

Thanks guys.