Rotten Suspended Floors & Improving Sub-Floor Ventilation

[njwilson]

I've recently bought a narrow c.1860 mid-terrace that has significant issues with wet and dry rot to the ground floor, as well as woodworm (all hidden under laminate, with no obvious smells or 'bouncy' floors despite large holes in the floorboards below). The amount of rotten broken wood scraps left under the floor indicate that this isn't a new problem for the house, and that the existing floor (probably Edwardian - 1930s) is at least the second floor to fail.

The air-bricks (just four of them, with not much room to greatly expand their number) are clear.

All of the internal walls are solid right down to the soil, as are the sleeper/dwarf walls, so the only airflow is between the joists and underneath where the internal doors are. Much of the hallway is cut off from the natural airflow completely, with no obvious way of venting it without knocking holes through internal sub-floor walls.

I've done a quick floorplan of the ground floor. The dining room floor is, as yet, unexplored because all the living room furniture had to be piled in there - but I anticipate it'll be a similar state.



The fireplace in the living room has been bricked up - but not vented. Is it possible/probable that this is exacerbating the rot around it, or are they just naturally 'dead' spaces?

Rebuilding the sleeper walls honeycombed would be an obvious way of getting a little more air moving about, but are there any other ways of helping it along without knocking through under the internal walls? Would, for example, a grille vent set into the hallway floor near the front door do much? (There's no risk of radon in this area)

Any advice gratefully received.

 

[John D v2.0]

I'm wondering whether the problem is that the oversite is saturated and the floor timbers are not adequately isolated from the ground.
What kind of dpc separates the masonry from the timber wall plates?
The timber shouldn't spontaneously get damp if it's really clear of the ground as long as there's a little ventilation.
How about next door, is the dry rot on their side too?

 

[njwilson]

Thanks for the reply.

The area is on an estuary - so the soil is prone to being quite moist at the best of times.

The DPC is slate on the dwarf walls that support the joists at the walls, and blue engineering bricks on those in the middle of the room, but there appears to be no obvious bridging. The gap between soil and underside of the joist is ~40cm.

No similar issues on either neighbours' side, as far as I've been told.

 

[bobasd]

who says its dry rot orwet rot,have you had a survey report done?first thing to do is clear all stuff and floor coverings out of the ground floor. you are in for quiet a bit of work. i can help you and tell you how to go about things if you want?we do this stuff all the time,none of it is a big deal.

edwardian ended in 1910

 

[John D v2.0]

Slate dpc will always let some through the mortar joint, it's more of a damp reducing course. We had a lot of damp timber in our slate dpced wall plates. We either replaced or simply jacked up and inserted modern dpc under the timber. It seemed to be drying out well.
Sounds like Bob knows a lot more about this topic so I'll let him advise better!

 

[njwilson]

Thanks for the response, bobasd. And for the offer of help.

Well, the wispy white fungus covering the joists and board undersides and the small tendrils on the topsides look a lot like dry rot, and the bit that looks like wet rot is just sodden, spongy and crumbly - but, admittedly, I'm not an expert.



Also, I meant the floor was probably renewed somewhere between the Edwardian period and the 1930s, rather than the Edwardian period extending to the 1930s - they're just much narrower boards than you'd expect to find in an early-/mid-Victorian house.

 

[TicTac]

The sleeper walls could do with a few bricks being removed, and a couple more air bricks being installed. All the rotten woods needs stripping out and burning, and then you need to treat all the walls, ideally for a metre beyond any rot, which may require some plaster being removed as dry rot creeps bhind the plaster. You've mentioned the areas damp, but has the outside path been raised over the DPC. If the ground realy is that damp, then it might help to lay a 1400 membrane over the ground, and hold it down with a few bricks.

 

[bobasd]

ok well i asked you have you had a survey, maybe a mortgage survey?
post photos of both rooms after removing skirtingandlifting boards in afected areas
you want to open up all the fireplaces onboth floors so you can get a camera down the flues and then sweep them. the camera will spot damage to the flues and any fungal stuff inside the flues spreading from floor to floor.
get on the roof and examine any stacks,take photos and post.

whats the photo showing, is it a knee wal with a partition over? the joists have been sistered for repair support and the plate is fungal afected as wellas the jists.
the grill in the hall ideawont work. there are air bricks or narrow vents that can go under front door sills and side wall can be honey combed.your existing airbricks could be to small or blocked.
if remove any hallway boards to look below screw them back down for now,dont leave any traps.

 

[njwilson]

TicTac, there's no DPC - but all of the outside ground is below the height of the suspended floor. The skirting seems unaffected, so hopefully there'll be no need to hack away any plaster - it's all the original lovely lime plaster.

 

[TicTac]

If the rot hasn't come from the raised ground, then it has to have come from inside (unless there's been a massive water leak at some stage) and there's slate on top of the sleeper walls, and that hasn't stopped it, so excess moisture may well be the cause. Make sure you use tanalised joist.

 

[njwilson]

ok well i asked you have you had a survey, maybe a mortgage survey?

The mortgage survey was rudimentary - plus the entire floor was covered in laminate, and there were none of the usual signs of damp or rot from above it. So there was no indication of an issue before we bought it and ripped up the laminate in the living room and hallway earlier in the week.

The photograph is the dwarf wall that's built just in front of a solid interior wall (there's a ~15cm gap between the dwarf and the interior wall)

This is a photo of the dwarf wall in the middle of the room:

 

[bobasd]

the photo above shows lots ofbeatle frass caught in cobwebs. beatles love damp and lack of ventilation places. i think your problem is lack of strong through ventilation.

you want to get under those floors and crawl all the way round esp in the chimney breast alcoves an hallway.this way youlfind the full extent.

 

[njwilson]

All of the floors are coming up anyway - those that aren't dangerously fragile have been chopped up and relaid several times over. It's a horrible squeaky mess of a jigsaw.

They're being swapped out for T&G maritime pine when the root of the damp is solved.

Are there any ways of encouraging ventilation without getting too extreme with alterations? Pretty much every other part of house needs some urgent work, so the budget for any given project is slight. I'm not sure there's much space to add many more air bricks (whilst the living room could take another, the dining room only has ~2m of external wall and the two that are in it have already been enlarged, and venting under the front doorstep would be a big job - it's a solid limestone step up to the door).

 

[John D v2.0]

Based on our place, if I were you I'd take up the floor and clean everything I possibly could from under there, especially any organic matter ie wood. You can get a pin moisture meter cheaply which work well on timber but not on anything else. Check all timber for moisture levels.
Strip out anything over 18% or near to timber visibly affected by rot. Perhaps the whole lot. Remove and rebed any loose bricks in the sleeper wall or rebuild as necessary.
Lay modern plastic dpc on a mortar bed providing a clear separation between the masonry and the new timber. Lay new treated timber on the dpc and tap it level. Lay new treated joists and noggins on the wall plates, packed level if necessary and screw together.
Make sure all timber has a clear air path all round and is not hard against any masonry other than restin on the dpc.
Add air brick every metre in the front and rear external wall. Ensuring no air can go on top of any insulation under the floor.

Note that this is purely what I've done here, I'm in no way trained for this.

 

[TicTac]

You could blag it well from that description. The only things missing from that list, is the wood rot treatment to all surrounding areas, and extra ventilation holes in the sleeper wall.