What we found under the floor - Dry Rot?

[JohnArmer]

The middle room in our late 19C house has a suspended wood floor that's about an inch lower than the hall floor. This floor was chipboard and obviously a replacement; we know that the previous owners actually lowered the floor by about an inch to 'try and stop the damp'. One of the jobs we've decided to do was to lift up the chipboard and raise the joists so it's level coming out of the hall.



The joists are supported on three timber bearers that are sited directly onto the ground; they have rotted in a couple of places and there is a little white fungal growth at the point the timber is in contact with dampish earth and some growth on the ground:





The joiners were initially very concerned by the mould and there was lots of talk of dry rot. I am of the opinion that it could be, but most of the timbers are in pretty good nick and not rotten at all - only at the point they touch wet earth. After much discussion and research on here, the plan is to take the joists out, clean the mould and rubbish from the earth floor (which includes lots of scraps of old wet timber) and refit the joists at the correct height by hanging them off joist hangers that are attached to the walls, rather than supported by timber bearers that are sat directly on the floor.

The floor is dampish but not completely sodden; we found that the main airbrick that provides ventilation under the floor had been almost completely bricked up from the inside, so we are going to open that up to help dry things out.

Does this seem an appropriate approach? Is it advisable to spray a fungicide down there too?

 

[Feltwell]

I'll let others with more experience comment on the rot, but it doesn't look too bad at all to me. Joist hangers off the wall seems like a sensible bet - or fit a wall plate. It might just be the angle of the picture but those joists look a bit shallow to me - are you putting deeper ones in?

You can still support them in the centre of the room if needed, but from a masonry sleeper wall with a DPC built in rather than timber. Is the sub-floor ventilation OK with that airbrick you mention unblocked? Something must have caused the original floor to go - it may have been blocked airbricks but don't assume that even when that is unblocked that the ventilation the house was built with is adequate. If you do put a sleeper wall in then a common practice is to lay the bricks in a "honeycomb" pattern to allow airflow through it.

 

[JohnArmer]

The joiners are fitting deeper joists that won't need any central support.

I didn't think the mould was as bad as I was expecting either... I was expecting massive dinner plate like mushrooms, but there were just a few little white tendrils. There isn't much smell either.

 

[MatthewC]

the previous owners actually lowered the floor by about an inch to 'try and stop the damp'.

I'm still puzzling as to exactly what they thought that would do!

In my experience of doing my front room floor last year, I reckon that's wet rot. My floor had sleeper walls giving about a foot of space underneath, but was otherwise pretty much the same. Apart from opening up a similarly blocked air brick we also had a floor vent added on the far side of the room to help the ventilation underneath.

You've certainly got to get the bearers off the damp earth so your joist hangers sound like a good idea, but I'm a bit worried about the fact that you say the earth is slightly damp. Are you able to remove some and see if it gets wetter or drier as you go down? Personally I'd be thinking of increasing the air space there by removing a few inches of the soil and building sleeper walls for the bearers.

I wouldn't spray fungicide underneath but I would treat the re-used joists with something like Cupr*nol 5 Star.

There's more about my floor on my blog at this link.

 

[MdB]

In addition to what other people are saying, I would do the following:

1) Clear out the ground area and potentially drop its height a bit to give more room.
2) Potentially add brick pillars for support if needed with a piece of dpm between bricks and joists.
3) Add insulation between or beneath joists.
4) Add loads more ventilation into the floor void such that you can have as much airflow as possible under the floor.

The last two being the most important.

 

[AMc]

I'd back up what MdB says they all sound like good things to do, esp. the insulation.
Even if you add thicker joists you will get more movement than you had with solid bearers. My joiner was keen to replace the original thin joists under our bathroom with new thicker timber. I was keen to keep them as they are part of the history of the house so we "doubled" them adding a new joist next to them and bolting them through (IIRC). My structural engineer did the calculations on the size of supplemental joists (which were bigger than the originals) and the joiner went up to the nearest stock size we could source but the floor still flexes a little. The span is only about 4 metres and the double joists are very substantial, by the time you realise the floor is a little bouncy it will be too late to do anything about it. I would add at least a central pillar if it were my floor.

 

[Matt Green]

Weevils!

It's mostly weevil damage and an unknown brown rot (Antrodia (Fibroporia) vaillantii, or some other poroid type). Weevils like Euophryum confine, Euophryum rufum and Pentarthrum huttoni love those sort of conditions. Just damp enough to make the wood soft to eat and on right on the low end of the moisture content for fungi to decay it. The decay progresses quite slowly and the weevils chow down on your bearers. You can tell it's weevils- they leave ragged exit holes along the grain of the wood and avoid knots.

Clear the sub-floor void out, isolate any replacement timbers from ground contact and improve the amount of ventilation where you can.

 

[MdB]

John, if you haven't worked it out already, Matt is an expert in rot and stuff that munches wood.

(at least he tells us he is an expert - he uses lots of long important sounding words anyway....)

 

[Matt Green]

Indubitably

 

[MdB]

 

[JohnArmer]

Thanks very much for all the replies - I suspected that the pictures might whet Matt's appetite for rot and bugs!

I didn't mention that there are little piles of dust under all the joists - from wood boring insects - so Matt's identification of Weevils is absolutely spot on.

I think I will push the joiners to go for a central bearer to reduce bounce as per your suggestions.

 

[Penners]

I suspected that the pictures might whet Matt's appetite for rot and bugs!

Quite right - he has an appetite for whet rot.




I most humbly beg forgiveness.

 

[Matt Green]

Thanks very much for all the replies - I suspected that the pictures might whet Matt's appetite for rot and bugs!

Oh dear, have I become that predictable?

Penners, I have a McVitie* of biscuit beetles at my disposal that could be trained to messily devour specific types of patisserie within your locale. Were I to release them, the results could be Katastrophic.


*the collective noun for Stegobium paniceum

 

[Penners]

Penners, I have a McVitie* of biscuit beetles at my disposal that could be trained to messily devour specific types of patisserie within your locale. Were I to release them, the results could be Katastrophic.


*the collective noun for Stegobium paniceum

In which case, Matt, I'd better continue paying you the protection money.

I love your collective noun. By the same principle, should we refer to an "Andrex" of dung beetles?