failed floor

[MrT]

I have a floor problems with my Victorian mid terrace house. I have just removed the floorboards with intent to insulate but have found serious rotting of the joists and the timber on top of dwarf walls. The joists sit on top of three dwarf wall and do not interface the walls, falling short by a few mm. The dwarf walls, single brick in height, provide the support for the floor. The entire void was filled with rubble and soil up to and covering in some parts the joists. I've cleared out the rubble and excavated down to the dwarf walls which are now visible. The timber on top 2 of the dwarf walls has disintegrated into a woody mush. Ventilation, a single air brick through the front cavity wall was seriously inadequate. There is no air flow as the rear room(2up2down type) is solid floor type, which looking at a cross section seems to be a failed concrete slab and screed of about 60mm laid on top of clay tiles covering compacted clay earthen floor. I will remove the concrete slab (I could probably use a child's beach bucket and spade, given the crumbling nature of the concrete). The floor is at street level so all the timbers and void are below.

Should I reinstate the timbers and improve the airflow. I could use a duct to
the rear of the property after removing the concrete slab. The ducting would
be around 4 meters in length and could terminate at an air brick. Or should I remove the rest of the rear floor and go for a very un-green concrete slab. Limecrete would take to long to dry as we live in the property.

What to do with the old joists. I've taken the really rotten bits down to the recycling center as they smell and were full of beetle infection.

Any ideas ideas would be very welcome.
Appologies for cross-posting on the greenbuildingforum.

 

[MdB]

Mr T, I thought you used duct tape to fix everything? (sorry, just had to say that)


It sounds like you know quite a lot about this already and are doing the right thing but my thoughts are as follows: A new concrete floor with DPC could possibly give you problems with damp being forced up the walls. Could you not replace the current concrete floor with a suspended wooden floor to run through the entire building with lots of air bricks either end?

 

[Penners]

I agree with MdB. It'll be better for the building (and probably cheaper for you) to replace the rotted joists.

This time, I should put a strip of DPM along the tops of the dwarf walls, to prevent any damp rising into the new joists, and use pressure-treated timber for the joists.

You also need to get the ventilation problem sorted out. The decision about removing the concrete slab (and what you replace it with) is yours, but whether you reinstate blocked vent(s) adjacent to that slab, or create new ones elsewhere, just be sure to have a good airflow under the floor.

Keep in mind also that the dwarf walls will in effect create sealed areas on each side of themselves, so you'll have to make sure that there's an airflow through each of these areas. That may also mean that you need to install additional airbricks or vents in the outside walls or in the dwarf walls.

 

[chuckey]

As said the dwarf walls will restrict/block the airflow, before putting your joists back remove every 4th brick on the top of the wall. It would be a good idea to remove a brick at ground level as well, but a tedious job to do. It is quite likely that there is a "spine" wall running right down throught the house between back and front, so this will need underfloor vents as well. I would go for a floating floor in the backroom, with air vents offset from the one in the front.
If the walls go front to back, if you put a vent in the back wall, there might be a tendency for the draft to blow straight down the centre of the house. Providing sufficient air holes have been put in the walls, there is a good argument to put an obstruction between the walls that are directly in line with the vents, but in the centre of each room. This is to encourage the air to flow right around the outer walls. A suitable obstruction could be a plastic sack with some glass fibre in it (or bricks).
Frank

 

[Matt Green]

Insane. Ducting that distance to an airbrick is not ideal for any useful amount of airflow.

A lack of ventilation did not cause the joists to rot- ground water transferred through the in-filled earth did. Clearing the soil will have solved the problem. Isolating the new (pre-treated) timbers from the masonry with a DPM or similar should preclude any further decay.

Put a trap door in to check the conditions periodically. Seriously: a sense of proportion is in order here.


Matt

 

[MrT]

Thanks for all the suggestion.

One extra possibily is to keep the earth floor and tiles in the back room after removing the slab and inserting a 80 cm lenth of 100mm ducting from the suspended floor to the operational woodburning stove. The stove should create a stack effect and pull air through the cavity and give the stove the air supply it needs. This room will be a kitchen and I plan to put in a vertical passive vent inserted into a redundant ventpipe already installed through the roof. The front room also has a wood burning stove which I can add an additional air brick and grill for.

MdB, I has thought about putting in a timber floor in the back but after discovering the earth floor and tiles under the screen I'm keen on preserving this.

Some good ides about keeping timbers dry and ventillation. I'm going to forget about the concrete and put some new joists in.

Thanks all.

 

[saddened1]

Put polythene sheet on the ground under the floor to lower the moisture load in the air. This will help keep the wood dry

 

[MdB]

Put polythene sheet on the ground under the floor to lower the moisture load in the air. This will help keep the wood dry

Agreed this would reduce the moisture load in the air but it would also increase the moisture in the soil and hence may cause problems in the walls.

 

[Matt Green]

Put polythene sheet on the ground under the floor to lower the moisture load in the air. This will help keep the wood dry

Agreed this would reduce the moisture load in the air but it would also increase the moisture in the soil and hence may cause problems in the walls.

Unless the sub-floor regularly floods, the air moisture loading beneath the floor should never get high enough that insect damage to treated timber becomes a significant risk. Isolate the bearings that are in contact with the masonry though, the risk to those areas is much higher.


Matt