Replacement for damp-prone timber floor

[rdw1985]

Hi All,

 

First post on here so bear with me..

 

I have just purchased a terraced victorian house and found dry rot in the timber ground floor in the extension. The house is late 1800s and is of stone construction. The foundations are also stone and the building is on clay soil. The extension adjoins my neighbours extension and looks to be original as the construction matches the main house, and every house in the row has a similar extension. The extension is 2 stories, with the ground floor consisting of a 2x4m lounge and a 2x2.5m kitchen, with the kitchen farthest from the main house. The kitchen has a solid floor while the lounge has a suspended timber floor. This post concerns the lounge.

 

I have removed half of the rotten timber floor. The floor consist of 2m wooden joists running side to side and 4m floorboards running front to back of the property. The ends of the joist rest on a continuous ledge of 150mm running down both sides of the room. This has revealed a void of roughly 1.3m down to bare clay. This clay is damp on the side facing the outside (as opposed to the side facing my neighbour). I suspect there has been a leaking drain and whilst I am employing drain inspectors to check the condition of the drain, the clay looks likely to remain damp for a long time in any case. The dry rot was obvious above the damp soil on the inside of the foundation wall below floor level and extended across the underside of the suspended timber floor for around 1m.

 

Now I have commenced works to fix sources of damp, find the extent of dry rot damage, remove all damaged materials, and begin repairs, which leads me to my main question:

 

What shall I replace the floor with?

I want to replace the floor of the lounge with something that meets this criteria:

-Not wooden. I can't ensure that the void will be dry enough for fungal damage to be guaranteed not to occur in future and would like to use materials that can't rot.

-Supported by existing 150mm ledges. It seems obvious to make use of the ledges as they are level and solid.

-Does not raise the overall height of the floor. The doorways leading out of the room are low enough already so I don't want to raise the floor level at all. The current floorboards plus joists are 130mm thick in total so the replacement would ideally be a similar thickness.

-Maintains the void. I want to allow the clay earth and foundation walls to dry out and allow airflow so would like to keep the void. 

 

I've looked at beam and block, which seems like it might work except I'm struggling to find a system that will provide a thickness to match my current floor if I rest the beams on the existing ledge. Most inverted T beams suitable for beam and block seem to be 150/155mm in height. They also seem to need screed/insulation etc on top thus raising the floor even higher.

 

I'm open minded to any construction but will prefer those that fit the criteria above.

 

Any suggestions?

 

Cheers,

 

Rich

[Ferdinand]

Hi Rich

 

I do not know very much about block and beam, but my first thought on your situation was "fill, damp proof and insulated slab", given how far you have already gone in removing the floor. You would also get the opportunity to do a really well insulated floor. As a belt and braces a French drain along the walls outside if possible might be beneficial. 

 

If you are damp-proofed then it surely does not matter that the clay underneath remains damp.

 

Suggest a post in the Introduction section, and put a link to this thread in it for wider input .. as it seems slightly to have been missed.

 

Ferdinand

 

[Mr Punter]

Consider filling the void with LECA expanded clay beads and top with 150mm concrete.

[PeterW]

Will full fill be acceptable on clay ..??

 

You could create a steel ring beam sat on the brick and then insert timber joists into this - It would  isolate the damp but still be the same depth. 

 

Alternative is to tank the top side and then use a normal treated timber build up - doesn’t remove your damp issue but does make it easier to manage. 

 

I would find the source of the water first though as doing this could make the solution very simple. 

[rdw1985]

Hi All,

 

Thanks for the replies. I was expecting emails when I had replies but as none appeared I have only just spotted your comments.

 

PeterW I think I am inclined to follow your advice and replace with treated timber having tanked the surfaces where the joists rest. I'll coat joist ends in boron gel and wrap joist ends in dpm. Does that all sound reasonable?

 

Next question:

Would chipboard, 18mm ply (exterior?), floorboards, or something else be better to use with regards to damp?

 

Many thanks,

 

Rich