subfloor ventilation of wooden floor

[benrab]

Hi all

Hope you can help...
The rear ventilation to my suspended wooden floor has been blocked up for years by a solid floor extension. Because of this (and rubble up to the joists in some areas), the joists and boards have become rotten in localised areas.
The house is a small midterrace cottage.
There is a public sewer running under the floor.

As I see it, I have two options

1) ventilate the floor to the roof space via the (disused) chimney/ newly installed vertical duct with low voltage extractor fan combined with more sub floor venting at the front and patch up the rotten joists/ boards (couple sq m)

2) install new concrete floor - allowing for protection of public sewer e.g.by encasing in concrete

I would prefer the first option but dont know if ventilation like this would work. Any ideas?

Thanks in advance

 

[noseall]

the floor void should ideally have cross-flow ventilation. it will require more than one air vent in order to get any movement of air.

i can't see the point in venting into the roof space though.

i think i would bite the bullet in this instance and concrete the floor.

 

[benrab]

thanks noseall

I think you might be right. However because of the sewer complications it will be pretty expensive.
Do you not think that 2 air brinks at front and fan assisted venting to rear (to roof) would be equivalent to cross flow ventilation?
The floor is only 7x4m approx but have been quoted £5000 to replace!

 

[anobium]

You say rotten in localised ares, do you mean just the ends where they go in the solid floor, or is the decay elsewhere?

 

[benrab]

there are two areas:
by front door - sub floor is cut off in this area by brick wall (must have been entrance hall) and filled with rubble up to joists.

and to lesser extent where floor meets solid floor extension and under stairs - area furthest away from vent.

 

[anobium]

If the decay is just in the ends of the joists were they go into the wall all you really need to do is cut back the ends clear of said wall and insert a carrier beam beneath the joists.
With regard to the joists in the hallway how do you know they are rotten if there is rubble upto the joists.
Before you start rippping out the floor I would suggest the you remove a few boards from the hall and examine the joists thoroughly.
A bit difficult to be specific on the info you have given but I would be inclined to have a further look before you decide on complete renewal.
By the way I would also get another quote that £5k seems way over the top.
Oh I nearly forgot what type of decay have they said you have, dry rot or wet rot, very important let us know

 

[benrab]

thanks anobium

Its wet rot. Iv pulled up a couple of boards and the rubble covers half of the joists - and this is the part that is wet, causing the end of the joists to give way.
I think the floor can be patched up but I dont know if the venting idea will work to stop it happening again.
Also, any idea how much a concrete floor should cost?

 

[anobium]

If it's just the ends that are affected then the problem is not caused by lack of ventilation.
It's caused by either rising or lateral damp and as long as the joists are clear of the wall you should be ok.
On the other hand if the joist are affected in the middle then it is probably due to inadaquate ventilation.
Pics could help give a better diagnosis.
No idea how much it would cost for concrete floor.

 

[noseall]

Also, any idea how much a concrete floor should cost?

certainly not £5000 rips for a 7m x 4m floor.

unless of course there are circumstances.....?

 

[noseall]

1) ventilate the floor to the roof space via the (disused) chimney/ newly installed vertical duct with low voltage extractor fan combined with more sub floor venting at the front and patch up the rotten joists/ boards (couple sq m)

 

[benrab]

noseall: yep.. thats the sort of thing I was thinking about (although to the roof space instead of outside). Just wondering if it would be equivalent to airbrick at groundfloor level.

Anobium: There is varying amounts of damp around the walls above the floor, but its hard to tell if this is caused by condensation from the floor being unventilated for such a long time.
It could be damp in the wall contributing to the deterioration of the joists and boards at the edges or the floor creating the damp.