New oak boards on suspended ground floor

[ajbaker]

Hi.

I am about to replace the carpeted flooring in the living room of my Victorian cottage with a new exposed oak floor. A quarter of the floor is currently original oak and the remainder modern pine flooring. My thinking is to salvage the original oak to replace damaged sections elswhere in the house while improving the flooring in the room.

My question is with regard laying. The floor is suspended on joists perhaps 30-40cm off something - and I would love to know what! It is more like really thick soil than a hard substance. The walls show evidence of damp but I do not believe anything to be concerned about in such an old property. All walls have recently been lime plastered so that should certainly help.

New flooring will be 18cm wide by 2cm deep.

My intention is to lay the new oak directly on the joists, but have read a lot recommending this should not be done to avoid the boards cupping.

What are your thoughts? The oak that is there shows no sign of cupping, but I guess new oak may be more susceptible to humidty than the old? I am intending on introducing a few air vents in the floor (beneath radiator and behin sofa for example) to increase ventilation - may this be sufficient? Is there anything else I can do to avoid problems - for example laying a damp proof membrane beneath the boards?

I am aware laying the new boards over the existing is an option but I would like to avoid this.

Thoughts appreciated.

 

[biffvernon]

The diagram on the left shows how to suspend Thermafleece under your suspended oak floor. The longer you can keep the boards lying about in your house before fixing, the better. They tend to cup if one side is dried out more quickly than the other, but whether you have any problems depends on how dry the timber is when you get it, the humidity in the room and the humidity in the space under the floor. Thermafleece is good at absorbing and releasing water with changes in air humidity so may help to produce a more stable environment for the floor as well as keeping you warm, your heating bills low and the planet a tad safer.

...something - and I would love to know what! It is more like really thick soil than a hard substance.

Yes, that'll be soil then.

 

[Lime]

If you have the floor up or are going to take it up ensure that the underfloor ventilators are clear of debris and perhaps add a few more if you think the ventilation is insufficient at present.
For me the experience of ventilators in the floor when it is suspended over soil is not good. The rooms had an earthy smell about them all the time.

Also (you probably already know this) don't use ordinary steel nails or screws in the oak, they will corrode very badly.

Newly felled oak is notorious for twisting and warping so I hope the "new" oak has been seasoned before it was cut.

 

[biffvernon]

Newly felled oak is notorious for twisting and warping so I hope the "new" oak has been seasoned before it was cut.

Good advice. The timber is best left stacked in the open air, with sticks separating the boards, for two or three years at least. You can tell whether this has happened because the top of the boards will be much dirtier than the bottoms. Of course, if you buy the wood already planed then you will just have to trust the supplier.

 

[Me!]

Also (you probably already know this) don't use ordinary steel nails or screws in the oak, they will corrode very badly.

Is this true in dry oak? I know that iron and steel can make oak turn a black colour, but I have some oak doors that are hundreds of years old and still held together with their original nails that I assume are made of iron. I did notice though that Biff makes his oak doors using stainless steel screws. Is this to stop discolouration of the wood or to prevent corrosion of the screws?

 

[Lime]

Oak is acidic and causes iron to corrode within the wood whereas in pine, for instance, iron nails tend to rust only where they are exposed to air with water.
When used for a ground floor (I assume) there will be some moisture about so the oak will never be dry.

 

[biffvernon]

We've had some discussion about this before. I doubt whether there is much to worry about with ferrous metals and oak at the dampness levels found in doors indoors. I probably use stainless steel screws because I have a big box of them, unless someone particularly asks for hand knitted wrought iron cut nails hot from the forge. There are different types of ferrous metals and some corrode faster than others. You might get a bit of dark staining round the hole but I should think it would take a very long time before a fastener rusted to the point of failure.

 

[ajbaker]

Thank you for the replies and advice. The flooring has been put on hold due to a rather large and unexpected invoice for plastering (I'll be doing it myself from now on!)

The Thermafleece was indeed a good idea, though a little concern has been cast due to the possible smell caused by damp sheeps wool (personally I like the smell!). Presumably this would be avoided by the breather membrane as shown in the diagram.

The 'new' oak I was referring to will have been sanded, so I will have to trust the supplier that it has been dried adequately and thoroughly. Their spec states 8% +/- 2%.

One final problem, the underfloor cavity currently has no ventilation. It is assumed it did have at one point in time, but the lower 50cm of the external has been rendered 'recently' (not by me I hasten to add) with cement. It is assumed this has covered any ventilation bricks. As regards ventilation I was considering boxing in a small corner of the room, allowing the underfloor cavity to ventilate into this 'box' and provide external ventilation for this too. Is this a workable solution?

 

[Lime]

I'm assuming this to be the ground floor.

The floor will need proper ventilation by means of several air vents to avoid damp conditions and damage to the timbers.
Quite simply the more air vents the better.
Just one or two will be insufficient.

Anything that restricts a clear flow of air under the floor will lead to expensive trouble in the future.
I do not think your suggestion is a satisfactory way to ventilate the floor.

 

[ajbaker]

Lime.. I don't know if you consider this relevant to your thoughts above, but the room is only 12 square metres. It is therefore a small room and cavity.

I have read the minimum necessary area of ventilation is 1.5% of the area to be ventilated. That would be around 18 square cm. (I appreciae this is a minimum.)

 

[Lime]

Lime.. I don't know if you consider this relevant to your thoughts above, but the room is only 12 square metres. It is therefore a small room and cavity.

I have read the minimum necessary area of ventilation is 1.5% of the area to be ventilated. That would be around 18 square cm. (I appreciae this is a minimum.)

Indirect ventilation via your box would work if it had enough through-put of air.
However unless the house is in a windy position the air under the floor would not be stirred sufficiently to ensure the subfloor remained dry.

The minimum you quote is a rule of thumb for a direct opening into the subfloor and even this is not really applicable with small floors.
If the air has other obstacles to overcome, such as twists and turns in a narrow chamber, the flow will be further reduced.

It is up to you.
You know the situation:- how dry the subfloor is and how much the house is exposed.
In my experience the longer the restriction the more likely it is to be choked by cobwebs and other detritus over time too.

 

[Penners]

ajbaker - I've just read your earlier post about the dryness of the new oak boards that you're planning to lay.

8% moisture content (+/- 2%) gives me slight cause for concern.

Firstly, I don't believe the supplier can get oak down to that moisture content with "seasoning" (ie: air drying). It's almost certainly been kiln dried, which tends to cause more stresses in the wood than air drying.

Secondly, I'm a bit worried that 8% is a bit too dry. Excessive dryness can cause instability just as much as excessive dampness, because the timber then has a tendency to take up moisture from the atmosphere.

For the best stability, the timber should be at 10% - 12% moisture content.

Hopefully Biff will read this and let you know whether he agrees or not. If he doesn't - go with his view rather than mine.

 

[biffvernon]

If they did kiln it to 8% it wouldn't stay like that for long. It would soon get up to the ambient humidity of 12-14% indoors. No point in excessive kilnng and really for many purposes it is better to use air dried timber and leave it in the room for some while before nailing it down.

 

[NT]

Storing timber indoors for a while is a very effective way to weed out the stuff that will warp. And some of what warps before use can be cut to give straight pieces.

NT

 

[David Dyer]

8/10% seems to be quite common for hardwood floor boards lately I think the suppliers think it will all go in centrally heated houses, A friend recently built a new house (shame but that's what he wanted)The ground floor is oak supplied at 8% moisture content fine last winter with the heat (under floor) on but in the spring with the heat off the boards started to pick up moisture and lifted. It has now been refitted but it will be interesting to see if the same thing happens next spring.

 

[Lime]

A friend recently built a new house (shame but that's what he wanted)The ground floor is oak supplied at 8% moisture content fine last winter with the heat (under floor) on but in the spring with the heat off the boards started to pick up moisture and lifted. It has now been refitted but it will be interesting to see if the same thing happens next spring.

I would be hard pushed to think of a better way to get new oak to warp than to heat it intermittently on one side and exposed the other side to the varying humidity of the house.

 

[Penners]

A friend recently built a new house (shame but that's what he wanted).

Yes, a terrible shame. Why didn't he build an old one?

 

[David Dyer]

I would be hard pushed to think of a better way to get new oak to warp than to heat it intermittently on one side and exposed the other side to the varying humidity of the house.

I entirely agree with you Lime I tried to stop him but the flooring 'experts' said they had done it dozens of times before without any problems, so much for 'experts' at least they haven't walked away from it.

Penners behave.

 

[Penners]

Yes sir. Sorry sir.

 

[NT]

A friend recently built a new house (shame but that's what he wanted).

Yes, a terrible shame. Why didn't he build an old one?

Some poeple have done that , but it remains very uncommon. Strikes me as the best of both worlds to a useful extent.


NT