french floor

[BillH]

thanks FF Penners and AMC

best said the way it is FF

Anyway will revisit this over the weekend , which is the first chance to talk with the builders.

I am intending to make the piers out of concrete to at least try and keep some semblence of ""trying" . Will put a
damp course liner between the concrete and the timber.

we may well put a membrane in the piers.

Will wrap/protect the the end of the base plate as well nearest the wall end. .

just to be clear in my mind..

I intend to make the piers ... say 40 cm wide ???? and 7m long , stopping just short of the walls by an inch or two.

1.2 m between each one, so say ten all allowing one for each end. ( The room is about 10x7m )

(roughly ) What size timber to use.. for the base /sole plate and the joists

The gap between floor and earth is about 24 inches..

anything else I need to think about.. before the weekend to spec this job ?

Thanks to all for your frank input..

rgds

Bill

 

[BillH]

does anyone know if

coffrage

is the term for shuttering in french ?

and I use the word pier which I suspect is even wrong in english..

rgds
Bill

 

[Moo]

I think 1200mm piers sounds about right, I certainly wouldn't go less than that unless the spans are pretty small as bouncy floors are rubbish.

And that brings me to my next two questions:

1 I will need to span about 9ft in my new kitchen and would like to avoid the complication of building piers. How hefty should the joists be?

2 I'm thinking of fixing chicken wire to the underside of the joists, to provide a base for some sort of thermal insulation. The question is - what sort of insulation should I use?


and a ps to BillH: Why not just lay the joists the other way round (across the 7ft span)?

 

[BillH]

I think 1200mm piers sounds about right, I certainly wouldn't go less than that unless the spans are pretty small as bouncy floors are rubbish.

and a ps to BillH: Why not just lay the joists the other way round (across the 7ft span)?

tis 7 m x 10 m , it's a lumpy room.. so spanning the shortest width with the piers

rgds

Bill

 

[Moo]

Doh! I didn't read your post carefully enough and assumed that you, too, were talking about feet (small room) rather than metres (practically a ballroom!)

I hang my head in shame.

 

[Penners]

does anyone know if

coffrage

is the term for shuttering in french ?

Yes, I think it is. Shuttering or formwork.

The English word pier, as applied to brickwork, normally applies to a post built of bricks. Like this:

I think what you're talking about is what I normally hear described as a dwarf wall. Your flooring joists can be supported on a series of individual piers, or laid at 90 degrees across a series of supporting dwarf walls.

If you opt for the latter, just be sure that your dwarf walls don't impede the flow of air through the sub-floor void. If you think they might, be sure to include plenty of air bricks or vents in them.

 

[BillH]

If you opt for the latter, just be sure that your dwarf walls don't impede the flow of air through the sub-floor void. If you think they might, be sure to include plenty of air bricks or vents in them.

dwarfs they are... good point re air flow past them... I really hadn't thought about it.. I assumed air would be
moving tween the floor boards and the joists and the sole plate on the dwarf .

thankyou again ..

rgds
Bill

 

[BillH]

sorry to trouble you again..

just doing my sums..

if I reduce the dwarf walls to 30 cm wide instead of 40 cm .. x ten of them.. that means, I am covering about three of the ten meters with dwarf walls.. each 7 m wide across the room, that's then 21 out of the 70 sq m of floor space..


is that OK ?


rgds
Bill

 

[BillH]

[

I think what you're talking about is what I normally hear described as a dwarf wall. Your flooring joists can be supported on a series of individual piers, or laid at 90 degrees across a series of supporting dwarf walls.

.

I was intending to lay a base plate across the dwarf wall, then on top of that, a joist onto which I would lay the
floor boards.

rgds
bill

 

[Penners]

Bill - the number of dwarf walls you need is dependent to a considerable extent on the section of timber that you intend to use for the joists. But (if I understand you correctly) your plan to build a dwarf wall for every metre of width seems considerably over-specified to me.

I can't quote you any engineer's calculations of building standards - I'm afraid I tend to rely on gut instinct for such things - but given a floor area of 7m x 10m I think I would place 7m joists across the shorter dimension, supported on 10-metre long dwarf walls. I would probably specify joists at least 50mm x 150mm (175mm depth would be even better, if there's the depth of void to accommodate them). And along the longer dimension I would build three dwarf walls, dividing the space into four, meaning that each joist would be supported every 1.75m* of length. I reckon a roughly 200mm thick wall (ie: in UK terms 2 bricks thick) is ample, provided it's well founded. At the ends of the joists, I would either fit joist hangers to the walls or (if they're straight enough) bolt a supporting timber plate to the walls, upon which the joists would rest.

Obviously if you need to join the joists, in order to gain a 7m length, this should be done on top of a dwarf wall.

There's no particular reason why you can't turn this whole structure around by 90 degrees if there's some reason why you want to do so. Obviously this would involve you in constructing a larger number of shorter dwarf walls.

Be sure to used pressure treated timber for your joists and wall plates. Rest the joists on a piece of damp-proof membrane on top of each dwarf wall.


* I'd be interested in other people's views on the 1.75m unsupported span that I have suggested for a 50 x 150 or 175 joist.

 

[Moo]

I'd be interested in other people's views on the 1.75m unsupported span that I have suggested for a 50 x 150 or 175 joist.

So would I !

I need to span approximately 9ft.

 

[Feltwell]

If you opt for the latter, just be sure that your dwarf walls don't impede the flow of air through the sub-floor void. If you think they might, be sure to include plenty of air bricks or vents in them.

I think they're more commonly called sleeper walls, the ones that I've seen are built in a honeycomb pattern for precisely the reason of airflow. No need for airbricks, just build them with gaps between the bricks - it'll still be strong enough. See link below - do always fit a wall plate on top of them, beneath your joists, as it spreads the load properly across the bricks.

http://nuweb.northumbria.ac.uk/bedemo/domestic_floors/page_08.htm

Don't bother with oversite (as it says on that link). Their comment on T&G boards being introduced in the 1940's is out as well - my house has them (1899) and whilst I don't doubt they are better than square edge boards, they still shrink and let draughts through!

Dwarf walls tends to be the term used for the low brick walls on some conservatories and greenhouses.

 

[Feltwell]

I'd be interested in other people's views on the 1.75m unsupported span that I have suggested for a 50 x 150 or 175 joist.

So would I !

I need to span approximately 9ft.

Moo, sleeper walls are very easy to build and will definitely stop the bounce.

However, both my lounge and dining room have spans in excess of 9' with no sleeper walls and very little bounce. But they aren't "high traffic" rooms, you don't walk through them to get to any other rooms - for a kitchen that people will walk through a lot I'd want to really make sure there was next to no bounce.

 

[Penners]

Needing plenty of support and not much bounce...

Sounds like me.

 

[BillH]

thankyou penners.. absorbing your reply.. will report back..


rgds

bill

 

[Feltwell]

Moo, here's the table for allowable joist sizes:-

http://www.tameside.gov.uk/tmbc8/note07.pdf

So, if your span is 9' - which is near enough 2.7m - you could use 145 x 47 joists (6" x 2" in old money) joists at 450mm / 18" spacings. Only if you use C16 graded timber that is (all builders merchants will have this if you ask).

However, personally I'd be tempted to put at least 8" x 2" joists in at 12" spacings - make sure the bounce is minimised! Or put in a sleeper wall. But then I always have been rather "Victorian" in my building style, over-engineering everything!

 

[BillH]

I think they're more commonly called sleeper walls, the ones that I've seen are built in a honeycomb pattern for precisely the reason of airflow. No need for airbricks, just build them with gaps between the bricks - it'll still be strong enough. See link below - do always fit a wall plate on top of them, beneath your joists, as it spreads the load properly across the bricks.

http://nuweb.northumbria.ac.uk/bedemo/domestic_floors/page_08.htm

Don't bother with oversite (as it says on that link). Their comment on T&G boards being introduced in the 1940's is out as well - my house has them (1899) and whilst I don't doubt they are better than square edge boards, they still shrink and let draughts through!

.

thankyoiu for that Feltwell , I was talking to someone and they said honeycomb walls, and I was sturuggling to understand them really.. This helps a lot .. and there was I hoping tongue and grove would help with the draughts


I just told my french bloke to postpone start of work, and we will talk it thru when I am there.. I need to be in a position of knowing all the answers for the proposal I will make , so many thanks to all for gifting me your knowledge so far.. .




Bill

 

[Penners]

Bonne chance, Bill!

 

[BillH]

Bonne chance, Bill!

thanks penners... just going thru your mail , with feltwells , with a toothcomb , so may well hit you with another question.. (by sunday me thinks)


rgds
Bill

 

[AMc]

BillH - The joist size, pier spacing is basically a trade off. If you think about a 1st floor room the floor is necessarily suspended for several metres with no support, so you need substantial joists with a significant depth to avoid movement when you walk around.
If your piers are thick and frequent the joist can be quite small, if you imagine a continuous pier (or screed) you'd only need tiny battens to fix your floorboards to as there is no option for them to move up and down. As you shrink the support structure then you need deeper joists to cope with the variation in load across them as you move around. You can in most circumstances keep shrinking the support and increasing the joist until end up with a massive joist and no interim support - a fully suspended floor.

My ground floor has several suspended wood floors with brick piers and walls under them to prevent deflection. Most of them are either single brick walls or piers with 2 bricks per level. The walls are almost continuous every 600mm (the old imperial 2') with small ventilation gaps. They look to have a timber bearer over the top but then they're original and DPM didn't exist and more importantly these floors have always been bone dry thanks to the way the house is raised up. The joists are substantial laid at 90 degrees to the walls. The upstairs rooms above them have very similar joists and no support, so them move a little but not too much and probably always did.

Structural engineers, timber merchants and builders often have ready reckoners that allow you to work out the joist sizes, spacings and timber grades to cover a particular span. In the UK building control would probably want to see calculations that showed the joists were up to the job.

We fitted underfloor heating and insulation under the ground floor rooms. This consisted of a tile batten screwed to the lower edges of the joists, foam board insulation cut for a snug fit between the joists and expanding foamed to seal the edges then an layer of OSB on top. Then the pipework was fixed to the insulation and covered in a 25mm light screed to the top of the joist. Then finally the floorboards over the top.
If you don't want the underfloor heating then the tile batten screwed to the joists to support foam board insulation across the top will provide a good level of thermal insulation. I wouldn't bother with cheaper insulation methods (myself) as you only want to do this once so spend a few quid more to get the best result you can. Obviously if you have skinny joists you can only fit minimal insulation so it needs to work better - for rough reference a 100mm of foam board has the same thermal conductivity of 300mm of mineral wool insulation.

Before we did our floors the cross ventilation and gaps between the boards meant on a windy day you could feel the draught coming up under your feet and your heat being blown into the night!

I would size the joists to allow for at least 50mm of board insulation (celotex, ecotherm, kingspan) but consult a professional for the best sizing/spacing of joists and piers and I would avoid continuous piers as they will affect ventilation worse for no clear benefit.

Doh! just seen another page of comments which give you most of what I've said already!

Still pictures are worth a 1000 words - here's how they did it in the olden days and what we did afterwards.