marmox on joists

[johntree]

Is it possible to lay 20mm marmox directly onto floor joists or does it have to be supported. I don't want to lay ply or anything over joists 1st as this messes up levels.

Will I have to put battens on side of joists then ply to meet top of joists then marmox.
Cheers

 

[Richard C]

Marmox (or any tile baker) boards are designed to be laid over & supported by an existing floor, concrete or suspended timber, you can’t just lay them over the joists. Apart from the sheer amount of work involved, a problem you have in laying support ply between the joists rather than over them is that it will do nothing to stiffen the overall floor structure. The joists will be free to flex individually & I fear the Marmox board on it’s own will have insufficient rigidity the prevent the floor flexing with disastrous consequences for your tiled floor.

 

[johntree]

Thanks Richard,

I will lay a floor first, either ply or it seems I may be able to use osb board. My only queery is the thickness. 18mm is the most but do you reckon we can go thinner, just trying to reduce step into the room. Was going to check with marmox head office on tuesday.

 

[Richard C]

I would not advise you use OSB, it’s nowhere near as rigid as WBP ply. The minimum thickness is really 18mm in a light use/load area such as a bath/shower room, don’t be tempted to go thinner; sometimes 22mm or even 25mm is necessary, depending on joist size/spacing/span. Marmox may even tell you 25mm as that is British Standards recommendation.

Why are you over laying with 20mm Marmox? if you use 18mm or 22mm WBP ply you can tile straight onto that; if you want additional insulation you can always put that between the joists under the ply.

 

[johntree]

Using 20mm marmox cos we have a marmox shower former which are 20mm as we want a level access shower. Figured we would have extra security of water resistant / proof floor just in case anything escapes through the enclosure door. Also good ammount of insulation for floor heating. Using 20mm all over seemed sensible option when it comes to tiling.

Thanks for advice we will have a nice and solid floor soon

 

[geraint]

Marmox (or any tile baker) boards are designed to be laid over & supported by an existing floor, concrete or suspended timber, you can’t just lay them over the joists. Apart from the sheer amount of work involved, a problem you have in laying support ply between the joists rather than over them is that it will do nothing to stiffen the overall floor structure. The joists will be free to flex individually & I fear the Marmox board on it’s own will have insufficient rigidity the prevent the floor flexing with disastrous consequences for your tiled floor.

so battening and fitting ply between the joists is not structural....

 

[Richard C]

so battening and fitting ply between the joists is not structural....

Sticking sections of ply between the joist swill never make the floor as rigid as overlaying the joists, I could spend a great deal of time producing calculations to prove it wont be as rigid but I will leave it to you to produce calculations as to why you think it will be!

The joists will be free to flex individually & I fear the Marmox board on it’s own will have insufficient rigidity the prevent the floor flexing with disastrous consequences for your tiled floor.

 

[Richard C]

I’m not a structural engineer by profession but in a previous life I was (& still am) a very well qualified Mechanical Engineer with experience in the design, stress calculation & construction of significant steel structures; but not specifically in the building trade.

The strength/rigidity of a multiple component structure is dependant on many factors which includes the strength of materials involved, the physical size of components, method of construction &, critically, tolerances & method of fixing. As I said previously, I have neither the desire nor time to provide calculations (which I'm sure you will be aware would be meaningless if it wasn't correctly constructed) to justify some basic DIY advice on tiling a suspended floor which is going to constructed using DIY tools & experience. If you wish to justify your post of what seems to be over reactive & pointless criticism of my advice to the OP & disprove my advice as to how he should build his floor to the best effect, please do so & I will come back & comment.

No tiler (or even structural engineer) in their right mind would board between the joists over battens & expect overall floor rigidity to be as good as boarding over the joists with the same floor thickness but I look forward to your calculations to prove it.

 

[geraint]

why is it that you make false comments, and when i question them, i am a troll......

you stated in your earlier post that over boarding is more structually sound than boarding in between the joists... and stated you could prove....

and then have the audacity to mention a structural engineer....

i am only asking how you come to that conclusion....

a very simple question

in my enlightened opinion, a floor with joists battened with a ply floor in between is far stronger......

i have answered your post with manners, and i also expect the same in return.......

 

[Richard C]

why is it that you make false comments, and when i question them, i am a troll......

Then prove they are false; don’t post opinionated argument, it counts for nothing! It’s the way you question without any substantiation that makes you a Troll.

you stated in your earlier post that over boarding is more structually sound than boarding in between the joists... and stated you could prove....

As I’ve already said; over boarding with sheet flooring will always be more structurally rigid than cutting strips of flooring & laying them between joists & fixing over battens. I’ve already said I’m more than willing to look at & comment on your calculations to prove me wrong.

and then have the audacity to mention a structural engineer....

Why on earth do you consider that as audacious?

i am only asking how you come to that conclusion.... a very simple question

Experience & knowing it as fact & I’ve already given you a very simple answer, twice.

in my enlightened opinion, a floor with joists battened with a ply floor in between is far stronger......

Your enlightened opinion doesn’t really count for much unless you can back it up with facts so lets have them, either put up or shut up! We are talking about a DIY boarded floor here, not designing the 5th road bridge.

i have answered your post with manners, and i also expect the same in return.......

So the “another jo boy” comment in your original post was complementary was it? Perhaps that’s why I’m being just a little spiky! I’ve even Googled it & still don’t understand what it means but my guess is it wasn’t intended as complementary so just giving back a little in return what you gave me; but I can get really ill mannered if you want me to be.

See you got thanks from paul348 which improves your total but I seem to remember he also 3rd party trolled advice I was giving & didn’t like my response either so I guess it’s his way of getting back; I donlt think he likes me very much either. What is it with the Forum “lurkers”; never contributing much but feel the need to snipe at regular contributors efforts! If you think something isn’t right then say so & give your reasons, you have a perfect right to do so & I will be the first to bow when I’m proven wrong. But don’t Troll & make snide or sideswipe comments you won't or can’t substantiate, Christ even joe90 has more manners than that.

 

[tictic]

c24 joist @190x50(187x47mm)..maybe even c16..

max span (outside span) 2.5m internal size 2.4m

min cc 400mm

no need for herringbone struts(BS standards)

double joists under internal walls a must..

 

[Richard C]

Right, to simply matters; if you treat each floor joist with its associated piece of floor ply attached as a simply supported, homogeneous “T” beam on fixed supports (which it won’t be due to methods of construction). The cross sectional area of the beam is used to determine the moment of inertia (I value) of the beam, In this case, the larger cross sectional area of the beam with the floor over the top of the joist will translate into a correspondingly greater I value than the beam with the floor attached to the sides of the joist which has a smaller cross sectional area. This I value is used in calculation formulae to calculate beam deflection; the greater the I value, the smaller deflection for any given point load.

Thanks for all your support lads, lets hope that’s the end of it.