Thoughts on this

[Gazzer]

A while back I went to quote for a floor. The substrate was 18mm ply on joists. Very solid and no problem except they wanted backerboard, UFH and Travertine or limestone...maybe polished marble. The first thing I noticed was 4 doors were too low and You wouldnt be able to get the depth needed.
Adjusting the doors is a no go as one door is a folding door , about 2 metres wide. The others are UPVC and to adjust would be major work.
They left it at that . I now have an email stating that a "builder " will remove the ply and cut it into the joists, thereby giving an extra 18mm.
This will still be overboarded with backerboards but I am not too keen on the idea. In theory it may be fine but until I see how well this "builder" is then I dont feel I can give a definite answer.
How would you play it ?

 

[Dougs Third Go]

but how is the ply going to be supported? with battens attached to joists? if so it'll be seriously under strength.

 

[Gazzer]

Exactly Doug, my thoughts too but as you know " Our builder says it will be fine" I think it would reduce overall strength.

 

[Ian]

I suppose in theory if extra noggins were added 18mm below finished joist height, with battens, to support the ply, it may work. I suppose you would have to see what the deflection was like after this was done. As Doug says, it would surely be under strength as there would be continuous breaks in the substrate, essentially all this would be doing would be in filling the joists.

 

[Gazzer]

Well this is the problem. It could be done , if they take a belt and braces approach but will they ? I wont know until its done and even then I still dont like the sound of it.......all those straight joints every 450mm etc

 

[Ian]

Maybe achievable on a small floor, this sounds like quite a big area. What's the m2?

 

[Gazzer]

Its about 21m2 in total.

 

[Mr T]

Couldnt you plain the folding door to the correct depth? and then raise the bottom section somehow? Or am I way off the mark lol

 

[BoldreChris]

It could be done by reducing the joists themselves by 18mm - b1tch of a job but that would leave you with a continuous ply base to tile onto.

 

[Gazzer]

Couldnt you plain the folding door to the correct depth? and then raise the bottom section somehow? Or am I way off the mark lol

Miles off the mark !

 

[Ian]

Its about 21m2 in total.

That's way too big an area I reckon, it'll be interesting too see if it works.

 

[Dougs Third Go]

It could be done by reducing the joists themselves by 18mm - b1tch of a job but that would leave you with a continuous ply base to tile onto.

but then you'd be weakening the joists.

 

[ian158]

I take it its downstairs given the UPVC doors..the bifold will be easy enough to plain, but UPVC, be easier to move them up if the brick work can be cut above door,,less work in making good than taking floor up, cutting each joist ! nightmare job that would be, even with a festoon plunge saw.

 

[Ajax123]

I don't think weakening the floor would be too much of a concern in this instance. If they were taking lumps off the joists then yes but just in filling is not really much of a worry for me.

of greater concern would be the level of movement in the floor. Bear in mind that the joists will move at a different rate to the ply and if it is cut in really tightly it will try to belly when heated.

With trav this one sounds fraud with danger.

 

[Gazzer]

Bifold doors are four doors made from UPVC...about 1800mm

 

[ian158]

Bifold doors are four doors made from UPVC...about 1800mm

steer clear then, you that desperate for the agro it may become

 

[Sean SML Tiling]

if the joist get trimmed and become weaker they will also become more flexible and there's a good chance that the flexible adh wont cope. in my opinion joist are there for a reason and shouldn't be touched.

 

[Dougs Third Go]

I don't think weakening the floor would be too much of a concern in this instance. If they were taking lumps off the joists then yes but just in filling is not really much of a worry for me.

of greater concern would be the level of movement in the floor. Bear in mind that the joists will move at a different rate to the ply and if it is cut in really tightly it will try to belly when heated.

With trav this one sounds fraud with danger.

but you're actually taking the floor joists out of the equation. Assuming they're specc'ed to carry the existing floor if you batten to them the tensile strength will be reduced surely?

 

[Jeff the tiler]

It does not work. The joist are generally specified in relation the span of the area. Any adjustment will weaken them and you are increasing the load being put on them. Also, are you going to use a decoupling membrane, as this is going to ad further height issues.
Would love to see the joist after they have been reduced exactly by 18mm???
Just my observation

 

[Ajax123]

Joist are specified to carry a standard load plus a margin. The span is irrelevent in as much as it only affects he joist depth. they are usually designed to display a deflection no greater that the length divided by 300 or 12mm. When we put flowing screeds onto joists they are designed at L/333. in modern domestic housing this load is usually a minimum of 1.5kN/m2 but most builders architects and engineers over specify to future proof the dwelling. Most of the ones I come across these days are specified at 5kN which is a pretty hefty load. Provided the battens are attached at sufficiently regular intervals they will act compositely and the overall lad bearing capacity will not be appreciably affected. If the joists are add thinner then this will affect the load bearing but from reading your op this is not what is intended... Or have I read it wrong.

1.5kN = 1500N = 150kg per m2
5.0kN = 500kg per m2

 

[big george]

could you drop the joists?

 

[ian158]

Joist are specified to carry a standard load plus a margin. The span is irrelevent in as much as it only affects he joist depth. they are usually designed to display a deflection no greater that the length divided by 300 or 12mm. When we put flowing screeds onto joists they are designed at L/333. in modern domestic housing this load is usually a minimum of 1.5kN/m2 but most builders architects and engineers over specify to future proof the dwelling. Most of the ones I come across these days are specified at 5kN which is a pretty hefty load. Provided the battens are attached at sufficiently regular intervals they will act compositely and the overall lad bearing capacity will not be appreciably affected. If the joists are add thinner then this will affect the load bearing but from reading your op this is not what is intended... Or have I read it wrong.

1.5kN = 1500N = 150kg per m2
5.0kN = 500kg per m2

too much information !

 

[Gazzer]

I emailed client with my concerns and they just replied that they agree !!!!! they will now see if the joists could be lowered.........i am going to lie down now !

 

[Sean SML Tiling]

This job must be costing them a bomb. Most likely rip it up in 5 years as women get bored

 

[White Room]

I emailed client with my concerns and they just replied that they agree !!!!! they will now see if the joists could be lowered.........i am going to lie down now !

I can't see that happening, the work involved would be so expensive...