Strengthening suspended floor for large tiles, advice please on idea

happyhero

I want to run large tiles from concrete floor to suspended timber floor without using an expansion joint.

To explain the layout first we have a house with an existing single storey extension across the width of the back of the house, which is about 5.5m wide internally and 3.8m deep towards the back garden. Standing in the extension facing into the rest of the house with your back to the back garden, the kitchen is on the left side and the lounge on the right. The kitchen has units both sides and is 3m long with about 1.3m between the units. So if you walked out of the extension into the kitchen you would walk between the units through the kitchen into the rest of the house. Hope that all makes sense.

I have bought 600mm x 900mm x 10mm porcelain tiles but in the actual extension we wanted a heated floor so I also bought 10mm insulation backerboards, a Warmup DPM plastic self-adhesive mat and loose heating cable.

The self-adhesive mat does 2 jobs it contains the cable when laid and allows the tile some movement (you can buy this sort of sheet for non-heated floor to allow for movement/expansion/contraction etc it was explained to me). What I am getting at is it is good to have under the tiles even if you don’t have a heated floor. Several stores wanted to sell us this type of mat regardless of a heated floor we eventually chose to add.

I want to run the tiles all through the concrete floor extension and into the timber floor kitchen and so normally you would need an expansion joint but I would like to avoid an expansion joint and have consulted some tilers in one store as to how I could do this. They said ideally you fit an expansion joint but if you want to avoid it you must do a lot of work to try and make the suspended wooden floor in the kitchen as strong and rigid as possible. They said the insulation board and plastic sheet will help disconnect the tiles from the timber floor.

So I was thinking how to make my timber floor rigid. It has a gap under the joist of about 250mm(10in). I had this idea so tell me what you think about it and if you think it will work. I was going to get some plastic pipe, 4” diameter or maybe better 6”. Clean the area of rubble under the joist and cut the pipe to fit tight between the joist and the cleaned area below which seems to be concrete. Then I would put some rebar down into the pipe the same length as the pipe and pour concrete into this pipe. I also thought to stop bounce I would drill the top of the joist over the pipe and put a recessed screw or bolt to go into the concrete thus securing the joist to the concrete post I make. I was thinking of making these posts every 18” or 24” maybe, its fairly easy to do and it seems a strong idea to me plus with the 10mm backerboards and plastic movement sheet I think that would do it, what do you think?

Can this work?

Any feedback appreciated.

ic

So you want to put legs under the joists to stop any downward bounce in the floor?

happyhero

Yes I think thats the way to go to achieve what I need.

Hasbeen

Timber floor as in floorboards or chipboard? Floorboards expand / contract etc, most would put say 20mm exterior  ply screwed down. But would guess this would raise the height to much?

Legs at regular intervals would support the joists, but are you sure it is actually a concrete base ?

My biggest concern would be the cost of running the electric underfloor heating.  

Doozergirl

All this to not have a relatively innocuous expansion joint at the threshold? Why not just plan the tiles so a natural break line falls at the join between the two floors? 

The usual route is to put concrete board over the floorboards to make them rigid, then your decoupling membrane works to absorb some movement. 

 If the wooden floor isn't already tiled and the concrete wasn't laid specifically to get the right levels for the proposed flooring, there's enough prep to be done to try and get the floors level to one another, without adding this to it. 

I can't say I've ever heard of what you're doing.  The usual route for inadequate joists is to replace them with bigger ones!  Then you can have bigger, stable boards as well. 

The benefit of that, as well as being a lot less faff, is is that you'd get your levels right by sitting the joists at the correct level for your sub-base to fall level with the concrete.  

stuart45

A similar idea is sometimes done in the USA using Sonatubes. The rebar is tied into the base though. It's also done before the floor is laid. Never seen this done here, although I'd be interested to know if it worked.