Tiling and UFH over 100mm joists

[severnside]

We are in design phase of our renovation. Our current ground floor is suspended floor with 100 X 50 mm joists spaced around 350mm. This is sloping site with void ranging from 30cms to 160cms, so not possible to change to concrete. 

We would like to fit insulation between joists, ufh (spreader plates or Pre routed floorboard) and finally finish with tiles. We had discussed it with Architect technician and structural engineer, they believe joists are too thin to support tiles and their might be cracked tiles in future. 

Is there a way to improve strength of the floor to support tiles e.g. Ply instead of chipboard and overboarding with No More Ply before tiling?

If changing joists to deeper one is the only option, how much would it cost for 80m2 ground floor. 

 

Thanks 

[Gus Potter]

2 hours ago, severnside said:

We are in design phase of our renovation. Our current ground floor is suspended floor with 100 X 50 mm joists spaced around 350mm. This is sloping site with void ranging from 30cms to 160cms, so not possible to change to concrete. 

We would like to fit insulation between joists, ufh (spreader plates or Pre routed floorboard) and finally finish with tiles. We had discussed it with Architect technician and structural engineer, they believe joists are too thin to support tiles and their might be cracked tiles in future. 

Is there a way to improve strength of the floor to support tiles e.g. Ply instead of chipboard and overboarding with No More Ply before tiling?

If changing joists to deeper one is the only option, how much would it cost for 80m2 ground floor. 

 

Thanks 

Ask your SE how they feel about the following.

 

Lift the existing boards, clean the top of the floor joists, wire brush , hoover the top so that you get a good rough clean surface. Apply a liberal amount of structural glue to top of joist. Pilot drill and counter sink holes in ply and screw fix down to top of joists at 200mm centres. Glue to be a true structural glue such as Cascamite.. not D4 as a popular on BH.

 

The screws are to provide the compression while the glue hardens, the glue does the work. Now you have turned the joists into a quasi "T" beam. The SE will probably need to do some calcs by hand (a few pages mind) as you don't get off the shelf software for this, and also, as it's a bit off the wall they may need to spend some time "researching ".. great job for a graduate though wanting to build their portfolio as you have to go back to basics.

 

Now you have your structural deck. Give it a bit of time to condition, get the heating working. Next tile but use a decoupling matt? ... plenty info on BH from the pro tilers on how you do it.

 

Lastly make it clear to SE and Architect that you will accept any liability if the tiles crack and offer to pay SE as you have "heard" that it can be quite complex to design. The SE may just say.. oh I have an old spread sheet that I used to use, give me a couple of hundered quid and you take responsibility if tiles crack.

 

On short span floor like this tile cracking is much more likely to occur due to shrinkage (drying and swelling) of the substrate rather than deflection of the structural members.

 

 

 

 

 

[ProDave]

The deflection of the 100mm joists will depend on the distance between the sleeper walls that will be supporting it. You need to measure that.

[severnside]

So far I have opened only one room and found sleeper wall at 1.2m distance. So joist span looks safe. However both Architecture technician and SE made comment solely based on joist thickness (100mm). 

Our intention is to check if someone has resolved this problem in real life and what are their results. Some of tiler forums suggest overboarding and noggings to fix these issues.

[saveasteading]

I can confirm what Gus says. Had on old house with v skinny joists. Added Plywood with ribbed nails at close centres, and was approved by bc.

The difference was huge and deflection matched that of new joists adjacent. 

All other comments agreed too. 

[severnside]

Thanks for all the comments. We will use plywood in place of chipboard to strengthenn the joists. I checked joist span tables and it seems with 100mm joists they are within safe limits and also deflection calculation show suitable for porcelain tiles. It could be a generic comment from SE without looking at finer details. I will have another discussion with him.

[Temp]

+1 to plywood.

 

I would seal the ply with a sealer recommended by the tile adhesive company (not PVA). Perhaps also consider a decoupling mat.