How much compressive strength is enough for a subfloor

[Adsibob]

On our first and second floors, our build up is as follows:

 

1. 14mm engineered oak herringbone (small pieces) OR 14.5mm engineered oak long planks

2. 6mm ply

3. 25mm warmup branded expanded polystyrene foil faced UFH heating boards

4. 5mm dense rubber matting

5. 22mm Egger chipboard flooring

 

In the bathrooms we are actually using a Cellecta extruded polystyrene product that has a 250kpa compressive strength and can be tiled onto immediately, but for the hallways and bedrooms my builder said I could save money by going with the warmup product. The spec of this product says this

I tried squeezing it between my fingers and was surprised that I could reduce it’s thickness by 20% or 25% relatively easily, whereas I cannot do that with the Cellecta product. I told my builder to return it because I thought it was bad quality. He explained that in reality it will never be subjected to those sorts of pinpoint forces because the only and engineered wood will spread the loads. I’m sure he is right, to an extent, but I’m still nervous using something so flimsy feeling. I’m a heavy person 105kg, and I don’t want the floors to squeak underneath me.

Am I worrying about nothing or should I insist he return it? He has confirmed he can return it, but he thinks anything stronger is going to be significantly more expensive and that I’m wasting money on it. 

Edited December 12, 2021 by Adsibob

[Conor]

100kPa is standard for under floor insualtion, you won't have any issues.

[SteamyTea]

11 hours ago, Adsibob said:

Am I worrying about nothing or should I insist he return it?

Trust your gut on this one.

Or reduce your gut and wear oversize shoes.

Edited December 13, 2021 by SteamyTea

[SteamyTea]

Being a bit more serious, and as much as I like SI units, some times it is easier for us to visualise things in imperial measurements.

45 kPa is 6.5 lb per square inch.

Now I have just measured my size shoe and estimated that the floor contact area is 3 inches by 9 inches, so two of them will be 54 inch².

Now taking a mass of 105 kg, that is 230 lb.

Assuming that larger people have bigger feet, shall call it 60 inch².

 

230 [lb] / 60 [inch²] = 3.8 lb.inch^-2

 

Now on the face of it, all seems fine.  But is engineering, we don't work at the limit of the materials, and a safety margin is used.

A safety margin of 2 is not very large in my opinion.

I am also not sure if the @1% and @10% numbers are what happens when a load is applied, and it springs back, or after a load is applied and it is permanently deformed.  That would make a difference to floor movement.

[Dudda]

Have you detailed drawings with a specification and does it specify Cellecta or a compressive strength of 250 kPa? If it does I'd probably stick with it. First question I'd be asking the builder is what's the saving in going for warmup instead of Cellecta as he's proposing. Its rare a builder suggests something that doesn't benefit his own pocket rather than yours. 

 

[Jilly]

Go with your gut cos it will p*** you off and be so hard to rectify if it happens.

 

Building work seems to have soooo many challenges where you need the benefit of hindsight, so I change things when it's pretty easy to, and try to forget about the rest when it has been done or cant be changed/helped.  

[Gordo]

7 hours ago, SteamyTea said:

Being a bit more serious, and as much as I like SI units, some times it is easier for us to visualise things in imperial measurements.

45 kPa is 6.5 lb per square inch.

Now I have just measured my size shoe and estimated that the floor contact area is 3 inches by 9 inches, so two of them will be 54 inch².

Now taking a mass of 105 kg, that is 230 lb.

Assuming that larger people have bigger feet, shall call it 60 inch².

 

230 [lb] / 60 [inch²] = 3.8 lb.inch^-2

 

Now on the face of it, all seems fine.  But is engineering, we don't work at the limit of the materials, and a safety margin is used.

A safety margin of 2 is not very large in my opinion.

I am also not sure if the @1% and @10% numbers are what happens when a load is applied, and it springs back, or after a load is applied and it is permanently deformed.  That would make a difference to floor movement.

I see a big flaw in you calculations lol. No one walks flat footed, they walk heal to toe and the pressure would be quite extreme at point of impact from the heal. Warm deck flat roofs have often be the subject of concern where infrequent pedestrian traffic happen ie window cleaning. The insulation can often be seen to have compressed where trafficked 
 

but the 6mm ply will adequately spread the load so as not to deform insulation IMO.

[Adsibob]

2 hours ago, Gordo said:

I see a big flaw in you calculations lol. No one walks flat footed, they walk heal to toe and the pressure would be quite extreme at point of impact from the heal.

Indeed. Another problem with @SteamyTea’s assumptions is that he forgets that whilst walking all weight will be oscillating from one foot to the other.

[SteamyTea]

3 hours ago, Gordo said:

No one walks flat footed

True, but they are not walking directly on the insulation, as you point out.

3 hours ago, Gordo said:

but the 6mm ply will adequately spread the load

 

2 minutes ago, Adsibob said:

Another problem with @SteamyTea’s assumptions is that he forgets that whilst walking all weight will be oscillating from one foot to the other.

As above.

But

22 hours ago, Adsibob said:

In the bathrooms we are actually using a Cellecta extruded polystyrene product that has a 250kpa compressive strength and can be tiled onto immediately, but for the hallways and bedrooms my builder said I could save money by going with the warmup product.

The hallway will take much more peculiar loads than a tiled bathroom floor, but I was really agreeing with you that a gut feeling is that it is not going to be robust enough.