Do I need to 'float' timber floor?

[Joe87]

Hi all. 

I'm close to starting floor construction for my garden room. 

 

I have a concrete base about 8 inches deep and was planning on putting down DPM over the top and up the walls to the DPC level in the walls. Then I was planning to put the joists on top of this, leveling and then insulating in between the joists with rigid insulation boards and then a wood floor over the top. 

 

Is this OK? I've read about having a floated timber floor so there is a gap below the joists. Is this a must? I was hoping the DPM would prevent damp getting in? 

 

Thanks for any help. 

[Adsibob]

When you say “wood floor” what exactly do you mean? Engineered wooden flooring? Solid wooden flooring? Laminate?

Typically, you would have a solid flat layer under the finished flooring.  Laying directly onto joists is not a good idea.

Edited December 25, 2022 by Adsibob

[Joe87]

3 hours ago, Adsibob said:

When you say “wood floor” what exactly do you mean? Engineered wooden flooring? Solid wooden flooring? Laminate?

Typically, you would have a solid flat layer under the finished flooring.  Laying directly onto joists is not a good idea.

 

The plan is to put osb on top of the joists and then a laminate or wood floor or something on top of the osb. 

[Iceverge]

Why the joists if you have a concrete base? 

 

Level with sand. 2 layers of insulation crossed. Then 2 layers of OSB crossing all joints and final flooring. 

 

 

 

[Joe87]

2 minutes ago, Iceverge said:

Why the joists if you have a concrete base? 

 

Level with sand. 2 layers of insulation crossed. Then 2 layers of OSB crossing all joints and final flooring. 

 

 

 

Really!? Never heard that as a way of doing it, but would make life easier. The concrete slab is large enough that it was never leveled out when they poured it (probably laziness on their part). The concrete is about 10 meters by 5 meters. 

[Redbeard]

Agree with @Iceverge. I am a great fan of floating floors (base/insulation/OSB/finished floor, with or without some membranes along the way) but perhaps the degree of 'out-of-level-ness' will have a bit of a bearing. May be hard to level it all out with sand and then not spoil the level as you lay? (Envisages a series of pulleys on ceiling each supporting a piece of insulation, jigsaw-style!)

[Iceverge]

Using EPS will be more forgiving to bumps than PIR. Cheaper too if you have the height to accomodation the extra depth for the same U Value. 

 

You could mix some cement into the sand like a biscuit mix to prevent it moving when the boards go down. 

 

Alternatively.

 

DPM, 

Insulation boards in 2 layers. 

Slip membrane

Pumped screed, this can be as little as 35mm AFAIK and will remove any small undulations of the floor . 

 

[Adsibob]

Or you could level with a self levelling compound. Much thinner than screed.

[Adsibob]

3 hours ago, Iceverge said:

Why the joists if you have a concrete base? 

 

Level with sand. 2 layers of insulation crossed. Then 2 layers of OSB crossing all joints and final flooring. 

 

 

 

Plus 1

[Joe87]

16 hours ago, Iceverge said:

Why the joists if you have a concrete base? 

 

Level with sand. 2 layers of insulation crossed. Then 2 layers of OSB crossing all joints and final flooring. 

 

 

 

Would this floor have the strength to carry something like a pool table? What thickness osb should I go for. 2 layers up to 18mm or something?

 

Thanks for all the replies this is really helpful. Doing it this way would be alot simple without having to bother with joists etc. 

[PeterW]

1 minute ago, Joe87 said:

Would this floor have the strength to carry something like a pool table? What thickness osb should I go for. 2 layers up to 18mm or something?

Yes would be fine as the compressive strength of the insulation and the jointing of the floor would spread the load across a wide area. 2 layers of 18mm bonded and screwed together is going nowhere fast !

[Glenn]

15 hours ago, Adsibob said:

Plus 1

Plus another one. For my temporary accommodation in self build site I converted a farm shed with uneven concrete slab. Dpm, sand strategically placed to stop the 100mm butted insulation boards from rocking, then a raft of 2 layers criscrossed of 12mm construction ply screwed together, floating on the insulation boards. Solid.

[Iceverge]

On 26/12/2022 at 10:13, Joe87 said:

What thickness osb should I go for. 2 layers up to 18mm or something?

 

 

I would say 2 layers of 11mm would be fine. Screwed and glued. 

 

Kingspan recommend 18mm T&G but I feel properly overlapped sheets would be stronger. Don't forget to leave enough space for expansion and contraction. 

 

https://www.kingspan.com/gb/en/knowledge-articles/a-guide-to-constructing-a-floating-floor/

 

Here's a project from the states. 

https://youtu.be/NbC-EfQ_u3E

 

 

 

[Iceverge]

Here's an article. https://www.jlconline.com/how-to/foundations/insulating-over-a-structural-slab_o

 

 

I'll attach a PDF from Huber in the USA. 2-Layer-Floating-Subfloor-with-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf2-Layer-Floating-Subfloor-with-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf

 

 

[Joe87]

On 28/12/2022 at 07:47, Iceverge said:

Here's an article. https://www.jlconline.com/how-to/foundations/insulating-over-a-structural-slab_o

 

 

I'll attach a PDF from Huber in the USA. 2-Layer-Floating-Subfloor-with-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf2-Layer-Floating-Subfloor-with-AdvanTech-subflooring-Technical-Tip-Subflooring-AdvanTech.pdf

 

 

Thanks for all your help. This is brilliant information to mull over. Thanks again! 

Happy New year