Surely foundation insulation paper isn't needed?

[BORNER]

We are going to build a deck of pressure-treated wood with joists that rest floating on a metal balcony roof. We have now been advised to put strips of base isolation felt under the joists. However, since the joists will be resting on the metal roof (which can handle water), I think the base isolation felt serves no function and may even cause water to stand longer since you can't fold down the edges of the felt.

How would you do it?

 

[ArneTW]

You don't want water standing against wood whether it's treated or not.

Paper is used on top of concrete as a barrier for moisture migration coming up through the concrete.

On a metal roof, it wouldn't be necessary.

However, consider the path of the water if you don't have a roof. Leaves and debris will accumulate, and if water gets there that doesn't dry quickly, it will create a poor environment for both your wood and metal.

/ATW

 

[BORNER]

Thank you, ArneTW!

The house and the balcony are located very high, which means no leaves or needles have a chance to gather under the decking. The metal roof also slopes properly away from the facade and faces directly south, which generally makes it quite well-ventilated and dry. So I will follow your advice and skip the underlay.

 

[ArneTW]

Thank you, ArneTW!

The house and the balcony are situated very high, so no leaves or needles have any chance to gather under the decking. The metal roof also slopes well away from the facade and faces directly south, so it is generally quite well-ventilated and dry. So I'll follow your advice and skip the roofing felt.

I leave no guarantees for those who follow my advice .

It might be worth pondering for a moment the benefit of something between wood and metal.

It's probably not impossible that I would have put something there anyway.

/ATW