ᐅ Is a Wood Fiber Board Necessary Under the Roof?

Hello everyone,

We are planning the following roof/upper floor structure (townhouse with a hipped roof):

- Concrete slab instead of a wooden frame on the upper floor
- Insulation on top of the concrete slab
- NO roof insulation

--> Our general contractor also offered us a wood fiber board with a thickness of 35mm (about 1.4 inches) (this is probably the board placed under the roof underlay membrane?).
Reason: In case a roof tile is damaged during a storm and it rains, our entire attic space would not be flooded right away.

Cost for this wood fiber board: €4,000.

What do you think? Is this board necessary?

The third water-bearing layer then?
1. Brick
2. Underlay membrane
3. Wood fiber insulation board

Water is a problem when it gets through the roof, but I don’t see the risk as higher than with other “normal” buildings.

For summer heat protection, the 35mm (1.4 inches) doesn’t add much, but it’s better than nothing.

exto1791 schrieb:

--> Our general contractor has now offered us a wood fiber board with a thickness of 35mm (1.4 inches) (is this the board installed beneath the roofing membrane?).

If there is such extensive work being done, it might make sense to also consider insulating the roof. Otherwise, you can skip it.

I would also recommend properly insulating the roof and leaving out the insulation of the top floor ceiling...

exto1791 schrieb:

Reason: If a brick gets damaged due to a storm and it rains, our entire attic won’t immediately be flooded.

But assuming this (unlikely) scenario does happen… the water will have to go somewhere eventually. We have exactly the same type of roof—insulated ceiling floor, uninsulated roof. We only installed wood fiberboards above the ceiling insulation so that we can access the attic. After a storm, we simply check to make sure everything is still okay.

In case of brick damage, the underlay membrane is there, so no additional insulation is needed. However, for 4000€ I could think of several much better options. The benefit is close to zero.