Is limited sub-floor ventilation a real risk for rot in 1930s semi with concrete rear extension?

Eatmym

Hi all,

I'm currently in the process of buying a 1930s semi-detached house and had a Level 2 survey. One of the issues raised was potentially inadequate sub-floor ventilation, particularly at the rear.

This is the information that I have got so far:

• The original house has suspended timber floors (likely ventilated via 4 working air bricks at the front).

• There’s a large ground floor rear extension to the right of the rear of the house with a concrete slab floor, built by previous owners.

• There are 2 air bricks ducted from a newer extension to the left of the house, installed during a 2022 renovation.

• The current owners have put decking over the area of the original extension, making it difficult to assess the ventilation to the rear.

• The kitchen/living area between the front and rear sections is solid concrete and possibly blocks cross-ventilation.

From messages to the sellers, there seems to be no evidence of rot or damp currently, and the current owners say they’ve had no issues — but we’re concerned about the long-term risk of stale, trapped air under the suspended timber floors at the front of the house.

So, my questions are:

1. Is this setup likely to cause moisture build-up or rot over time?

2. How serious is the risk if cross-ventilation is disrupted in part of the suspended floor?

3. Has anyone had to deal with this after purchase — or improved ventilation later?

4. Would it be advisable to add air bricks to the sides or ducting under the slab?

5. Would you recommend a PCA timber/damp specialist before completion?

Any advice or experience would be hugely appreciated! We’re keen to move forward but want to be sensible and plan for any early preventative work.

Thanks in advance!

grumpy_codger

How long is a piece of string?
It all depends - ground water level, soil, drainage, place (top, slope,...).
If there is no source of moisture there is no reason for it to build up.
Is there a hatch to get under the floor and check the soil and the joists?

stuart45

If you can lift a few boards in different places and check the condition and moisture content of the timbers it will give you an idea of whether there's enough ventilation.