If you suspect mould growth on a specific surface, a pressure plate test is one of the most precise and reliable DIY methods available. Unlike air tests, a pressure plate is designed to detect active microbial growth directly on a surface.
In this guide, we explain exactly how to use a pressure plate correctly, how to interpret the results, and when this type of test is the right choice.
What Is a Pressure Plate Test?
A pressure plate test is a microbiological test that is pressed directly against a surface to collect mould spores, bacteria and other microorganisms.
It is particularly effective for:
-
Walls and ceilings
-
Window reveals and cold bridges
-
Areas behind furniture
-
Basements and bathrooms
-
Suspected problem spots with no visible mould
Because the test is pressed directly onto the surface, it provides a very targeted result.
When Should You Use a Pressure Plate?
A pressure plate is ideal if:
-
You suspect mould but can’t clearly see it
-
You want to check whether a surface is contaminated
-
Mould was previously cleaned but may still be present
-
You want to confirm whether staining is biological or not
It is often used alongside air testing for a more complete picture of indoor mould issues.
Before You Start – Important Preparation
To ensure a reliable result, follow these instructions carefully:
-
Store the pressure plate in the fridge until use
-
Remove the test from the fridge shortly before testing
-
Avoid touching the agar surface
-
Do not test immediately after cleaning
-
Use one pressure plate per surface
-
Wash or sanitise hands before use
How to Perform the Pressure Plate Test – Step by Step
Step 1: Choose the Test Area
Select the surface you want to test. This should be an area where mould is suspected or where moisture problems are known.
Step 2: Remove the Lid
Carefully remove the lid from the pressure plate. Avoid touching the agar surface inside.
Step 3: Press the Plate Against the Surface
Press the agar surface gently but firmly against the test area for approximately 5 seconds.
Do not slide or rub the plate — a single, steady press is sufficient.
Step 4: Close the Plate
Carefully place the lid back onto the plate and ensure it is properly closed.
Step 5: Incubate the Test
Place the closed pressure plate in a warm, dark location at normal room temperature.
Do not refrigerate after sampling.
Step 6: Read the Result
Check the plate after 3–5 days.
Microbial growth may appear as:
-
Dots or colonies
-
Fuzzy or powdery growth
-
Different colours (white, green, black, grey, yellow)
How to Interpret the Results
-
No growth or very minimal growth
→ Normal or insignificant surface contamination -
Visible colonies
→ Active mould or microbial presence -
Heavy or rapid growth
→ Strong indication of a moisture-related mould issue
The number, size and density of colonies all matter when assessing severity.
If you’re unsure, we recommend documenting the result with photos.
What to Do If the Test Is Positive
A positive pressure plate result typically indicates:
-
Excess moisture
-
Condensation problems
-
Cold surfaces or thermal bridging
-
Past or ongoing water damage
Recommended next steps:
-
Identify and address the moisture source
-
Improve ventilation and humidity control
-
Consider professional inspection if growth is extensive
-
Use air testing to assess overall indoor air quality
Disposal of the Test
Once the test period is complete, the pressure plate can be disposed of with normal household waste.
Important Limitations
A pressure plate:
-
Identifies surface contamination
-
Does not measure airborne spores
-
Does not identify mould species
-
Does not determine legal liability
For complex cases, professional sampling and laboratory analysis may be required.
Need Help Interpreting Your Result?
If you’re unsure about your result or would like guidance on next steps, we’re happy to help.
📞 +44 20 4638 9811
Clear answers make the next step easier.