Is a dark lintel stain from water or internal sweating?

Most of the time its the Lintels that are wet somewhere or common fault is that there is not enough air flow nearby to clear the moisture or that there is actually a leak from above.
This could be vents blocked or damp trays installed wrong.

It is coming from the rain, cause the wall gets wet and probably they dont have Dpc on Top of the steel lintel mate

It sounds like damp is getting in and causing it to mould, maybe damp course is failing but could be a number of things , would be better if I could call round to take a look and find the right answer for you, thanks

Dark stains on or above lintels are often indicative of an underlying issue, even if the area doesn’t feel damp. Here are potential causes and how to address them:

1. Possible Causes
a. Water Ingress Through Rendering
What Happens: If the external rendering is cracked or not sealed properly, water can penetrate, settling on the steel lintel and causing rust or staining. The water may then transfer the stain to the plaster.
Signs: You may notice cracks in the external render or discoloured patches near the affected area.
b. Condensation or "Sweating"
What Happens: In poorly insulated or poorly ventilated areas, condensation can form on cold surfaces like steel lintels. This might not feel damp but can still cause stains.
Signs: Common in homes with high humidity levels or where there’s inadequate thermal bridging.
c. Rusting Lintels
What Happens: If the lintel isn’t properly protected (e.g., not galvanized or poorly painted), it can rust, and the rust will seep through the plaster as a brown stain.
Signs: Look for reddish-brown streaks and signs of corrosion on the lintel (visible if you can access it).
d. Previous Water Damage
What Happens: Even if the area feels dry now, historical water ingress could have left mineral or rust deposits that "bleed" through new paint.
Signs: Stains reappear quickly after repainting, especially if only cosmetic fixes were applied.
2. Diagnosing the Issue
Inspect the external rendering for cracks, gaps, or damaged seals.
Check for rust on the exposed edges of the lintels (if accessible).
Look for signs of condensation in the room, such as moisture on windows or a musty smell.
Use a moisture meter to check if the plaster has any hidden dampness.
3. Fixing the Problem
a. External Repairs
Rendering: Repair cracks or re-render the area if necessary. Use a water-resistant render or sealant to prevent future water ingress.
Lintel Protection: Ensure lintels are properly painted or treated with an anti-rust coating.
b. Insulation and Ventilation
Add insulation above the lintel to reduce thermal bridging.
Improve ventilation in the room (e.g., extractor fans or dehumidifiers) to reduce humidity and condensation.
c. Treating the Stains
Strip back the affected plaster to expose the lintel.
Treat any rust on the lintel with a wire brush and rust-inhibiting primer.
Apply a stain-blocking primer before repainting the area.
d. Professional Help
If unsure, consult a surveyor or builder who specializes in damp issues. They can confirm whether the problem is structural or moisture-related.
4. Preventative Tips
Regularly inspect and maintain the external render.
Use breathable paint on interior walls to allow moisture to escape without trapping it in the plaster.
Address any signs of damp early to prevent long-term damage.
If the stains persist after these steps, it’s worth having a professional assess for hidden structural or damp issues.