UFH Screed Problems & Solutions
Underfloor heating installations can encounter a range of screed-related problems that compromise thermal performance, structural integrity, and floor finish quality. Understanding these issues — and knowing how to prevent or resolve them — is essential for reliable UFH performance. This guide covers the most common problems and their solutions.
Thermal Cracking
The most common UFH screed problem. Thermal cycling — repeated heating and cooling — creates expansion and contraction stresses that can crack the screed over time.
Causes: Insufficient curing before commissioning, excessive heating rate during commissioning, lack of reinforcement, inadequate movement joints at perimeters and large areas.
Solutions: Always observe the full 28-day curing period before commissioning. Follow the gradual commissioning procedure (starting at 25°C, increasing 5°C per day). Use fibre reinforcement — polypropylene at minimum 0.6kg/m³, or steel fibres for heavy-duty applications. Install perimeter isolation strips and movement joints in areas exceeding 40m².
Air Voids Around Pipes
When screed fails to fully encapsulate UFH pipes, air pockets form. Air is an excellent insulator — exactly what you don't want around heating pipes. Even small voids can reduce heat transfer by 30-50% in affected areas, creating cold spots on the floor surface.
Causes: Using traditional hand-laid screed that doesn't flow around pipes, insufficient compaction, pipes too close together (under 100mm spacing).
Solutions: Use flowing screeds — their liquid consistency naturally fills all gaps around pipes. Our Tekcem and Ardex flowing ranges eliminate void risk entirely. If traditional screed must be used, ensure thorough compaction with particular attention to areas between pipe loops.
Slow Heat Distribution
When the floor takes excessively long to warm up or shows uneven temperature distribution, the screed is likely the culprit.
Causes: Low thermal conductivity screed (traditional cement at 0.4-0.8 W/mK), excessive screed thickness over pipes, air voids reducing heat transfer.
Solutions: Specify anhydrite screeds (1.2-1.5 W/mK thermal conductivity) for optimal heat transfer. Maintain minimum pipe cover of 25-30mm — going thicker than necessary adds thermal mass without benefit. Ensure void-free installation with flowing screeds.
Surface Defects After Commissioning
Crazing, dusting, or surface delamination appearing after the UFH system is first operated.
Causes: Premature commissioning before full curing, excessive water in the original mix (particularly with traditional screeds), contamination of anhydrite screeds with water during curing.
Solutions: Strict adherence to curing times. For anhydrite screeds, protect from water exposure during the entire curing period. Use proprietary products with controlled water-to-powder ratios rather than site-mixed screeds where water content is variable.
Screed Curl and Edge Lifting
The screed edges lift away from the substrate, creating a dish-shaped profile. This is more common in unbonded and floating installations.
Causes: Differential drying — the top surface dries faster than the bottom, causing differential shrinkage. Commissioning too early exacerbates this by driving moisture from the top surface.
Solutions: Proper curing with polythene sheeting for the first 7 days. Gradual commissioning. Use products with low shrinkage characteristics — anhydrite screeds are inherently low-shrinkage compared to cement-based alternatives.
Reinforcement Requirements
UFH screeds benefit significantly from reinforcement to control thermal cracking:
Polypropylene fibres: The standard choice for domestic and light commercial UFH. Added at 0.6-0.9kg/m³, they distribute through the screed and control early-age cracking. Cost-effective and easy to incorporate.
Steel fibres: For heavy-duty applications — commercial, industrial, or areas with point loads. Added at 20-40kg/m³, they provide superior crack control and increased load-bearing capacity.
Mesh reinforcement: Wire mesh (typically A142 or A193) placed within the screed. Effective but more labour-intensive to install than fibres, and positioning is critical — it must be in the upper third of the screed to be effective.
Getting Expert Advice
Related Reading
UFH screed problems are almost always preventable with correct product selection and installation practice. Call us on 0118 370 2060 for advice on your specific installation. We offer free delivery on orders over £600 ex-VAT.
