Understanding Why Extensions Develop Internal Cracks

Minor cracking to internal plasterboard and finishes is a common and expected occurrence following the completion of a single-storey extension. These cracks typically result from drying shrinkage, thermal expansion and contraction, or minor settlement of materials — all of which are normal as a building acclimatises.

To ensure a consistent standard of assessment, Brackenwood Windows Ltd follows recognised industry guidance, including:

  • NHBC Standards (Chapters 9.2 & 9.3)
  • British Standard BS 8212 (Code of practice for dry lining using gypsum plasterboard)
  • BRE Digest 561 (Assessment of cracking in low-rise buildings)

Acceptable Cracking and Tolerances

Under these standards:

  • Hairline cracks up to 2mm wide are considered cosmetic and are not deemed defects.
  • These are expected within the first 12 months due to natural drying and movement.
  • Such cracks are the homeowner’s responsibility to repair using standard filler and redecorating.

Cracks exceeding 2mm, recurring cracks after repair, or cracks forming unusual patterns may indicate other issues and should be referred to a qualified surveyor.

Brackenwood's Responsibilities

Where Brackenwood Windows Ltd has constructed the foundation:

  • Brackenwood acknowledges the potential for minor cosmetic cracking.
  • These occurrences are not defects, and it is the customer's responsibility to fill, redecorate, and maintain internal finishes.
  • We defer to the NHBC, BS 8212 and BRE Digest 561 for guidance on what is acceptable.

Where Brackenwood Windows Ltd has not constructed the foundation:

  • Brackenwood accepts no responsibility for:
  • Internal or external cracking of any kind;
  • Settlement, movement or shrinkage of any structural elements;
  • Issues arising from ground conditions or foundation performance.

Responsibility for such matters lies with the party who completed or commissioned the groundwork or foundation.

Summary Table

 

Crack Width Likely Cause Responsibility Action
≤ 2mm Drying shrinkage / minor movement Customer Fill and redecorate
2–5mm Minor settlement / material change Assess; may still be cosmetic Monitor, fill, and redecorate
>5mm Possible structural issue Depends on foundation builder Independent inspection advised

 

Final Note

Cracking in new extensions, particularly where new materials interface with old, is an expected part of the building’s natural settlement and drying process. Brackenwood Windows Ltd adheres strictly to industry best practice and construction standards but cannot guarantee the absence of cosmetic cracking post-installation.

We appreciate your understanding and are happy to provide further guidance if you are unsure whether a crack is cosmetic or potentially structural.