Bridging and Buckling of Solid Wood Floors


When either the whole or portions of the flooring have become separated from the sub-floor, they form a bridge. This is often accompanied by various degrees of cupping. Buckling is a much rarer occurrence. This is when the flooring lifts from the sub-floor, reaching several millimetres in more extreme cases. The reaction to excessive moisture is the reason why this may occur.



Bridging and buckling are caused by the expansion of the flooring due to a rise in moisture content. When the flooring meets obstructions, such as walls and door frames for example, the horizontal movement of the floor is obstructed and expansion causes lifting.

Bridging and buckling are usually an indication of high levels of moisture. However, cumulative expansion can cause bridging and buckling from normal seasonal expansion when the floor has inadequate expansion especially in large floors.

Even where expansion gaps are present around the perimeter of the flooring and obstructions, excessive expansion can cause buckling within the main body of the floor.

Factors That Can Contribute To Bridging And Buckling

Primary Causes

  • Environmental moisture problems from sub-floors.
  • Inadequate expansion gaps.
  • Inappropriate filling of seasonal movement gaps.
  • Inappropriate filling of spaces between boards intended to accommodate expansion.
  • Excessively dry flooring materials.

Secondary Contributing Factors

  • Incorrect nailing.
  • Incorrect adhesive, application method or trowel type.
  • Weak screed leading to uplift of screed which is still attached to adhesive.
  • Contamination of screed surface causing failure of adhesion

If wood flooring is installed direct to an existing wooden base and where the existing wooden flooring is bonded to a direct finish concrete slab which does not have a DPM, once the wooden flooring is covered over, moisture can no longer escape the existing wooden flooring. The moisture content of the existing wooden sub-floor rises and expands, pushing up the new flooring.

General Diagnosis

  • In order to find the appropriate solution for the problem, you must look for a number of things, that should point you towards the issue.
  • Identify whether there is any cupping occurring in combination with the bridging / bucking indicating a moisture problem?
  • Check the size of the flooring, as larger floors sometimes require more expansion than can be provided by a perimeter expansion gap and provision for expansion must also be incorporated into the body of the floor itself.
  • Check the size of the expansion gap around the perimeter of the floor
  • Look into the humidity of the atmosphere in-situ
  • Check if the building permanently unheated, or unheated for long periods of time?
  • Look at the moisture content of flooring during installation?
  • If the concrete / screed does not have a damp proof membrane, it could be a cause of rising moisture.
  • Specifically, what was the moisture level of screed at the time of laying?
  • If precise moisture level is unknown - How thick is the screed, and how long has it had to dry?

Subfloor type - Wood Based Subfloor

  • What is the moisture content of the wood based subfloor?

On-Site Diagnosis

  • Check the moisture content for excessive moisture.
  • Uplift flooring element and check for elevated moisture level on the underside.
  • If no excessive moisture is found, is it possible that inadequate expansion gap was installed?


Bridging and buckling also represent a trip hazard which must be considered if natural drying is intended to play a role in the correction of the problem.

Initial Action

Find and remove the source of the moisture problem if one exists

On Nailed or Adhesive Fixed Floors

Where flooring is fitted by nail or adhesive fixing, bridging occurs where the fixing to the sub-floor has broken away. Spot repair / replacement or re-installation may be adequate in minor cases, whilst complete re-installation or replacement is sometimes required due to the fact that the floor has often shifted from the subfloor throughout the area. Even where flooring appears flat, fixings may be compromised due to cumulative horizontal movement.

Floated Floors (Engineered or Solid Wood)

If floated floors have bridged because of inadequate provision of expansion, the problem can often be remedied. This is because the performance of floated floors is not dependent on fixings to the subfloor which can affect bridged / buckled floors. Integrity of the tongue and grooves can however sometimes be affected by buckling, and to a lesser extend by bridging.


  • Proper testing of subfloor humidity and moisture before installation.
  • Use an appropriate vapour barrier or surface DPM.
  • Appropriate selection of moisture content of flooring materials.
  • Suitable provision for expansion.
  • Maintenance of suitable ambient conditions in-service.
  • Comply with manufacturers recommendations for the provision of expansion.
  • Install flooring in the correct direction, so that it runs parallel to the two longest walls.

When sanding and finishing existing floors during the season of low humidity, consider the possibility that small gaps between flooring elements are needed for expansion during the season of high humidity consequently it is sometimes inadvisable to fill gaps between these elements . This is especially significant in large floor areas.