Cavity walls

Party Wall Construction



In order to reduce the risk of fire spread and excessive sound transmission, it is important that care is taken in constructing party walls.

In accordance with the requirements of Technical Guidance Document E of the Building Regulations joists at right angles to a party wall should be fixed by single hangers.

If possible, joists should span the entire house from front to back. Where this is not possible, joists should be hung on single joist hangers that are fixed to the party wall and the joints between the joists and the hangers should be filled to reduce noise transmission and reduce the risk of fire spreading.


  • Ensure hangers are the appropriate size for the joists.

  • Ensure the hangers are fixed securely, tight to the party wall.

In situations where external walls consist of hollow blockwork and the internal party walls are solid blockwork, insulation should be provided for a minimum of 1 m from the external wall in order to reduce the likelihood of thermal bridging. Similar precautions should be taken for all internal masonry walls.


Below details the typical arrangement using single hangers to support joists off the party wall.

Diagram C99 - Typical party wall detail - joist fixing

NOTE: Never build joists into party walls.

Diagram C100 - Timber joists should not extend through the wall. Do not construct the party wall this way

Finishing & Fire-Stopping

The party wall should be finished along the line of the slope of the roof in such a way to restrict the spread of fire from one dwelling to another. It is important to ensure that the junction of the party wall and the roof is capable of restricting fire spread between dwellings.

The party wall should only be brought up to 25 mm below the level of the rafter; this is to account for settlement of the roof.

Technical Guidance Document B of the Building Regulations provides greater detail with respect to fire stopping and typical details in common use. This document also allows the use of other systems provided they have been tested to ensure they effectively restrict the spread of fire through a party wall and roof junction.

An example of a typical fire stopping detail along the junction of the party wall and the roof is provided below.

Method Cut blocks to follow the general line of the rafter. In order to create a smooth surface, fill gaps and level top of block line with mortar. Leave a 25 mm difference in level between the rafter and the party wall to account for roof settlement. This gap will later be filled with some compressible material such as mineral fibre quilt when the roof is being constructed.

Diagram C101 - Typical fire stopping detail along a party wall

Diagram C102 - Typical fire stopping detail along a party wall

IMPORTANT There should be no holes or ope in the party wall.

External works Fire safety Roof External insulation Cavity wall insulation Underfloor insulation Wall ties Blocks Air to water heat pump Air to air heat pump Air to ground heat pump Modular build Timber frame IS 440 Ceiling insulation Roof insulation Mineral wool Joists Time and temperature Fire stopping Fire mastic Fire wraps Fire board Sound insulation Tongue and groove Building energy rating Bead insulation Pumped insulation External wall insulation Roof lights External doors Fire doors Internal doors Wall tiles Air tight membrane Fire cable