This document discusses cavity walls and how to prevent dampness. Cavity walls are designed to prevent dampness but can experience problems if the cavity is bridged by things like excessive mortar droppings or incorrect wall tie placement. Mortar droppings must be cleaned from the cavity as they can form bridges. Cavity battens can be used to help prevent excessive mortar from dropping down by being raised as construction progresses. Proper installation of damp proof courses and membranes is also important to prevent moisture penetration.

1. Cavity Walls
Preventing
dampness in
cavity walls

2. Tutor Copy

3. When constructing cavity walls it is essential
that good building practices are applied.
Dampness and the penetration of water into
a building must be avoided at all costs.

4. Cavity walls
Cavity walls are designed to prevent this from
occurring as well as providing better thermal
insulation to the building. However, dampness
or moisture penetration can occur if the cavity
is bridged. The most common situations which
can form a bridge are:
• Excessive mortar droppings forming a
bridge within the cavity.
• Incorrect placement of wall ties
• Lack of, or badly placed, DPC membranes
at crucial openings in the building.

5. Cleanliness of cavities
When building cavity walls, waste mortar should
never be allowed to drop or collect in the wall cavity.
Excessive dropping may form bridges by:
• Collecting at the
bottom of the cavity
and building up at the
base of the cavity to a
level which is above
the horizontal DPC.

6. Problems
• Collecting on top of wall ties, making the
tie drip ineffective.

7. • Collecting on top of lintels or cavity trays
over door and window openings.
Problems

8. Use of cavity battens to prevent
excessive mortar droppings
A cavity batten is a timber
batten slightly narrower than
the cavity which is placed and
supported on top of the last
row of horizontal wall ties.
Two lengths of wire or rope
are attached to the batten
which allows it to be raised to
the next row of wall ties before
they are placed. The batten is
cleaned off after each lift and
before repositioning on top of
the new row of wall ties.
Rope
Wall tie
Cavity
batten

9. Cavity batten being used
for partial fill cavity
insulation
Cavity batten being used
for full fill cavity Insulation

10. Cavity batten being used
for partial fill cavity
insulation
Cavity batten being used
for full fill cavity Insulation