MMA Yearbook 2007 10 . For RD cavity walls there are several points to note. The performance of cavity-separating walls is dependent on the degree of isolation achieved between the masonry leaves. Wall ties, which are necessary to provide structural stability, tend to reduce the isolation properties between the wall leaves. Therefore, very stiff connections must be avoided. Type-A ties should be used, including butterfly ties, as described in BS1243. Alternatively, wall ties with an appropriate measured dynamic stiffness, kxmm in MN/m with a cavity width of Xmm and n ties/m2 is n. kxmm <4.8MN/m3. Care should be taken to ensure that mortar does not bridge the cavity. Excessive bridging can result in significant sound transmission between attached dwellings, by-passing the isolation effect that the cavity provides. Particular attention should be given to bridging that may occur at the base of the wall. This could be aggravated by the installation of radon barriers, continuous damp-proof courses and horizontal cavity trays at ground-floor level, preventing mortar from falling to the base of the cavity. The cavity for both separating and external walls must continue at least 225mm below the underside of where the ground-floor structure would or does break the vertical continuity of the wall. The reason for this is two-fold: . to minimise the risk of sound transmission via the foundation connection; and . to allow for some collection of mortar droppings below the floor level. Separating floors Effective sound insulation to separating floors relies on controlling flanking sound transmission. Therefore, the specification of inner leaves and separating walls is vital to the overall specification. There are seven RD separating floors. Five of these include precast concrete floor planks, and two are the beam and block type. They all include concrete blockwork to the flanking and separating walls. Aircrete, lightweight and dense block types can be used provided they comply with the range of densities quoted for each RD. The beam and block separating floor is more restricted in that only blocks with a density range of 1,850-2,300kg/m3 can be used to the inner leaf. To avoid PCT completely, it is necessary to specify RD constructions to both the separating floor and wall. However, there may be circumstances where it is necessary to specify RD construction to either the floor or the wall. In such cases, this is provided for by the non-robust detail element being subject to PCT. However, designers must ensure that any non-robust detail construction is capable of meeting the required level of performance. Acoustic details The RD Handbook provides construction advice to a number of critical areas, Plaster or drylining depending on block density Block density range 600-2,300kg/m3 Minimum 75mm cavity Block thickness 100mm minimum to each leaf . . . . . Masonry separating wall Ground floor not continuous between dwellings 225mm (min) Foundation . . . . . Basic construction of robust details for masonry Ground floor junction with separating wall beyond the scope of Approved Document E. These include details on cavity stops, a specification for movement joints in separating walls, and details for building-in of timber joists and steel beams, detailing of flues in separating walls, and services and chases in separating walls. The adoption of RDs has been welcomed by the masonry sector. Their use avoids the disruption and uncertainties associated with site testing. The monitoring process has shown that RDs work in practice, thus achieving the regulatory aim of improved compliance. Development of new RDs continues via the agreed protocol, although this is a rigorous process to ensure that quality is maintained and requires a lot of commitment and significant cost to be successful. . Reader enquiries: info@modernmasonry.co.uk. .

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