Dry Bedding – Envelopment within Mortar is Essential
Author/Issuer: Cavity Tray Standards Added:
Design Considerations – Identification Report (DC/06/18/3p)
The use of damp courses and trays in cavity construction and the necessity to envelope all placements within mortar.
This identification report concerns the use of conventional roll dpc and preformed dpc trays (cavity trays) when used within cavity wall construction. The observations and recommendations identified refer to use at all levels of construction appertaining to the building envelope, from ground level to plate level. Unless otherwise stated all are applicable to both skins of a conventional cavity wall where such skins are constructed of masonry.
In the United Kingdom construction work must be executed in accordance with the Building Regulations. The Building Regulation publications include a list of relevant British Standards.
BS 5628-3:2005 is the Code of Practice for the Use of Masonry and provides information and guidance regarding how dpc and trays are to be incorporated:
‘All dpc and cavity trays must be laid on a smooth bed of fresh mortar’. ((BS 5628 extract)
This is not a new requirement but one that has applied for decades. The two leading providers of UK construction and standards warranties stipulate the following:
“The structural design must be in accordance with Building Regulations and BS 5628 and additionally the need is emphasised in some locations to use only a dpc that can achieve a good key with the mortar.”
(NHBC Standards 6.1)
The NHBC Standards includes the following requirements regarding use of dpc’s:
Dpc’s should be laid on a surface, free from projections that could puncture or adversely affect the dpc material.
Dpc’s should be fully bedded on fresh mortar.
To ensure structural robustness and weather resistance, masonry units should be laid on a full bed of mortar. (NHBC Standards extracts)
Zurich Insurances enforces the same requirement, but in one sentence:
‘A dpc should be laid between a smooth mortar bed and lapped at junctions’
(Zurich Insurances – superstructure extract)
Additionally Zurich Insurances refer to observance of BS 8000:4.
Therefore to construct in accordance with the Building Regulations, to construct in accordance with the relevant British Standards, and to comply with the requirements of both the NHBC and Zurich Insurances, no dpc or tray must be laid dry-bedded.
If a masonry cavity wall is to be deemed compliant, all dpc’s and trays incorporated therein must be bedded on mortar and the subsequent masonry laid must be bedded on mortar also.
Consequences of Dry Bedding
Dry bedding means elements of the building envelope are not structurally enveloped. This can adversely affect the performance and the life of the structure.
Where dry bedding occurs within the inner skin (most commonly at ground floor level) any absence of mortar may subsequently influence the airtightness of the structure. Where the dpc is built into the inside skin at a higher level (for instance to provide support for a dpc that rises and crosses within the cavity), the absence of mortar similarly does not promote structural or airtightness qualities. The thermal behaviour qualities anticipated of an inner skin can be reduced where there is intermittent bonding or an absence of bonding material that would normally be present to provide a continuance of structural adhesion.
(Recommended reading BRE Thermal Insulation: Avoiding Risks)
Where dry bedding occurs in the external skin, the structural adhesion of the weathering wall is discontinuous. This gives rise to subsequent behaviour witnessed during severe rain conditions:
Dry-bedded features are liable to capillary action once saturation point of the masonry/mortar in the immediate vicinity of the dry bedding is reached. Because a dpc must extend through the thickness of a masonry skin, the commencing point for such behaviour is on the exterior masonry face. The movement of liquid in the interstices of a porous material (as a result of surface tension) promotes the phenomenon responsible for dry materials sucking moisture beyond or above normal water permeating depths. Water ingress is the result of a combination of adhesion and cohesion of intermolecular forces between the liquid and a substance stronger than thecohesive intermolecular forces inside the liquid. Hence the necessity to lay all dpc’s and trays within a bed of mortar so envelopment is achieved plus the accompanying structural bonding.
Dry bedding can also promote spalling. In a bed joint lacking mortar, any retained or pooling water is susceptible to freezing when a wet wall is exposed to a temeperature drop. The concentration of liquid along a bedding joint is neither structurally or aesthetically beneficial.
If this occurs at higher level such as within a parapet or running coping, there is the added exposure of thermal and moisture temperature swings manifesting in parapet ratcheting. Hence at coping level there is the added requirement for a minimum mass (25kg per metre run) to promote stability. Where dry bedding occurs behind an external render coat, the structure is susceptible to water rentention and tell-tale signs of normally manifest within a few years. The render coat will eventually crack and delaminate. Such risks are present if dpc and trays are dry bedded – hence the need to observe the regulations, requirements and reccommendations of the authoratitive bodies.
All dpc manufacturers contacted confirmed the instructions for use of their material stipulated bedding within mortar. This was in printed form and in many instances available on the internet also.
All cavity tray manufacturers contacted confirmed identical instructions. This was in printed form and in many instances available on the internet also. Some manufactuers pointed out the instruction ‘Do not dry bed’ is additionally moulded into their product so it is facing the installer every time.
Frost / Sulphate Attack of the Outer Leaf
The frost resistance of masonry is diminished if the masonry skin permits retention of water within voids, intermittent joins or if pooling exists within a structure and is not readily evacuated. The degree of exposure to wind driven rain combined with the frequency of freeze and thaw cycles are the main factors affecting frost attack. Dry bedding of a dpc or tray increases the risk because water is both retained and channelled by capillary action.
The length of time the masonry is wet, and the interaction of soluble salts used in masonry units and cement used for the mortar directly influences sulphate damage of mortar. If water retention is present because it cannot readily evacuate, the exposure to risk is increased. Dry bedding of a dpc or tray increases the risk because water is both retained and channelled by capillary action.
Failure to bed dpc’s and trays in mortar introduces deficiencies and as a consequence the structure is susceptible to numerous deficiencies in the immediate and long term, some of which will affect the ability of the construction to provide the requisite protection against damp and thermal elements, as demanded by the Building Regulations applicable in England, Scotland and Wales. If a masonry wall is to be deemed compliant, all dpc’s and trays incorporated therein must be bedded on mortar and the subsequent masonry laid must also be bedded on mortar.
Perhaps the situation is best summed up by a site agent given the task of ‘trouble shooting’ various properties in which the dpc’s had been dry bedded. The properties were suffering water ingress at various high level intersections because of the omission.
‘If a glazier came on to my site to bed and putty the glass into the window frames but used no putty at all and put the glass in dry, he would hardly expect the rain not to come in – would he? ….. As for cavity construction, it’s in the requirements and it’s in the instructions, (to bed dpc and trays in mortar) so there’s no excuse’.
- James Long Design (background information)
- NHBC Technical Manual
- Robust Details
- Zurich Insurances
- British Standard 5628-3:2005
- British Standard 8000:4.
- Cavity Trays Ltd Technical Manual 17.
- BRE: Thermal Insulation: Avoiding Risks.