6.11.7 Detailing
Rendering shall be detailed to ensure appropriate weathertightness and durability. Issues to be considered include:
- copings, cappings and sills
- abutments and interfaces
- weepholes
- detailing at openings
- exposed elements
- ancillary items
- render below the DPC
- resistance to sulfate attack.
6.11.7.1 Copings, cappings and sills
Render should be protected from damage by copings, cappings or sills made of a material of low permeability or with suitably detailed DPCs. All copings, cappings and sills should have an efficient throat or drip on the underside which should be not less than 40mm away from the face of the rendered wall.
Extending sills or sub-sills beyond window reveals can help to disperse water and prevent staining
6.11.7.2 Abutments and interfaces
Where raked roof abutments occur against a rendered masonry wall, preformed cavity trays and appropriate flashings should be provided. Clauses 6.1.17 and 7.2.20 contain guidance for stepped cavity trays and flashings to masonry walls.
Cavity trays with stop ends are required above all openings. Where required, a minimum of two weepholes per opening should be provided.
A site-made or preformed cavity tray should be provided at horizontal abutments with weepholes at maximum 1m centres.
Render abutting exposed features, such as stone string courses or quoins, should be finished neatly without gaps.
Abutments between render and exposed masonry should be detailed to prevent moisture passing in behind the render or adversely affecting the building.
When rendering into window or door frames, the render should be stopped against a bead and sealed, or a bead of sealant applied between the frame and render.
6.11.7.3 Weepholes
Weepholes should be provided:
- where required for ventilation to timber frame construction
- to cavity trays in parapet walls and horizontal roof abutments at not more than 1m centres
- to the last tray at stepped abutments
- in severe or very severe exposure zones where rendering is returned into the window or door head (weepholes are not required where the render is not returned).
To prevent staining, weepholes should be of a type which restricts the entry of wind-driven rain.
6.11.7.4 Detailing at openings
Design features around openings and at the head of the rendering should provide shelter and help shed water away from the surface below.
Checked rebates should be used in:
• Scotland
• Northern Ireland
• Isle of Man
• areas where the exposure to driving rain is very severe (severe/very severe exposure when building using timber frame).
Alternatively proprietary cavity closers which hold a satisfactory assessment by an appropriate independent technical approvals authority, accepted by NHBC, for the exposure zone may be acceptable.
Proprietary render systems should be detailed at abutments in accordance with the manufacturer’s recommendations.
6.11.7.5 Exposed elements
Render to exposed masonry elements, such as parapets, freestanding walls, pillars, retaining walls or chimneys, should be of a type appropriate for severe exposure conditions.
When rendering both sides of freestanding or parapet walls of single leaf construction, care should be taken to prevent damage caused by moisture becoming trapped. For example:
- the detailing should prevent the masonry from becoming saturated
- the wall should be protected from rain during construction
- rendering both sides of single leaf walls in areas of very severe exposure to frost attack should be avoided (see Clause 6.1.6c).
Bricks with S1 or S0 designation are not recommended for exposed elements that are to be rendered.
6.11.7.6 Ancillary items
Stop beads and render stops should be austenitic stainless steel or PVC. Long runs of steel beads and stops should be avoided due to their expansion potential.
Corner beads should have an appropriate projection to prevent thin tapering of the render which reduces its overall thickness.
Beads should be:
- adhesive-fixed using a material appropriate for external use and in accordance with the manufacturer’s recommendations, or
- mechanically fixed using suitably durable fixings.
6.11.7.7 Render below the DPC
To prevent damage caused by prolonged periods of wetting, it is preferable to stop the render at DPC level. Where rendering is continued below the DPC, the following precautions should be taken:
- for site-made render, use a stronger mix (M4) that is sulfate resisting, or
- use factory-made render in accordance with the manufacturer’s recommendations.
Consideration should be given to providing:
- appropriate drainage installed along the perimeter or ground falling away from the building
- adjacent surface finishes which do not promote splashing.
Admixtures may be required to enhance performance.
6.11.7.8 Resistance to sulfate attack
To prevent sulfate attack, the wall construction should restrict moisture from entering into the background and having a detrimental effect on the performance of the render.
When detailing between the render and exposed brickwork, it is advisable to use appropriate materials resistant to, or without sources of, sulfate.
Last updated: 2nd January 2025