Render

6.11

6.11.7Detailing

Rendering shall be detailed to ensure appropriate weathertightness and durability. Issues to be taken into account include:

copings, cappings and sills
abutments and interfaces
weepholes
detailing at openings
exposed elements
ancillary items
render below the DPC
resistance to sulfate attack.

Copings, cappings or sills

Render should be protected from damage by copings, cappings or sills made of a material of low permeability or with suitably detailed DPCs. A minimum 40mm projection with a throating or drip detail should be provided to all copings, cappings and sills.

Extending sills or sub-sills beyond window reveals can help to disperse water and prevent staining.

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.

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.

Weepholes

Weepholes should be provided:

where required for ventilation to timber frame construction

to the last tray at stepped abutments

in severe or very severe exposure zones where rendering is returned back into the window or door head (weepholes are not required where the render is not returned)

to cavity trays on parapet walls.

To prevent staining, weepholes should be of a type which restricts the entry of wind-driven rain.

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.

In areas of very severe exposure, and in Scotland, a check reveal should be provided at openings.

Proprietary render systems should be detailed at abutments in accordance with the manufacturer’s recommendations.

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.

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.

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

factory-made render used 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.

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.