Lime Rendering / Lime Wash

Lime rendering is a traditional method of smoothing over imperfections on external walls of a structure or building. Render can be applied to follow the undulations of an underlying substrate, or applied in layers to provide a smooth final coat. The finish can be left as is, or further enhanced with a pigmented Lime Wash to add some colour and further weather protection.

The advantage of using lime over modern cement based materials is that it remains flexible enough to allow slight movement in a buildings structural core, and is breathable allowing the ingress and egress of moisture with the seasonal variations of precipitation and temperature. Cement based renders on the other hand set hard providing a rigid and waterproof coat, which is fine until there is structural movement and the finish cracks allowing the ingress of moisture via capillary action through these fissures. Movement and thermal cracks in cement render can be minimised by applying a layer of expanded metal or plastic lathe to the substrate prior to rendering, but this is often omitted on cost grounds.

The characteristic of cement renders to maintain a waterproof layer tends to work against it when it cracks as moisture ingress becomes trapped behind it keeping the structural elements damp and making them prone to degradation either through being continually damp or the freeze - thaw action created cold weather. From an environmental perspective a damp wall also has a greater ability to transmit heat through it thus reducing its insulating properties.

Ultimately a cement based render will become 'blown' and drop off a structure and if constructed in a soft material taking a chunk of substrate with it. Lime render will deteriate with age and erode away from a building but leave the basic substrate intact. As lime is a relatively soft material patch repairs can easily be carried out.

 

 Restoration of Lime Rendered Finish to a Gable End on a Late Victorian Property in Norwich

The property at some point in its life had the gable wall rendered in cement, possibly to provide a waterproof exterior coating over very porous soft Norfolk red bricks. The client initially engaged us to remove the offending, delaminating and cracked cement render with a view to leaving the wall as exposed brickwork and allow it to breath once again. Unfortunately, as with most unsympathetic historical repairs on buildings constructed of traditional materials combined with poor treatment of the roof verge and parapet finishes resulted in major reconstruction, strengthening and application of a more sympathetic lime render and lime wash system to the wall.

The following set of photographs take you through the renovation and reinforcement process applied to the wall.

 

Wall condition on initial survey illustrating cracks and de-bonding of rendered finish.

Close up photo of de-bonding render.

Test patch of render strip over a crack exposing brickwork substrate and illustrating problems to come. 

A period of several months past between initial survey and start on site, and as can be seen from this illustration the property was loosing its render and becoming a danger to the public and their vehicles.  

 

View of rear elevation showing unsympathetic historical repairs to brickwork. Note the poorly capped parapet wall that does not provide a drip each side for water to be shed clear of render or lead flashing.
Front and rear elevation corners required reinforcement of cracks with Thor Helical Remedial stainless steel reinforcing bar.  

 

Above and below a series of photographs illustrating the destructive effect of trying to remove cement render from soft red bricks. In addition to damaging brickwork when removed it can weaken the lime mortar bonding bricks together. Part of the problem associated with this wall is the lack of parapet coping stones and adequate verge detail to the roof providing drip features that shed water away from the wall.

 

Unless a cement render has de-bonded of its own accord it will take at least the face off of a soft red brick.

Here we have an example of where the lime mortar is weaker than the brick resulting in the almost complete removal of a brick that is bonded to the cement render. 

An example of cement render systematically removing the whole face of bricks and in some cases part of the brick.

 

 

When we started exposing finishes on this building historical structural repair was evident in the form of mild steel straps tying the wall to first floor joist timbers to alleviate outward movement of the gable wall, so it had been an ongoing problem. We had initially allowed for Thor Helical Lateral Restraint Ties into floor and ceiling joists as from experience we knew that a wall of this height and age would only be restrained by internal wall partitions connected to it via a weak brick / lime mortar bond. Upon removal of the cement render the wall was found to be in a far more critical state of repair than first envisaged requiring the addition of Crack Stitch and Grout Ties to reconnect the external gable wall with internal partition walls, plus large areas of the gable wall were judged to be too weak to take reinforcement and required rebuilding. As a further precaution stainless steel expanded metal lath was fixed to the wall over major cracks to reinforce those prior to re-rendering in lime.       

 

As can be seen from the photographs below a considerable amount of the original wall had to be taken down and rebuilt. Although modern concrete blocks were used to minimize rebuild cost they were bonded with an NHL3.5 lime mortar.

 

 

Roof line with verge tiles stripped back ready to take new verge board.

Roof line with verge tiles stripped back ready to take new verge board.

Laying of first coping stone on parapet wall.

Completed coping stone cap to parapet wall. 

 

As stated above part of the problems for this wall stemmed from the fact that the parapet and verge details had been poorly finished in that they did not allow for water to be shed away from the wall. To remedy this problem, apart from removing the soft red bricks, we fitted concrete coping stones to overhang the parapet wall to provide a drip on each side and a waterproof cap.

The roof verge detail had been poorly executed at some time in the past whereby no verge board had been installed following a re-felting of the roof in a breathable felt, and a cement edge fillet fudged in between tiles and roofing felt relying on the render underneath to support this finishing detail. The result being that there was very little, if any, overhang of the verge to provide a drip away from the wall. To solve this issue a verge board was installed to extend the roof line away from the wall by 50mm. The roof covering is single Roman tiles which left little room for maneuver as this type of tile interlock with very little lateral tolerance requiring one leading edge replacement of single Roman tile with a wider standard tile to cover the extension of the verge.    

 

Completed parapet coping stone detail.

Verge detail awaiting final infill of lime mortar.

Verge detail illustrating overhang prior to render application.

Verge detail illustrating partial final infill with lime mortar. 

 

Completed application of Thermalime render product. 

Lime render returned onto rear gable to provide additional stiffening for that corner.

Lime render after Yellow Ochre lime wash application.

Lime render after Yellow Ochre lime wash application.

 

A lime render product called Thermalime supplied by the Anglia Lime Company was chosen by the client because it not only has breathable properties, but as the name suggests has greater thermal insulation properties than a standard lime render. All major holes and indentations left by the cement render removal were filled in with a standard lime mix prior to the Thermalime product being applied in a two coat finish. Following this a four coat Yellow Ochre lime wash was applied to provide a breathable but waterproof finish.      

 

A four coat lime wash application provides a breathable waterproof finish to the wall.

 

Restoration of Lime Rendered Finish to Monastery Court, Elm Hill, Norwich

Photographs depict renovation of a gable wall adjacent to the River Wensum. Works involved infilling of large wholes and deformations within wall followed by a three coat lime render to provide a flat surface to which a three coat burnt sienna pigmented lime wash was added to produce the desired finished colour.

 

Overview of workface

Rough finish prior to render application

View of rough brickwork to be rendered and stainless steel stop bead at corner of wall

Eroded brickwork and lime pointing to be coverd by render

 

Finished wall with burnt sienna lime wash. Grey finish below is a cement render from a previous construction that the client just required a patch repair on

Finished wall complete with lead flashing at top to provide weather protection

Overview from across the River Wensum

 

Lime Rendering

Resurspec Structural Repair Specialists Norwich Norfolk Thor Helical Remedial

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