28 Dec 2012

House Rendering Bristol – Case Study

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When we inspect re rendering jobs we look for reasons why the render has failed and what we can do differently to prevent it happening again, whilst also making it visually attractive.

We were first called to Gladstone Street to patch up some render which had blown, and was about to fall away. On close inspection we noticed that all the render was in poor condition and needed replacing.

Large cracks could be seen running through the render and tapping it produced a hollow sound, which indicated it had ‘blown’ and was no longer bonded to the brickwork.  On our recommendation, the customer decided to have the whole elevation re rendered.

When we started removing the render we realised the extent of the job. The brickwork underneath was in a terrible condition; it was soaking wet and crumbling to the touch. It became clear the outer skin of this solid wall would have to be rebuilt.

The brickwork had sustained damage over 20 years and it was clear there were a few contributory factors.  Firstly the coping stones on top of the wall were not wide enough and had not been maintained; there was no mortar in the joints between them, and no DPC underneath, allowing water to pass down through the gaps into the wall. The drip grooves on the underside of the copings had been bridged with render, allowing a passage for water back into the wall; wider copings would have prevented this.

Secondly the render had been applied too thickly, in some areas 2 inches thick, and the mix was too strong for the brickwork.  Excessive thickness causes weakness in the render especially whilst curing, as it cracks allowing a passage for water.  A 2-coat render should not exceed 20mm thickness.

The strength of the render should never be stronger than the background it is being applied to; in this case clay bricks. If the mix is too strong and water penetrates the render, it cannot escape due to the dense nature of the render mix. During freeze/thaw cycles the weaker substrate will be damaged. This was the main cause of extensive damage at Gladstone Street; water had been trapped behind the render for years, gradually destroying the bricks.

We removed all the damaged bricks down to a solid course of bricks, and then rebuilt the wall with concrete blocks. Wider coping stones were bedded on DPC on top of the new wall. New lead flashings and stainless steel bell cast were installed above the lean-to.

We re rendered with Weber materials to specification. A first coat of Weber Rend aid was applied with crack resisting mesh embedded. Weber OCR 3 was used for the second coat, a 1:1:6 (cement/lime/sand) render mix that is factory blended for consistency.  For the final coat Weber Cullamix Tyrolean was applied which is a pre coloured decorative finish that does not require painting.

Our experience in general building allowed us to recognize the faults in the original works, and to rectify them. Having carried out the works with good building practice, using quality materials ensures that the customer will have a long lasting, protective and attractive render.

Do you have cracks in your render? Does your render sound hollow when you tap it? Has your render come away from the wall? Are you getting damp patches internally?

If so, call ADCAR Bristol rendering specialists today for free advice/quotations. Freephone 0800 862 0201 or via email using the online contact form.

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