8 September 2016
I writing to you on the issue that is occurring in North Donegal, the recent media have focused on the concrete blocks. As an engineer who regularly visits houses, I found that the choice of render along with the designed block strength have a adverse effect on masonry walls in areas that are designated as having severe weather exposure. In my research on the render I found the following:
- I.S 325 part 2:1995 ‘Code of practice for use of masonry. Part 2 Masonry Construction’ gives advice on weather exposure and the entire Donegal/Inishowen peninsula is in a severe exposure zone with a high driving rain index. There is a guidance in respect of the required render specification being its type and suitability to the exposure conditions. Smooth render “woodfloat” is not recommended.
- B.S 5262 :1991 outlines similar recommended codes of practice for areas of severe exposure again smooth render “woodfloat” finish is not recommended.
Both codes of practice pay close attention to the drying time of each layer and the significance of this in the prevention of cracking. Speedy onsite work now, and in the past does not allow for this drying time which inevitably will cause render cracking.
The specified and recommended roughcast, buttercoat and tyrolean render type for severe exposure conditions are coarse by nature and this rough texture helps dissipate the rain water in various directions therefore not concentrating the water in one location.
The woodfloat (smooth render) type dose not dissipate the rain water and this water can run down a wall and be drawn into cracks by capillary action. Thus the material behind the wall may become very wet and the trapped water cannot escape readily if the plaster is strong and impervious and can lead to frost damage if subjected to prolonged low temperatures as seen in the winter of 2010.
Houses are better insulated in the last 20 years and less heat can easily escape past the insulation whichin turn will prevent the external block drying out. Previously houses were poorly insulated and heat loss through the cavity was high. This lost heat helped dry any masonry that was saturated on the external leaf. Now that leaf is constantly left saturated.
Traditionally rough texture render was used in the past but from time to time woodfloat had been used which contained lime. Over the last 15 – 20 years woodfloat has become more common and with the introduction of plasticisers instead of the traditional lime, the self-healing “Autogenous Healing” properties of the lime was lost.
Lime accommodates movement but plasticisers do not. Like lime, roughcast renders have been superseded by woodfloat renders with plasticisers which are neither suitable nor have crack reducing properties. Unfortunately, there is little to no standards to adhere to in relation to render but if one where to take note of the BS 5262:1991 the render choice for Donegal would change drastically.
Please note the issue with crumbling blocks is mainly occurring in smooth rendered houses.
A further good bit of reading was produced by Donegal County Council on this matter is Here. The media are focusing in on mica in the rock and how this has an effect on initial strength but they fail to mention the internal blocks of the house which from all tests carried out are sound.
The purpose of mentioning this to you is that from inspections, I can conclude the workmanship on a lot of theses self build houses is awful and the need for strict building control is necessary, our neighbors twenty miles over the border have strict guidelines which are adhered too and I feel we need them too.
Other posts of interest: