The demolition of the wall required careful disassembly, copious note-taking, and almost a roll of film. Notes and photos were taken of the external appearance, and then the render was chipped away piece by piece. This allowed us to work out what was going right with the wall, not just what had failed. As the wall was dismantled, observations were recorded for final analysis, and various pieces kept for further study. Eventually the wall was completely down, what was reusable put to one side and the rest ditched. All up this took about five hours.The one thing that I would like to point out here is that this wall is a freestanding courtyard/garden wall. As a result it is completely exposed to the elements - all the wind, driving rain and other weather that a building wall with roof eave or verandah protection normally doesn't have to contend with. However many newcomers to strawbale construction use these types of walls as their first project, and as a result build a structure that has the most potential to fail. Many of the safeguards that need to be taken with garden walls are not an issue if a roof is involved. So think of this report as the worst-case scenario that a building could suffer. And if a situation as this did arise, then I would suspect a total building failure, not a wall failure. The wall had been built on a concrete footing, with the render skin overhanging the footing. All-thread had been used to secure the top-plate through the bales to the footing. Treated bamboo pins secured the bales to each other. As the wall was an "L" shape, the top-plate had been gussetted at the corner with plywood. Expanded metal lath had been used as corner reinforcing in some places, and chicken wire in others. The top of the wall had been rounded off and reinforced with chicken wire, and bitumen impregnated kraft paper underneath as the waterproof barrier. The footing had been painted with bitumenous paint as the dampcourse. |
Having had time to analyse the results, I can now present the following findings. Because the wall ran north-south and the inclement weather ranged from the north-west to the south-west, the west side of the wall had copped a hammering. It was that side and the top that displayed the most damage. In fact, moisture had been driven through the wall almost to the other side.The render, which was a lime and sand mix, had been applied unevenly and with variations in its thickness. There was separation of the layers in places indicating poor bonding, and the mix had a sandy/powdery consistency in areas. This is a result of too little lime in the mix. Substantial vertical cracking, which is a hallmark of strawbale problems, was evident, due to structural movement in the wall. The render adhered well to the expanded metal lath, but not so well to the chicken wire. The chicken wire had not been tied closely enough to the bales. |
The wall itself had continued to compress after rendering had started, in some places by over an inch, or 25mm. This had not only contributed to the cracking, but because the render skin was not supported by the footing, skin sheer and creep was noticable.The kraft paper in the top of the wall had completely rotted away. In fact very little of it could even be found. The plywood gusset was also rotten with the result that the structural strength of the corner was no more. There was cracking horizontally along the top of the wall where the kraft paper and chicken wire had stopped. This indicated that the render had been unable to bridge across dissimilar materials. The bamboo pins had become brittle, and I suspect that's because they were treated. |
The most worrying aspect were the pools of water found lying on top of the footing. This was causing the bales to rot out at the bottom, and had caused the whole wall to lean. The bottom bales were totally shot. Because the water had been penetrating the top of the wall, it had trickled down to the footing and had been unable to escape. This suggests that dampcoursing is less important than a self drainage capacity.So what are the lessons to be drawn from this? I would suggest that the most important element is patience. Don't rush the building. Give the wall plenty of time to settle and keep the tension on it. If it is a building, put the roof on to add extra load before rendering. Take your time with the rendering. Get the mix right, apply it evenly and give it time to harden before applying the next coat. Make sure that the render layers adhere well to the previous layers. Provide mechanical keying and reinforce at least the first layer with fibre. If required, finish the wall in a water-resistant, not water-proof, limewash. In the case of a garden wall, make sure that the top is WATERPROOF, even if it means putting a tile roof on it as the Spanish do with their adobe walls. Treat it with the limewash as mentioned. Also allow for drainage on the top of the footing. |
From the structural point of view, the weight of the render skin needs to be supported by the footing. The timber used in the top-plate needs to be durable, and where mesh is used as reinforcement it needs to be pulled tight to the bales to avoid gaps and hollows. Drainage of the footing in a building shouldn't be required, but it is something that may need to be considered. There are a number of technical options available in that area.Conclusion - so the wall failed. So what! Any building material will fail if appropriate measures are not taken to ensure its structural viability. Even modern brick homes are not immune to settling, cracking and rising damp. This wall provided a vital piece in the jigsaw puzzle of practical knowledge of strawbale construction. By seeing how something goes wrong, we can then work out how to do it right. For me this exercise confirmed some suspicions, dispelled some myths and fired an even greater determination to provide further evidence that Strawbale construction is indeed a viable form of building. Back to MENU |