Hi John We have used woodwool boards (cement) on our TF extension on our random rubble building conversion. Beware! We are using Savolit boards, screwed to wood battens at 600 centres, in turn screwed through sarking boards to timber frame. We decided to ignore manufacturers recommendation to fix rectangular boards horizontally and fixed vertically – don’t ask! We used a scratch coat and top coat about 16/20mm total thickness. It cracked big-time😂. Main cracks were along line of vertical battens which probably wouldn’t have happened if boards fitted correctly. However after a few weeks cracks also appeared horizontally along board joints. At this stage we found out we should have used a fibreglass mesh between render layers. We had to remove the latest rendered mortar on one part of buildings for other reasons so had the opportunity to start again. It still cracked although only very fine hairline cracks in both directions but still along board joint lines. We can live with them. Lime wash in due course will hopefully completely disguise them. Anybody have experience of filling render cracks up to 3 mms wide. Although cracked the mortar is very strongly adhered to the render boards so we want to avoid having to remove it. We are pretty sure the cracking is mainly due to boards being mounted on a timber frame which got very wet for extended periods before being covered. We think the drying timber shrunk a lot .
Mike Alcock Fife
Happy to discuss07740679304