Silvertop Ash Cladding on Montacute

The timber used to clad Montacute is Silvertop Ash. This comes from Radial Timbers. I looked at a whole range of cladding. Weatherboard is traditional and cheap (about $25 a square metre) but not at all durable and I used it in unimportant places like the side of the carport. There are some very fancy timbers like Queensland Spotted gum that seem very stable but cost over $100 a sq. m. (about 110 mm cover). In the end I used this for the front and bacl doors which were made by Pickerings Joinery in Geelong, who also did the windows. These doors are incredibly heavy (although there are only two layers of spotted gum cladding on a steel frame) and it took all three carpenters to hang them. But they feel really good and solid. I hate to think what would happen if the whole building were clad in it. Cypress pine is cheaper but nasty. Cedar is very, very expensive and because of the cocktatoos everywhere not a good choice. (Not that the cockatoos are choosy: the ate halfway through one of the treated pine trusses, but seem to have balked at the Accoya battlements.) There is a whole range around $100 per sq. m., including Accoya. This would have been a serious choice if it had been half the price. The obvious material, treated pine shiplap, is strangely expensive. I used it for the eaves and behind the modillions, putting it on back to front, as I needed a durable timber and did not want the emphatic lines of shiplap. The usual material for such places, cement sheet, would not have taken well to having all sorts of things nailed into it and is hard to finish well or to repair. But all of these shiplap claddings were too wide for such an intricate building. Narrow boards look better.

I went for the Radial Timbers shiplap boards this because they have a shiplap that is 70 mm wide which you can screw through the tongues and conceal the fasteners. It comes in other widths (e.g. 90 mm) which are thicker that for some reason are not supposed to fixed with concealed concealed attachments. Perhaps this is because I have heard of whole walls of it or something similar just peeling off because the nails just can’t hold it. Hardwood cladding like this is very strong and although it is supposed to be seasoned or kiln-dried it still moves a lot and cups easily. I had experimented with this on the Gothic Fabrique, where I used 7 gauge stainless steel square head (Robinson) screws fixed through the tongues when possible. (On the front face I couldn’t use concealed fixings because of the pointed shape, as the boards had to be fixed from the top down.) Although these did not split the tongues they were difficult to use as they sheared off easily and it was easy to strip the head.

Ben the carpenter wanted to use stainless steel screws by Würth. These have Torx star heads, two different threads, and rounded heads that sit just below the surface but are not flush. They are about 65 mm long. I cannot find them on the Wü website. I have used them and they are absolutely brilliant. You have to be very clumsy to strip the heads. They don’t bite that easily but when they do they pull away strongly. Although they are at least 8 gauge and possibly thicker, they cut their way through the hard and rather split-prone Silvertop ash without any splitting. So all the cladding on Montacute is put on with these. It was fairly slow work, as the boards are narrow any had to be screwed to every stud, and because the design is dense the studs are much closer than standard. I am sure that cladding could never have been attached without these unless we had gone for face mounting. Ben had to be careful not to put the first one to tightly down on the flashing, as it was a wet winter and they expanded more than expected. We had to remove several and trim them to give a 10 mm clearance.

Silvertop Ash is heavy in tannins. The painters put on a tannins-suppressing oil undercoat but after they had finished the top coat there were tannins streaking everywhere over the new paintwork. It is still strongly visible even now around the air-conditioning heat exchangers where, because the painters were faffing around, I could only slap on one coat of primer before the airconditioning people arrived. The Dulux rep happened to be there and we worked out that the tannin was not coming through the paint (which was what mattered to him) but from the unpainted tongues. For this reason the tannin streaking was worse after rain, when the wind had pushed the rain up into the tongues and grooves and caused the raw wood to release its tannins which ran down over the painted surface. With a bit of washing down and touching up we got rid of most of it, but I have to be careful about applying a pressure spray to the walls or it will start up again (Fig. 1).

On the carport, which is being finished with a clear wood finish, the boards have had a year and a half to lose their tannins and are mounted vertically, so I don’t expect the same problem, which in any case would not be noticeable.


Fig. 1. Tannins coming out of tongue grooves when washed.


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