The Concrete Hollow Block House

Thanks @Dave, do you have any pictures
?

That's the extension plan above:

Probably get 2 upright for the wall out of 1 C-Purlin - so very roughly assuming wall width is (14m) 550 inches:

20 C-Purlins for the walls plus 60 pieces of Hardiflex => 20 x P572 + 60 x P 1131 = P79,300 ?

Hi John, yes I could send on some Pics but it would be easier to do this by email here is mine; ddenholm@btinternet.com,
This is quite a small extension you are doing and with two cross bracing interior walls this will be very stable, for a plain internal wall you can use the 3''x2''s but still of 1.2mm thickness, clad with 4.5mm Hardie this will be strong enough, also you would be OK to use the 4.5mm Hardie on the inside of the exterior walls, whatever you do use for this go for one of the cheaper brands for the interior and only use the Hardie brand for the exterior, there will be around P200 difference in price.

Tools required; about the most efficient way to cut this cement fibre board is to use a Grinder with diamond disc, in my build of 100M2 we only used two of these they last forever, to cut the steel we used a 14 inch Chop saw of Lotus brand P6K on line, however a Grinder with a 125mm disc will do it but it takes that bit longer, for a small extension that is all you need, the 4x2 channels have to be adapted and the flange of the top and bottom rails checked out, this gives a very tight fit and you can build a complete wall without any fixings in this steel, the fixings are the self tapping countersunk Hardiedrive screws, when the cladding is fitted, you aim to get this screw through the Hardie and into the join between the steel studs, we always had two grinders on the go and two batt drills, when one of these failed halfway through the job I bought a cheapo on line for p1,500, this fired in a few thousand screws and did not fail us, however you do need one really good one and I favour De-walt, steel cutting discs are between p35 and P50 to buy local, you can get a box of 50 Lotus brand on line and this worked out at P11 per disc you will need 15 to 20, when I was stuck I bought two Bosch discs at P79 they were no better than the Lotus, another brand I would recommend for tools is Tolsen I only ever saw them on line.

Filling and fairing the joints in the Hardie, the method that worked for us was to leave a minimum gap of 2mm between sheets I think 3mm is better this was filled with concrete Epoxy, (Island brand I found to be the best for this) all crew heads were also coated with this and the final finish was a Hardieflex putty, both these are applied with a basic putty knife available from all the hardware stores and only around P10 to 20 each, 4inch for the Epoxy, this allows you to press it well into the joints and 6inch for the putty where less pressure is required, any sheets that are butted hard together will crack later on, this applies to just about any crack you must open it out before any insertion, this same epoxy is good for a wood glue and many other things with the exception of plastic.

Something I forgot to mention in reply to your concerns regarding the strength of the steel stud and hardie construction method and that is Plywood box beams, these are often used in house construction as a substitute for steel beams, easier to work with than steel, easy to attach to and have amazing strength to weight ratio, so the SS&H method has similar properties and could be used in the same way as in the likes of roof trusses, if you were to google box beams you will see for yourself just how functional they, are using steel and hardie your walls are just tall box beams.

Thanks for that @HONEST DAVE

Just to clarify - not two cross bracing walls - although that was partially my thinking, a half and two arches from another angle:



thinking of support for the roof more than the interior.

I will send you an email.

Anyone interested I do the planning and pictures using

Sweet Home 3D

Sweet Home 3D is a free interior design application which helps you draw the plan of your house, arrange furniture on it and view the results in 3D.