Graham, you have built an excellent shed and the documentation if anything outdoes the build which is excellent. I am in the final stages of completing a 105 sq. ft. extension to the rear of my garage and was beginning to think my budget had run away but believe I have built it to a very high standard, perhaps at a much lower cost per sq. ft. mainly because I have very cheap high quality timber suppliers.
One wall is formed from the garage wall and a second wall has 18 inches formed from another wall I have not had to build which obviously saved on cost but against that I had to lay the slab (4 inches mainly but 6 inches where I have inserted very thick walled steel sections to take a leg vice and as immovable holes for bending). The base layers are DPM, 2 inches of Kingspan,Vapour barrier . 4 inches concrete with steel mesh). I used modified metal post stubs for supporting the 3 X 3 verticals,set into the concrete. I have also fireproofed the entire structure (inside and out ) with Ecosol (excellent service technically and always next day delivery to me in Scotland) and treated all tongues and grooves ,newly cut faces and the main structural members (all 3 X 3 s ) with Jotun Visir. I think this is probably the best rorproofer altho whether it does what it says on the tin I can't tell you as its all in Norwegian)..
I obtained 36 mm tongue and groove cladding (finished thickness about 30mm thick) cladding, excellent quality, Scandanavian grown timber (would have preferred Siberian) but as I paid £12 per sq. metre I was happy to settle for Scandanavian). I would have preferred lapped timber to T and G as I would have then controlled the overlap but the T and G price was too tempting. My initial plan was to use tanalised timber but I got fed up with arranging a load to Scotland from the Yorkshire tanalised timber mafia. This proved fortunate in hindsight as tanalised timber is expensive and difficult to fireproof and at £12 a sq. metre that pays for an awful lot of visir. All timber fixings are screws or ss ring nails (60mm) My bottom layer of cladding consists of thermalite blocks, profiled by me, they cut very easily , to take a sloping ,watershedding, fibre cement cladding (expensive but rot proof and slightly more insulating than timber.All thermal bridges,such as the metal post stubs have been "broken " with kings span. I have 3 inch kings span for the pent roof (8 ft 6 inches falling to 6 feet 5 inches) and 25mm Kingspan for the walls to be installed this winter. I have still to make the smallish windows (security) and doors (a double door about 5 feet wide.
One wall has two zigs and a zag to preserve the view from my lounge window giving 6 corners in all all of which I have mitred for extra security (i did buy a compound chop saw for the job which makes good mitre joints quicker than hand sawing butt joints).
The roof is clad in half inch sarking, fireproofed this twice, and will then have two mm glass fibre mat with bitumen "torched on" as my roof slope is only 12 degrees (the mat is meant for flat roofs) which will be covered with Wickes felt shingles. under this as stated 3 inches of kingspan for the highest half of the pent roof and two inches for the lower half to be installed before Xmas,I hope.
I have procured all the materials except , double glazing panes,Butinox 3, window fittings. locks and bolts..electrical fittings,compressed air fittings (the zigzag wall gives a convenient housing for a vertical compressor, sound insulated by the kingspan. ) Probably a few other bits as well. Visir is a bit expensive and slow to apply to the main area of cladding which will have to be content with a double spraying of modern gutless wood preserver followed by 3 coats of Butinox 3 (12 years life they say even in Scotland–but pricey ).
I have all receipts and had avoided doing a total due to a suspicion I had run over my estimate of £2000 but will do an add up when I have procured all the fittings I have still to get). There are also complications in that I bought enough cladding to build my grandkids a hut so they can sleep out on the tree platform in summer -Has to hold 4 ) and I have vast quantities of DPM ,vapour barrier, and ss nails left over plus 4 fibre cement boards for the base of my other shed)
I think any saving I might have made is probably due to the cheapo prices I got for the cladding, other timber (I paid the guy £20 more than he asked as he's my last remaining cheap timber supplier and I don't want him going out of business. ) I believe the concrete was pretty cheap as well at £200 for cash , which included a man to help with the borrowing, use of barrows, and was mixed to my specification in one of those posh new lorries that mix it on the spot, so you only pay for exactly what you need. The man who supplies my timber (not the same as supplied the cladding) always supplies top grade timber and last time I costed some from him I worked price out per cc (to compare with Wickes as I could n't have got timber that size from Wickes) and it worked out under one sixth yes one sixth, Wickes price. Wickes by the way supply better timber IMO, than the other chain stores ,and are by no means the dearest.
I may yet get a shock when I do the final addition but my gut feeling is that for 105 sq. ft it will come in at under £2500 of which the Visir (4 litres at about £15 a litre), fireproofing (Eco-sol WD 15 litres at about £80 for 5 litres ) and the Butinox 3 (still to be procured) are not insignificant contributors.
The shed is 11 feet 6 X 11 feet , but only 105 sq. ft due to the zigzag wall.
erdLL HAS TWO ZIGS AND Apost stumetal