Classic workshop build - novice questions

[shedk]

Hello - a collection of rambling thoughts and a question or five!

My intent is to create a 5x3m outbuilding to be initially used as a shed/shop, and later with a stud wall down the middle to split into a home office once we run out of space in the house.

I was looking at pre-made log cabin type sheds but they’re quite expensive, and the builds here are quite inspiring. I have read through plenty of threads here (well - so far!) and both Mike’s guide and later build’s cross-sections are in my notes. I’m working through DBT85s thread.

I had some questions I was hopeful for advice on. My basic idea is the standard several courses of red brick, timber frame (2x4s), cladding/battens/membrane/insulation/OSB. 2.5m ridge, 2.1m eaves. Double doors one end, single uPVC door on the other. Replacing where I think an old potting shed structure used to be near the garden wall.

Base

Concrete slab (eg. Mike’s design) vs strip/trench footings + oversite?

• If slab, I see in several threads there are reservations about tying in the DPM under the slab with the DPC, but does the cross section not call for a separate floating floor that has it’s own DPM?
• If trench/strip, is there any advantage to blockwork to take from strip to ground level, or can I trench fill to ground level?
• I’d like to bump insulation to 100mm. Presumably if slab this would go under whole slab, or if strip then under the oversite as hardcore, sand, dpm, insulation, concrete?

Brick courses

• Half-brick or one-brick?
• If one brick, is there any advantage other than cosmetic to eg. 3 courses vs 6 (with either one or two rows of block)

Doors

• Thinking shed doors one side and a uPVC door the other. Should the uPVC door sit on a course of brickwork with DPC under? Obviously if doing strip/trench, but if slab?

Roof
Again novice, and this is a mixture of what I have seen or can Google. Would appreciate confirmation or whether I’m on totally the wrong track.

• My plan is to finish with box-profile metal roofing, ~5kg/m2 which looks like it can be attached straight to purlins.
• The span is <2m - I found some lookup tables here suggest I can use 44x95 for this but this wouldn’t fit 100mm Kingspan, so figure 44x145 instead would be acceptable.
• My intended design was the fairly (I think) standard rafter/ridge board/tie at 600mm spacing, and then purlins on the outside for the roof sheets. I have seen comments in other threads about a central principle trusses to avoid needing low ties everywhere, is this a thing I can do? I’d like to maximise headroom.

Thanks for any advice! I appreciate some of this may be answered as I go through more build threads. I’ll get the area dug out once I’ve settled on a plan. I readily accept that some of this I may have both overcomplicated and under-considered.