Every one of these has been made by someone before you. Read this, then don't make them yourself.
These aren't invented. These are the actual, genuine, "oh God why didn't anyone tell me" mistakes that people make and then spend years living with. Forewarned is forearmed.
If your base isn't square, nothing else will be. Walls won't fit, the roof will be a nightmare, doors won't hang right. Always, always check the diagonals. Spend ten extra minutes getting this right and it pays dividends throughout the entire build. It's the most common mistake and the most consequential.
Laying slabs or a concrete slab directly on bare earth. The earth settles. The slabs move. Your shed ends up twisted and damp. 100mm of compacted type 1 hardcore under everything is cheap insurance and takes two hours. Don't skip it.
This is a "save money now, pay later" mistake that always comes back to bite. Untreated timber outside in the British climate will start rotting within a couple of years, particularly at the base and anywhere water can pool. Always use tanalised (pressure-treated) timber for any outdoor structural work.
A lightweight shed can actually be lifted by a good gust of wind if it's not secured to the base. Use frame anchors or hold-down straps. A shed flying across the garden is genuinely dangerous and embarrassing in equal measure.
The green lightweight felt that comes in 10-metre rolls from B&Q is rubbish. It cracks in frost, tears in wind, and you'll be replacing it within three years. Spend a few extra quid on mineral felt (look for the one with the mineral chips embedded in it). It's not even significantly more expensive — just better.
The diagonal brace on a ledge-and-brace door must run from the bottom hinge corner to the top latch corner. Put it in the wrong direction and instead of supporting the door it actively helps it sag. This is one of those things where getting it wrong is as much effort as getting it right — so just get it right.
Everyone does this. "I only need a small shed." Two years later, you're standing outside it in the rain trying to find the lawnmower because there's not enough room to get past the bikes and the pressure washer. Build bigger than you think you need. This is probably the most universal piece of advice on this entire site.
A sealed shed is a condensation factory. Tools rust, wood swells, everything gets damp and musty. Fit a small vent high up on each gable end — even just a louvre vent from the DIY shop. The cross-flow of air makes a massive difference to the internal environment of the shed year-round.
The face of your cladding gets painted. The back doesn't, because once it's up on the wall you can't get to it. Except moisture gets behind it and the back rots while the front looks fine. Before fitting each board, give the back and all cut ends a good brush of wood preservative. It takes minutes and doubles the life of your cladding.
This might sound daft but it happens. People get excited, order the timber, and then start building in October in persistent rain. Wet timber is heavy, slippery, unpleasant to work with, and warps as it dries. You want dry weather for the build — even if that means waiting. A shed built in good conditions is better than one built in the wet.
Run through this before you start building:
The FAQ's got you covered. All the common questions, answered honestly.