A loft conversion diary: what I planned, what I found and what I wish I'd known

Hi all, I'm a long-time BuildHub lurker and stealer of others' good tips. I started a loft conversion in April this year and I've been meaning to start a BuildHub diary for a while, partly because I've found the forum enormously useful both prior to and while doing it, and partly because I've ended up learning more about loft conversions than I ever intended to and it may help others in future.

This isn't necessarily going to be a 'look what I built' thread (albeit I'll happily pepper it with photos where appropriate). There are plenty of those and, while they're very useful, I think my personal experience on this project is potentially more valuable in documenting the decisions, problems and compromises that happened between the drawing and the finished building. One of the biggest things I've learned is that you can plan a project to death and still have absolutely no idea what you're going to find when you start cutting holes in it.

The project

We have a detached house, built in 2001 on a typical family housing estate, with a Fink truss roof and the project is a loft conversion with a full-width rear dormer. I wrote and submitted the full plans Building Regulations application myself, mostly because I have been the primary builder and so I wanted to design something that I was comfortable actually building. Just on me briefly - I'm not a professional builder, but it's something I've been doing for a long time and I come from a proud lineage of stubborn people who would rather mess something up themselves and learn from it than pay someone else to do it. It's a dangerous mindset to inherit.

The loft conversion structure itself is based on the EcoTrus light gauge steel system, rather than the conventional approach of using steel beams and timber joists. I chose this for a number of reasons, which I'll detail in a later post, but the primary one was because it was a system that I could build largely by myself with minimal heavy plant and additional hands.

There was also quite a lot going on around the LSG structure: external wood-fibre insulation, mineral wool, a warm-roof approach but with a ventilation layer, a full-width dormer, new rooflights, new stairs, fire protection, upgraded acoustic performance, ventilation and a new bathroom downstairs.

The application therefore became a fairly substantial document covering everything from structure and fire safety through to thermal performance, ventilation, drainage and electrical work. I took several months to put it together and was quite pleased with it when it was done, and it passed inspection without any clarifications. I genuinely thought I'd worked through most of the major construction issues before I'd even got started...

The illusion that you can plan everything

Glibness aside, I'm not trying to say the planning wasn't worthwhile - it absolutely was. I think it has genuinely saved me enormous amounts of time and money so far. The Building Regulations drawings gave me something to work from. The structural calculations gave me a defined structural solution. The thermal details established how the insulation and airtightness strategy was supposed to work. The roof details dealt with things like ventilation, membranes, flashings and the dormer and the interfaces between them.

But the fundamental problem with trying to design an alteration to an existing building to the finest degree is that you don't actually know what you're dealing with until you expose it. I drew multiple scaled CAD illustrations, but ultimately there are parts of the building that haven't been visible since the day it was constructed. A good example of this was the cavity wall and the height offset between the wall plate and the brick outer skin - that relatively small piece of unknown geometry suddenly has consequences for several other things. Where does the fascia go? Where does the soffit finish? How does the gutter sit? How does that affect the external insulation plane? How do the various layers meet? How do you deal with the junction without creating an ugly or unnecessarily complicated detail? Those aren't really Building Regulations questions. They're 'you're standing on a scaffold looking at an actual building, what are you going to do now?' questions.

The value of my time

Over the last six months, I've often found myself weighing up several possible solutions to an unexpected problem and often spending far too long chasing the neatest or most technically elegant answer, particularly because it is my own house. I'm gradually learning that the perfect solution isn't always worth the extra time and complexity.

My time has value too. Sometimes a straightforward solution that works well is better than spending hours trying to make everything perfect.

I'd like this diary to cover both sides of the project: what I planned, using my Building Regulations application as the starting point, and what actually happened during construction. I'll cover the EcoTrus structure, insulation, roofing, windows and all the inevitable surprises along the way. If I waffle too much, please tell me!

Hopefully, it'll be useful to others considering a loft conversion, and an honest record of what happens when detailed plans meet the reality of an existing house. I've added a few illustrations of the project to date to set the scene.