John Vanstone, chair of Certass TA, on why an installer’s business can be going backwards, even when the diary is full.

There are few things more reassuring to a window fitter than opening the diary and finding the next six or eight weeks already full.

The phone is ringing, surveys are booked, installations are stacked up, and there is barely a spare day between now and half term. From the outside, and very often from the inside as well, that is exactly what a thriving business is supposed to look like.

The difficulty is that being busy and doing good business are not always the same thing, and anyone who has worked for themselves long enough tends to learn it eventually, usually through hard experience rather than a spreadsheet.

There are jobs that looked perfectly respectable the day they were sold and yet consume far more of the business than the price ever justified. An extra survey visit here, a change of specification there, two hours spent driving back to adjust something minor, a customer who needs six phone calls where another needed only one, or a remake that quietly turns what should have been a profitable Tuesday into a day you effectively worked for nothing.

None of that looks disastrous on its own. Put enough of it into a full diary, though, and you reach the strange, slightly demoralising experience of working harder than ever while wondering where the money went.

This is why I think of capacity as one of the most undervalued assets a small installation business has. If you run three fitting teams, there are only so many installation days you can sell in a year. If you are a sole trader the arithmetic is even more unforgiving, because there is only one of you, and every hour spent revisiting yesterday’s job is an hour you cannot spend surveying, installing or winning the work that pays for next month.

Once you look at the diary in that light, the question of which jobs to take on looks rather different. The highest-value job is not automatically the biggest quote, and the one with the healthiest margin on paper is not always the one that leaves the healthiest margin in the bank.

Distance matters, and so does complexity. The reliability of the products and suppliers behind them matters, as does the likelihood of being dragged back for repeat visits, and, whether we admit it or not, so does the customer.

Experienced installers become remarkably good at sensing these things. They develop a feel for the job that will run beautifully and the one where something does not quite add up, even when they could not tell you why. That instinct has real commercial value, and it should not be quietly overruled just because there is an empty Thursday in the diary that feels as though it ought to be filled.

In fact, sometimes the most profitable thing you can do with capacity is protect it. That is not the same as becoming precious about the work, or only ever taking the easy jobs. A genuinely difficult project can be excellent business when it is priced properly and its complexity is understood from the very beginning. The point is that turnover from using three days of capacity ought to be weighed against whatever else those same three days might have produced.

September is a particularly good moment to think along these lines, because this is when the year suddenly starts to feel shorter. The summer holidays are behind us, customers begin talking about having work finished before Christmas, and diaries can fill with astonishing speed. The temptation, understandably, is to cram everything in while the demand is there.

There is nothing wrong with being busy, and most businesses would far rather have that problem than the opposite one. But a full diary is best understood as the result of choosing enough good work, rather than as the goal in its own right. The strongest installers I know have this sorted in their day to day. They stop measuring success by how many weeks ahead they are booked and start paying much closer attention to what those weeks are worth.

It comes down to something very simple, which is that you cannot manufacture more days. There are only so many in a year, only so many in a working week, and no amount of enthusiasm conjures up another. Once that truly sinks in, saying yes to every job that comes along starts to look a good deal less ambitious than it first appears.