Why Your Cask Ale Is Flat

Last updated: 29 June 2026

Most licensees think flat cask ale is just bad luck—a dud from the brewery. It isn’t. Flat ale is almost always a cellar management failure, and it’s costing you money you don’t realise you’re losing. In my first three years running the pub, I served flat cask three or four times a month without understanding why. Once I started tracking cellar temperature, line cleaning, and cask age properly, it stopped. The problem wasn’t the ale. The problem was me not knowing what was happening in the cellar.

Flat cask ale is a symptom of poor cellar conditions, not a brewery fault. When you understand the five real causes and how to measure them, you can eliminate it—and claw back margin that’s disappearing into waste and customer complaints. This article walks you through exactly what causes it and what you need to do differently.

What Actually Causes Flat Cask Ale

Flat cask ale is caused by five things: cellar temperature above 55°F, CO2 escaping from the cask, dirty beer lines, cask damage or age, and poor measurement of what you’re actually serving. The brewery isn’t sending you flat ale. Your cellar is making it flat.

When I first took over the pub, I blamed the brewery every time a cask came out flat. I’d ring them up, complain, and they’d replace it. Then I realised the replacement went flat too, at the same rate. That’s when I started looking at what was actually happening in my cellar, not at the brewery’s warehouse.

The reason most licensees don’t catch this is simple: they don’t measure anything. They open a cask, start pouring, and hope it’s fizzy. By the time they notice it’s flat, twenty or thirty pints have gone out already, the customer’s complained, and the margin’s gone. You need a system that catches flatness before it reaches the tap.

Temperature: The Most Common Culprit

Real ale should be served at 50–55°F. Most pub cellars run 2–4°F warmer than that because the cellar door opens to the bar, the pub heating’s on, or there’s no temperature control at all. Every degree of warmth speeds up carbonation loss and yeast activity, flattening the ale faster.

The most effective way to keep cask ale properly carbonated is to maintain cellar temperature between 50 and 55°F at all times, checked daily with a thermometer you trust. If your cellar is running 57°F or higher, carbonation will be noticeably lower within 48 hours of putting a cask on tap.

Here’s what I do: I have a basic wall thermometer in my cellar that I check every morning before service. Takes thirty seconds. If it’s creeping up, I know I need to crack a window, move stock around to improve air flow, or ring my landlord about the cellar cooling. The difference between checking daily and checking once a month is the difference between serving flat ale occasionally and never.

A lot of pubs try to fix flatness by adjusting the breather or gas pressure. That’s backwards. Temperature is the foundation. You can’t fix a warm cellar with gas pressure.

CO2 Loss and Cask Condition

Cask ale naturally loses carbonation over time. A fresh cask has the right level; a cask that’s been on tap for three or four weeks loses it progressively. If your cask is dented, has a loose shive (the bung), or has a faulty vent, CO2 escapes faster.

I learned this the hard way. I was pulling casks out of storage that had been sat in the corner for months, putting them straight on, and wondering why they were flat immediately. Casks get damaged in delivery, during handling, or just from time. A small dent doesn’t look like much, but it compromises the seal slightly, and gas escapes.

A cask that is damaged, over four weeks old on tap, or shows visible dents should be taken off and replaced immediately, because CO2 loss accelerates and you cannot recover carbonation once it’s gone. Don’t try to nurse a dying cask through the week. The margin lost to slow sales and customer complaints is worse than the cost of pulling it early.

Before you put a cask on, inspect it. Look for dents, check the shive and keystone are sitting properly, and check the date it was delivered. If it’s been three weeks or longer, start watching it closely. By week four, start planning the swap.

Line Cleaning and Beer Quality

Dirty beer lines are not just about taste—they trap CO2 and bacteria that eat carbonation. If your lines haven’t been cleaned properly for a week or two, the ale coming through is already losing fizz before it reaches the glass.

Most pubs clean lines once a week. That’s the absolute minimum. If you’re pulling high volume through a line, you need to clean it twice a week or even daily. I clean my busiest lines every night because I run a high-turnover pub and the difference in taste and carbonation is visible.

The other thing I see constantly is licensees using the wrong cleaning chemicals or rinsing improperly. You need proper beer line cleaner, not general-purpose disinfectant. And you need to run enough rinse water through to flush the cleaner completely. Any residue left in the line will affect carbonation and taste.

Line cleaning directly affects carbonation because residue and bacteria in beer lines consume dissolved CO2, causing ale to arrive at the tap already partially flat. If your cask is carbonated properly but what comes out of the tap is flat, the line is the problem.

Invest in a proper line cleaning kit—a hand pump and the right chemicals—and do it weekly minimum. It costs about £30 to start and saves far more in wasted cask ale and customer complaints.

How to Track and Fix Flat Ale

You cannot fix what you don’t measure. Most pubs don’t know they have a flatness problem until someone complains or they notice sales dropping. By then, you’ve already served dozens of substandard pints and lost the margin.

Here’s what I track now: cellar temperature every morning, cask age and condition when it goes on tap, line cleaning dates, and how long each cask stays on before it comes off. That data tells me immediately if there’s a pattern—if flatness happens with certain suppliers, certain lines, or in certain seasons.

The most reliable way to catch flatness early is to use a StockTap pub stock app or a simple spreadsheet where you log cask details, temperature, and when you notice any quality issues. At the end of the week, reconcile that against your till data and your actual pour count. If you’re serving fewer pints than you should be from a cask (because the customer rejected it as flat), that shows up immediately.

I was running stock on a tangle of spreadsheets and still losing track of partial kegs and when casks went on tap. I built a simple count routine around a dipstick and a set of scales, and the weekly variance went from guesswork to a number I could trust within a fortnight. Once you have real data, fixing flatness becomes obvious.

The Weekly Cellar Check You Actually Need

You don’t need fancy equipment or hours of time. You need a fifteen-minute routine once a week that catches problems before they hit the bar.

Here’s exactly what I do every Friday morning:

• Temperature check: Thermometer on the cellar wall. If it’s above 55°F, note why and fix it (door left open, heating on, etc.).
• Cask inspection: Look at every cask on tap. Check the date it was put on. Check for visible damage. If any cask is over three weeks old or shows signs of flatness, note it.
• Line clean check: Confirm that lines were cleaned this week. If you’ve had customer complaints about taste or flatness, add a mid-week clean.
• Pour count: Dip-stick measurement of what’s left in each cask and compare it to what your till says you sold. Big gaps mean either waste or measurement error—both cost you money.

This takes me twelve to fifteen minutes. At the end of the week, I have data. If flatness has been an issue, I can see exactly where and why. If a cask is losing carbonation faster than it should, I can swap it. If a line is causing problems, I can clean it again.

The number that actually matters is wet gross profit by line, not a single headline stock figure. Draught hides losses in poor cellar temperature and bad line cleaning waste. Once you start tracking cellar conditions weekly—temperature, cask age, line cleaning—flatness becomes preventable, not accidental.

Most pubs that move from a messy spreadsheet to a disciplined cellar count claw back 1–2 GP points within a couple of months. That’s because they stop losing margin to flatness, waste, and mystery gaps.