How to repair stone mullions?

I spotted some fantastic before and after photographs of stone mullion repair work, so I thought I’d contact Paul Helmn, the master behind the stone mullion refurbishment and ask a few questions.

What is a mullion?

A mullion is a vertical section which partitions off parts of the window as seen below.

stone mullions

How did you get into repairing stone mullions?

I’ve been a plasterer since leaving school in 1979. My father and grandfather were both plasterers – so I was doomed. I’ve been lucky enough to live and work in the UK, USA and Australia, so I’ve learnt different techniques along the journey. I went into the repairing of stone mullions because I bought a house to renovate with rotting mullions and was flabbergasted at the prices being quoted to replace.

What is the main cause of decay to the stone mullions?

Nearly all stone mullions, heads and sills have been previously painted. Painting stone is not recommended and will be a factor in the rotting process although in my opinion, not the main cause. The main cause is water and although it keeps us alive it is the enemy when it comes to stone mullions. One thing homeowners need to keep a look out for is plastic trim around the mullions and window frames. This plastic trim is usually covering large voids and is one of the biggest culprits in letting water in. Mullions are not solid but more of a U shape so their weakest points are where this trim is positioned. Once water gets in, it cannot escape through the stone as it’s been painted so the stone mullions start to disintegrate.

What method do you use to repair the stone mullions?

Currently the finish I use benefits from being painted but it’s a product made to accept paint and the old stonework facade is totally covered by approximately 4mm of a polymer modified render. The stonework and old paint receives a sealer to stop the rot in its tracks at least from outside. All plastic trim is taken off and replaced with a render stop bead against the frames so you get a cement finish up to the frames which is the correct traditional look. When you look at how rotten some of my projects are, the transformation is quite good if I do say so myself. The cherry on the cake for the customers is how much money they save and considering most of my customers wouldn’t be able to afford replacement it’s a fantastic option. I will shortly be offering a white option that won’t require painting.

How long will the stone mullion last after refurbishment?

In many ways, the purists would have a heart attack about how I refurbish stone mullions but considering they’re already badly damaged, I breathe new life into them. I certainly extend the life of them for a fraction of the price of replacing. How long will my refurbishment last is an unanswerable question, I did a refurbishment over 5 years ago and it still looks as good today as the day it was finished.

stone mullion

After refurbishment, what maintenance do the stone mullions require?

As for ongoing maintenance, the main thing is to check downspouts and guttering above are clear, also keep the window frames clean.

Are there any stone mullion repairs you wouldn’t do?

If a homeowner has natural stone that has not been painted and needs repairing, my system is probably not for them. I also wouldn’t work on listed buildings or those in a conservation area.

What was your favourite refurbishment?

I’ve completed quite a few projects that I’m proud of from a skill set side of things, but my favourite stone mullion repair was for a customer who had big holes in the mullions and had been quoted over £4,000 to fix. She was an older, single lady and she was worried about what I was going to do as my quote was £450. I told her that no money exchanged until she was happy, so she let me loose. She got very emotional when she saw the finished article.

Do you work nationally?

No, I cover the Fylde coast, Lancashire.