Greater London · England
Window Mastic in Underhill
We cover Underhill for mastic around window and door frames, sills and patio doors. Old sealant removed completely, joints prepared properly.
- Water coming in at the bottom corners of a door
- Gaps between the frame and the inside reveal
- Old mastic that has gone black or come away
FAQs for Underhill
Do you work for landlords with property near New Barnet station?
Yes, we work for landlords and agents. You can ring 0345 464 0079 or drop us an email with the address and a few details, and we will give you a fixed price before we start.
Is parking a problem for a job in Underhill?
London parking needs a little planning, so let us know about permits, bays or a driveway when you book. The visit is then planned around what’s available.
Can you work on commercial sites?
Yes, regularly. We can plan bigger jobs in stages.
Do you have insurance?
Yes. Mr Mastic is fully insured, with over 20 years’ experience in sealant work.
Is the new sealant paintable?
Not normally, if it’s silicone. Tell us your decorating plans and we will pick a sealant to suit.
Do you come to Underhill?
Yes. We’re based in Crawley and work across South East England, London and Scotland, and that includes Underhill.
Is it OK to seal over old sealant?
No, it will not last. It may look fine at first, but the new bead soon lifts. Removing the old sealant completely is the only lasting way.
Should I try DIY first?
Sometimes. Preparation is everything: all the old sealant out, and a clean, dry joint. We can always put it right if it fails.
Window mastic in Underhill
Behind a thin line of old mastic there’s sometimes a deep void. Filling it with sealant alone leaves a joint that cannot move properly, so deep gaps get backing rod first. That way the new bead bonds to the frame and the wall only. Common call-outs are for hard, cracked sealant around windows and old mastic that has gone black or come away, on timber windows & doors, aluminium frames and window sills.
Many London homes are flats, with windows high above the street. Who looks after the outside of the frames can depend on the lease, so check that first, and access in Underhill is something to talk through when you book.
Our work is backed for twelve months.
- Place
- Underhill (EN5, N20)
- Wider area
- Greater London
- Guarantee
- A year, on every job
- Price
- A clear quote up front
- Telephone
- 0345 464 0079
- Page updated
- 26 October 2026
DIY versus a professional finish
The difference is mostly in the preparation and the product. We take joints back to clean, dry surfaces, and the sealant is chosen for how the joint moves and where it is, then guaranteed for a year.
The biggest DIY mistake is sealing over old sealant. Silicone doesn’t stick to old silicone, so it lifts and traps water and mould underneath. All the old material has to come out first.
Common problems we fix in Underhill
Water runs down the glass onto the sill, and the ends of the sill, where it meets the wall, are where it gets in. When that sealant fails, water gets into the wall or under the frame.
The problems people usually mention when they call include:
- A bead that doesn’t match the frame
- Hard, cracked sealant around windows
- Gaps where the sill meets the wall
- Draughts coming in around windows and doors
Schools such as St Catherine's RC School are buildings too, and its window and door frames need their mastic kept sound, just like a home’s.
What your quote should include
- Which colour the sealant will be
- What guarantee comes with the work
- What happens if the job turns out different
- Whether all the old sealant comes out first
What happens on the day
Once we’re on site, we compare the joints with what we discussed. If we find something unexpected, we talk it through with you first. Most jobs then take anything from an hour or two to a full day, depending on how many frames there are. Parking in much of London is by permit or pay-and-display, so it helps to know what’s near you in Underhill. A visitor permit or a nearby bay makes the visit run more smoothly.
- First we look at every frame and agree the list.
- We remove the old mastic and clean the frame and the wall.
- Where gaps are deep, backing rod goes in first.
- A flexible, weatherproof sealant is applied in a colour to suit the frame.
- Every joint is checked before we leave.
Before we arrive
- Keep children and pets away from the work area
- Mention any sign-in or site rules at work premises
- Tell us if parking is tricky
Ask for a quote
To get a quote, ring 0345 464 0079 or email us and tell us where the job is and what the problem looks like. You will have a clear quote before work begins.
Advice
Windows and doors in Underhill
Rainfall in the South East is lower than in much of the UK, though outdoor sealant still has to go onto a dry joint, so the forecast decides the day. In London conversions and blocks, the lease decides who is responsible for the frames, so it’s worth checking before booking.
If you let a flat or house near New Barnet station, renewing tired window mastic between tenants heads off draught and damp complaints. As a London address, Underhill can be tricky for parking: tell us about permits, visitor bays or off-street space when you book.
Underhill is about 1 mile west of Barnet, in Greater London. From the centre of Underhill to the outskirts, we cover window mastic for every kind of property, from flats and houses to offices and hotels. Underhill is one of 764 towns and villages in Greater London we cover. You can also see our page for London, about 11 miles away.
Postcode districts we cover here for window mastic: EN5 and N20. You’ll find streets such as Park Road, May's Lane and Manor Road, all of which we cover. We also cover the places around Underhill: Barnet (1 mile east), New Barnet (1 mile east), Monken Hadley (1 mile north), Hadley Wood (1 mile north-east) and Cockfosters (2 miles east). We also work in nearby towns such as London (11 miles south-east), City of Westminster (9 miles south), Marylebone (9 miles south), Kensington (10 miles south), St Pancras (9 miles south) and Shoreditch (10 miles south-east).
Near you in Underhill
In Underhill, that means homes and buildings near Susi Earnshaw Theatre School, Highlands Gardens and The Red Lion pub, among other places. Underhill’s nearest station is New Barnet station, roughly 1 mile east. Well-known places in the area include Francis Skaryna Belarusian Library and Museum (2.2 miles south) and Barnet General Hospital (0.9 miles west). Green spaces nearby include Greenhill Gardens and Darland's Lake.
Also near Underhill
What else we do in Underhill
When something else is needed
Sealant isn’t the answer to every problem, and we’ll tell you before we start. Sealant will not put right:
- Penetrating damp from a failed cavity tray or render
- Condensation trapped between the panes
- Worn gaskets, brush seals or a door that has dropped
If you are not sure, just ring 0345 464 0079 or email us and we’ll give you a straight answer.
Joints and surfaces we work on
- Composite doors
- uPVC windows
- Patio & French doors
- Timber windows & doors
- Window sills
- Aluminium frames
Work for landlords, hotels and businesses
Housing providers, facilities teams and property managers book us too in Underhill and across Greater London. Several sites can go on one quote, and we are fully insured for the work.
Whether it’s one flat or a portfolio, you get the same standard of work: the old material taken out, careful preparation and a 12-month guarantee.
Leave it or put it right?
Redecorating is a good moment to renew the mastic. A clean new bead makes new paint or render look finished, and it’s easier while the walls are being worked on anyway. Converted houses in London often have a bathroom above someone else’s living room, and a slow leak from a failed seal can end up in the flat below. Putting it right early is much less trouble for homes in Underhill.
What it costs depends on the size of the job, how hard the old material is to remove and access to the work. We give a clear quote before we start, so you know where you stand.
Why Underhill customers choose us
- A 12-month guarantee on all work
- Sealant colour chosen to suit
- Experience in homes, commercial sites and hotels
- A choice of colours where the product allows
See our cost guide and customer reviews.
What affects the cost of window mastic
We don’t publish a price list. What we look at:
- How many rooms, windows or areas are involved
- The type of sealant required
- Access and height
- The old material and how stubborn it is
Rather than a set price list, we price the job in front of us: how many joints, what’s in them now and how we reach them. The price is agreed before we begin.
Asking for a quote
To make quoting easier, please include:
- Roughly how big the job is
- The address or postcode of the job
- If it’s domestic, commercial or a hotel
- What the problem is, such as mould, gaps, leaks or cracking
For window mastic in Underhill, call 0345 464 0079 or drop us an email. A short description of the job and where it is helps us quote, and we’ll agree the price before any work begins.