Greater London · England
Underhill External Mastic
Our applicators work in Underhill on sealing brickwork movement joints, render, cladding and façade joints. Old material cut out, joints cleaned and sealed neatly and every job guaranteed for a year.
- Weeds or debris growing in open joints
- Joints pointed up with cement by mistake
- Movement joints with cracked, split or missing sealant
Why deal with it now?
Water gets in through a failed movement joint and can run behind render or cladding. Dealing with it early keeps the repair to the joint, instead of the wall and finishes around it. In a London flat, a failed joint rarely stays your problem alone: water can reach the neighbours below, and gaps in fire-rated joints affect the whole building.
A few things move the cost up or down: how much old sealant there is and how stubborn it is, any mould to treat, and whether we need extra access for height. Your quote takes all of that into account.
What you get with us
- Over 20 years’ experience
- Tidy, careful work
- Domestic, commercial and hotel work
- White, clear, grey or another colour, product permitting
Read what affects the cost or what customers say.
About our external mastic in Underhill
We cut out failed sealant, set the joint depth with closed-cell backing rod and reseal with a low-modulus exterior sealant. We pick a colour that sits well with the wall so the joint does not stand out. We are often asked to deal with failed sealant between panels on a façade and gaps where render, brick and frames meet, across brickwork movement joints, façade panels and copings & trims.
Newer London blocks are built with movement joints in brick and render, and with cladding joints sealed with mastic. Period houses have different weak points, such as gutter joints, trims and the line where an extension meets the old wall, and both kinds of building are found across London.
You are covered for a year from the day we finish. If anything is not right with it, phone 0345 464 0079 or send us an email.
- Where
- Underhill (EN5, N20)
- Wider area
- Greater London
- Guarantee
- 12 months on all our work
- Price
- A clear price before work begins
- Call
- 0345 464 0079
- Updated
- 26 October 2026
Outside joints around Underhill
London and the South East are among the drier parts of the UK, though outdoor sealant still has to go onto a dry joint, so the forecast decides the day. With historic houses such as Wyllyotts Manor (about 3.4 miles north) nearby, there’s older building stock in the area around Underhill as well as newer homes. Where a home is older, older brick and stone need a sealant that suits them, at the right joint depth.
As a London address, Underhill can be tricky for parking: tell us about permits, visitor bays or off-street space when you book. Homes close to St John the Baptist Church include some older properties, and the joints between older walls, newer extensions and the roofline need a sealant that can move.
Underhill is about 1 mile west of Barnet, in Greater London. Anywhere in EN5, we can carry out external mastic at your property. Our Greater London coverage runs to 764 places, Underhill among them. For nearby work, see our London page, about 11 miles away.
Work in EN5 and N20 is quoted and booked the same way as anywhere else. If you need external mastic at a property on Park Road, May's Lane and Manor Road or anywhere close by, our applicators can take on the job. If you are just outside Underhill, we cover 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). Larger places within reach include 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).
What’s near Underhill
Locally, jobs take us to homes and businesses close to Susi Earnshaw Theatre School, Highlands Gardens and The Red Lion pub. The closest station to Underhill is New Barnet station, about 1 mile east. Nearby you will also find Francis Skaryna Belarusian Library and Museum (2.2 miles south) and Barnet General Hospital (0.9 miles west). The countryside around Underhill includes Greenhill Gardens and Darland's Lake.
How the job changes by surface
Cladding and façade panels
Panel systems often rely on sealant between and around the panels. We reseal them with a sealant suited to the panel material.
Copings and trims
Coping joints that leak let water straight down into the wall. Resealing them is a small job that protects a lot.
Common questions in Underhill
Is parking an issue for external mastic in Underhill?
In much of London, parking needs planning: a visitor permit or a nearby bay makes the day run more smoothly, so mention what’s available when you book.
Can you do external mastic in Underhill?
Yes, Underhill is within the area we cover. Our work covers London and the South East. You can call 0345 464 0079 or drop us an email and tell us what needs doing, and we will send you a clear quote.
Does someone need to be in?
We need access to the joints, so we will agree that with you when we book. Indoors, that usually means someone at home.
Where are you based?
We’re based in Crawley, West Sussex, and we cover South East England, London and all of Scotland.
What does external mastic cost?
The cost depends on the size of the job, access and how much old sealant has to come out. Call for a quick quote and we will talk you through it.
How long does the job take?
A typical job takes anything from a few hours to several days, depending on the building. We’ll confirm how long when we quote.
Do you work at height?
It depends on the building. Tell us the height, the number of joints and what’s below them, and we will confirm what access is needed.
How long before the joint can be used?
That depends on the sealant used, so follow the cure time we give you when we finish.
What happens on the day
We bring everything the job needs with us, so there’s nothing for you to buy. Once we have agreed what’s being done, we protect the area and start. It normally takes a few hours for a handful of joints, longer for a whole elevation. In Underhill, as in most of London, parking needs a little planning. A visitor permit or a nearby bay makes the visit run more smoothly.
- We inspect the joints and agree which need resealing and how they’ll be reached.
- Old sealant and any debris are cut and raked out.
- We clean both faces of the joint and prime them where the product calls for it.
- Backing rod goes in next, setting the depth and keeping the sealant off the back of the joint.
- We gun in a flexible exterior sealant in a colour that suits the wall and tool it neatly.
Getting ready for the visit
- For indoor joints, keep the area dry beforehand if you can
- Keep children and pets away from the work area
- Mention any leaks or damp you’ve noticed
Ask for a quote
If you’re in Underhill and need external mastic, phone 0345 464 0079 or email us. Tell us what the problem is and where, and you’ll get a clear quote before any work starts.
Read more
For landlords and businesses
We also work for businesses, from offices to washrooms and kitchens. On commercial sites, working times can be discussed when we quote.
Water getting past a failed joint can damage the building, which matters even more when someone else lives there. Keeping joints in good order is simple maintenance.
Towns and villages near Underhill
Other services in Underhill
How to look after the new joints
Once the sealant has cured, a few habits help it last:
- Ask before painting over external sealant
- Deal with leaking gutters that soak the joints below
- Have failed joints looked at early
- Tell anyone working on the walls about the movement joints
Don’t paint over the new sealant unless we’ve said the product is paintable. Paint can crack on a flexible joint and spoil the finish.
An open movement joint lets rain into the wall every time it pours. That can lead to damp patches, staining and frost damage, all bigger jobs than a reseal.
How we quote for external mastic
We quote each job individually. These are the things that affect it:
- Mould in or behind the old joints
- The old material and how stubborn it is
- How easy the joints are to reach
- The number and length of the joints
The cost depends on how much there is to do, how much old material has to come out and access to the work. We give a clear quote before work begins, so you know where you stand.
Asking for a quote
When you contact us, these details help:
- Pictures of the joints, sent by email
- What the problem is, such as mould, gaps, leaks or cracking
- Access, such as stairs, a lift or height
- How many joints, rooms or windows are involved
Need a quote? Please phone 0345 464 0079 or drop us an email and tell us about the job. We will explain what we’d do and send you a clear quote before work begins.
What we’re called out for in Underhill
Buildings move more than people expect. Brickwork grows a little over the years, blockwork and concrete shrink, and all of it moves with heat and moisture. All of that ends up in the joint sealant.
Look out for these signs:
- Gaps where render, brick and frames meet
- Plants rooting in failed movement joints
- Sealant that has lost its bond to the brick or render
- Rigid filler in a joint that should flex
If you let a flat or house near New Barnet station, failed wall and movement joints let damp in, so they’re worth checking between tenancies.
What to look for in a quote
- Are they insured?
- Whether you can choose the colour
- When the joint can take water or movement
- Which sealant they’ll use, and why