Hertfordshire · England
External Mastic in Harpenden
Movement joints in Harpenden need attention? We cut out the old, prepare the joints and apply the right sealant.
- Joints pointed up with cement by mistake
- Weeds or debris growing in open joints
- Leaking joints around cladding and façade panels
External mastic for homes and businesses in Harpenden
A movement joint is only as good as its depth. Closed-cell backing rod sets the depth and stops the sealant sticking to the back, so it bonds to the two sides only and can stretch. We work on concrete & block, copings & trims and brickwork movement joints, and people most often call us about old sealant gone brittle and cracked and joints with sealant missing in places.
Around Harpenden, outside jobs take in garage door frames, porches, gutters and trims as well as walls. Where two materials meet, they move differently, and the sealant has to allow for it.
We stand behind the work with a one-year guarantee, so you’re covered after we’ve gone.
- Location
- Harpenden (AL5)
- Wider area
- Hertfordshire
- Cover
- 12 months on all our work
- Quotes
- A clear quote up front
- Phone
- 0345 464 0079
- Last updated
- 26 October 2026
How the job changes by surface
Cladding and façade panels
Where a cladding system has its own specified sealant, we follow it. Tell us what the building has when you get in touch.
Copings and trims
Copings expand and contract through the day, so the joints need a sealant that stays flexible in all weathers.
When something else is needed
Some problems need a builder, plumber or other trade rather than us:
- Failed render or damaged cladding panels
- Cracking across the wall rather than at a joint
- Cutting new joints into brickwork, which is building work rather than sealant work
If in doubt, phone 0345 464 0079 or send us an email and we’ll say what we think.
What we seal
- Brickwork movement joints
- Render & cladding joints
- Concrete & block
- Façade panels
- Joints between brick and frames
- Copings & trims
What external mastic looks like in Harpenden
Compared with the north and west, the South East gets less rain, though outdoor sealant still has to go onto a dry joint, so the forecast decides the day. With historic houses such as Childwickbury Manor (about 2.6 miles south) nearby, there’s older building stock in the area around Harpenden 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.
Schools such as Harpenden Academy are buildings too, and larger buildings have long runs of movement joint that eventually need resealing. If you let a flat or house near Harpenden station, failed wall and movement joints let damp in, so they’re worth checking between tenancies.
Harpenden is about 6 miles north-east of Hemel Hempstead, in Hertfordshire. Every kind of property in Harpenden is covered for external mastic. Every job comes with our 12-month guarantee. Harpenden is one of 381 towns and villages in Hertfordshire we cover. There’s also a page for London, about 25 miles away.
Here we cover the AL5 postcode area for external mastic. Streets in the area include Station Road, High Street and Connaught Road. You’ll find Harpenden on the B652 and B653, which makes it straightforward to reach with everything needed for external mastic. Nearby, we also carry out external mastic in Batford (1 mile north-east), Lea Valley (1 mile east), Bower Heath (1 mile north), Mackerye End (1 mile north-east) and The Folly (2 miles east). Further out, we also cover Watford (11 miles south), Hemel Hempstead (6 miles south-west), Stevenage (10 miles north-east), St Albans (5 miles south), Welwyn Garden City (7 miles east) and Hitchin (10 miles north). Being one of Hertfordshire’s larger places, Harpenden has homes, rented property and businesses of every kind.
Local landmarks in Harpenden
In Harpenden, that means homes and buildings near Lydekker Park, The Junction pub and Sir John Lawes School, among other places. Harpenden has its own railway station: Harpenden station. A little further out are Harpenden Museum (less than half a mile south-west) and Luton Castle (5 miles north-west). Green spaces nearby include Wheathampstead Local Nature Reserve and Verulamium Lake.
What happens if it’s left?
Running new sealant over the old bead rarely lasts. New sealant does not bond well to old, weathered sealant, so the old joint is cut out fully first. Gaps in sealant and mould in the bead never get better on their own, and in a busy household in Harpenden they spread with every shower and every downpour. Replacing the sealant now is the simplest fix.
Some things make a job bigger or smaller: the old material and how easily it comes out, any mould to treat, and whether the work is at height. We take all of that into account in your quote.
What you get with us
- Old sealant always removed completely
- We are fully insured
- Every job guaranteed for a year
- Neat joints and a tidy finish
Looking after it afterwards
So the new joints stay in good condition:
- Deal with leaking gutters that soak the joints below
- Avoid pressure-washing directly into sealant joints
- Keep movement joints as flexible joints, not mortar
- Show decorators and builders where the movement joints are
Give the sealant the cure time we tell you before anything rubs against it. Most exterior sealants skin fast but take longer to cure right through.
Once a joint can’t move, the wall finds another way, often as cracks near the joint. Resealing it early helps prevent that.
Places near Harpenden
Also in Harpenden
Signs you need external mastic in Harpenden
If plants are growing out of a joint, the sealant has usually failed or was never there. Dirt and roots then trap moisture in the joint and speed up the damage.
Signs it’s time to call us include:
- Rigid filler in a joint that should flex
- Joints with sealant missing in places
- Failed seals between different materials
- Plants rooting in failed movement joints
For an older house near a landmark such as St Nicholas Church, older walls, gutters and the joints where extensions meet them need a flexible sealant.
What to look for in a quote
- Whether VAT is included
- How the surfaces are prepared before sealing
- Who turns up on the day
- Which colour the sealant will be
How it works
We bring everything the job needs with us, so you don’t need to buy or prepare anything. After a quick look at the joints together, we protect the area and start. It normally takes anything from a few hours to several days, depending on the building. In Harpenden, parking is rarely complicated, though it helps to know about driveways or restrictions, and let us know if the work is upstairs, in a flat or up high outside.
- We inspect the joints and agree which need resealing and how they’ll be reached.
- The old mastic and anything lodged in the joint is removed.
- Both sides are cleaned back to sound material and primed if needed.
- Backing rod goes in next, setting the depth and keeping the sealant off the back of the joint.
- The new sealant, chosen to suit the brick or render, is applied and tooled to a neat finish.
Getting ready for the visit
- Tell us if parking is tricky
- For business premises, tell us about site rules or sign-in
- Where the joints are inside, avoid wetting them before we arrive
Getting a quote
For external mastic in Harpenden, phone 0345 464 0079 or send us an email. A short description of the job and where it is helps us quote, and you’ll get a clear quote before we start.
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What affects the quote in Harpenden
You’ll get a clear quote before work begins. It takes into account:
- The product the joint needs
- How much old sealant there is and how hard it is to remove
- How easy the joints are to reach
- How many areas need sealing
There are no prices on our website, because each job is priced on what it involves. Call for a quick quote.
What to tell us for a quote
To make quoting easier, please include:
- What’s wrong with the joints now
- Pictures of the joints, sent by email
- If it’s domestic, commercial or a hotel
- Any previous sealing or DIY work
A good description makes for an accurate quote. Let us know the location, the problem and roughly how many joints, and email pictures if that is easy.
Questions from Harpenden
Is there a guarantee on external mastic in Harpenden?
All our work, in Harpenden and everywhere else, comes with a 12-month guarantee. We’re also fully insured and ARBOMaster approved.
I let a flat near Harpenden station. Can you help?
Yes, we work for landlords and letting agents. Just phone 0345 464 0079 or email us and tell us about the property, and we will give you a clear quote.
What information do you need?
A short description and the location, plus pictures if you have them. We’ll ask anything else we need to know.
Can new sealant go over the old?
No, and we never do. It may look fine at first, but the new bead soon lifts. The old sealant has to come out completely.
Is there any odour?
That depends on the sealant. Some smell for a while, some hardly at all.
Are you an approved applicator?
Yes. We are ARBOMaster approved, the scheme for applicators of Arbo sealants.
Will you leave a mess?
Very little: the area around the joints is protected first, and we clear up the old sealant and our materials before we leave.
Will the new joint stand out?
The aim is a neat, even joint in a colour that suits the wall. Colour is agreed with you before we begin, matched as closely as the range allows.