Hertfordshire · England
External Mastic in Harmer Green
Specialist external mastic for Harmer Green: careful preparation, the right product and tidy work. Call for a quick quote.
- Cracked joints where brick meets render or frames
- Leaking joints around cladding and façade panels
- Weeds or debris growing in open joints
Why it’s worth doing now in Harmer Green
Sealing over the top of old mastic doesn’t hold for long. New sealant doesn’t bond well to old, weathered sealant, so the old joint is cut out fully first. Around Harmer Green, older homes often have timber in their floors and frames, where water from a failed joint can cause rot if it’s left.
Homes and businesses are quoted the same way: the amount of work, how we reach it and what it needs. Call for a quick quote and we’ll explain what it covers.
Why book with us
- Quality sealants from Arbo and Everbuild
- Homes, businesses and hotels
- Sealant colour chosen to suit
- Any old sealant cut out fully, not covered over
Take a look at our cost guide or customer reviews.
External mastic in Harmer Green
Joints in render and cladding are easy to overlook until a damp patch appears. We look over the joints, agree the ones to renew and reseal with a flexible, weatherproof product. Typical jobs include leaking joints around cladding and façade panels and plants rooting in failed movement joints, across joints between brick and frames, render & cladding joints and concrete & block.
In a village or hamlet like Harmer Green, older cottages and farm buildings often stand in the open, so walls and rooflines take the full force of wind and rain. Joints on those walls need a sealant that moves with the building and stays in place.
You are covered for a year from the day we finish. If there’s a problem with it, please call 0345 464 0079 or email us.
- Place
- Harmer Green (AL6)
- Region
- Hertfordshire
- Cover
- A year, on every job
- Quote
- A clear quote up front
- Phone
- 0345 464 0079
- Updated
- 26 October 2026
Work for landlords, hotels and businesses
We also work for businesses, from offices to washrooms and kitchens. For commercial premises, we can talk through working times when we quote.
Mouldy or split sealant often shows up at the end of a tenancy, and it’s easy to sort out between tenants. Several properties can go on one quote.
Keeping the joints in good condition
So the new joints stay in good condition:
- Keep movement joints as flexible joints, not mortar
- Keep soil, plants and ivy away from the joints
- Leave the joints unpainted unless we’ve said otherwise
- Tell anyone working on the walls about the movement joints
Avoid pressure-washing straight into the joints. A strong jet can damage the edges of the bead and let water in behind.
A joint that has stopped moving pushes the movement elsewhere, often as cracks near the joint. Resealing it early helps prevent that.
What affects the cost of external mastic
For a quick quote, just ask. The main things that affect it are:
- The number and length of the joints
- The product the joint needs
- Whether the work is at height
- The old material and how stubborn it is
The cost reflects the work involved, not guesswork. That is why we ask about the job first and give you a clear quote before any work starts.
Asking for a quote
What to tell us when you ask for a quote:
- Pictures of the joints, sent by email
- If someone has resealed it before
- How many joints, rooms or windows are involved
- Whether it’s a home or a business
Want a quote for external mastic? Please ring 0345 464 0079 or send us an email with a few details. You’ll know what we plan to do, and what it will cost, before any work begins.
The process
We work through the job in order: protect, prepare, seal, tidy. For most jobs that takes a few hours for a handful of joints, longer for a whole elevation and we leave things clean and tidy. Somewhere like Harmer Green needs a little journey planning: let us know about lanes, gates and parking. Everything for the job comes with us, so a second trip is avoided where possible.
- We start by walking round the building and agreeing the joints and the access.
- We cut out the old mastic and rake out anything in the joint.
- Joint faces are cleaned back and primed if the sealant needs it.
- Backing rod sized to the joint goes in to set the depth.
- The exterior sealant is applied, tooled and checked along its length.
Getting ready for the visit
- Let us know about earlier repairs to the joints
- Clear items away from the joints we are working on
- Tell us about access, such as stairs, a lift or high-level joints
Get a quote
Getting a quote is simple: tell us what needs doing and where. Photos by email help us understand the job, and we’ll come back to you with a quote.
Advice
How the job changes by surface
Render joints
Render has movement joints and stop beads that need sealing. They’re resealed in a sealant that suits the render.
Brickwork movement joints
Brick moves with heat and moisture, and grows a little over time. The movement joint takes that up, so its sealant has to stay flexible.
Places near Harmer Green
Other services in Harmer Green
What external mastic looks like in Harmer Green
Compared with the north and west, the South East gets less rain, yet outdoor joints still have to be sealed dry, so jobs are booked with the weather in mind. Historic houses like Digswell House (about 1.5 miles south-west) are a reminder that the area around Harmer Green has older buildings as well as new. In older buildings, older brick and stone need a sealant that suits them, at the right joint depth.
If you live in an older home near Digswell Village Church, older walls, gutters and the joints where extensions meet them need a flexible sealant. For site and facilities teams at buildings such as Oaklands Primary School, movement joints, cladding joints and gutters on larger buildings need the same care as a house’s.
Harmer Green is about 2 miles north of Welwyn Garden City, in Hertfordshire. Every kind of property in Harmer Green can book us for external mastic, old or new, large or small. The work is guaranteed for a year. We cover 381 towns and villages across Hertfordshire. We also work in London, around 23 miles away.
We work across AL6 for external mastic. We also cover Burnham Green (1 mile east), Tewin Wood (1 mile east), Oaklands (1 mile north-west), Bull's Green (1 mile north-east) and Digswell (1 mile south-west). We also work in nearby towns such as Watford (15 miles south-west), Hemel Hempstead (13 miles south-west), Stevenage (5 miles north), St Albans (8 miles south-west), Bishop's Stortford (15 miles east) and Harpenden (7 miles west).
Around Harmer Green
Around Harmer Green, our work takes us to homes and buildings near Danesbury Park, Monk's Walk School and The White Horse Public House. The nearest railway station is Welwyn North station, roughly 0.8 miles south-west. Nearby you will also find Hertford Castle (4.9 miles south-east) and Queen Elizabeth II Hospital (3.4 miles south). Open spaces near Harmer Green include Tewin Orchard and Hopkyns Wood.
Harmer Green: your questions answered
Will you come out to Harmer Green?
Yes, we do. Our work covers London and the South East, from towns to the smallest places. Longer journeys are simply planned in advance.
Do you cover Harmer Green for external mastic?
Yes, Harmer Green is within the area we cover. We work across London and the South East. You can phone 0345 464 0079 or send us an email and tell us what needs doing.
How long before the joint can be used?
Different products cure at different speeds, so follow the cure time we give you.
Are you ARBOMaster approved?
Yes. We’re ARBOMaster approved, the scheme for applicators of Arbo sealants.
Do you use backing rod?
The foam is closed-cell backing rod. It keeps the sealant at the right depth and bonded to the sides only.
Is there a guarantee?
Yes: twelve months. If anything is not right with the work within the year, let us know and we will sort it.
When will the joints need doing again?
It varies with how exposed the wall is and how much the joint moves. Look for splits, hardening or edges coming away and have them redone then.
What sealant do you use?
Quality sealants from brands including Arbo and Everbuild, picked for each joint. We will tell you what we’re using and why.
Signs you need external mastic in Harmer Green
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 make things worse.
The problems people normally mention when they call include:
- Plants rooting in failed movement joints
- Sealant that has lost its bond to the brick or render
- Damp patches traced to an open external joint
- Old sealant gone brittle and cracked
Letting a property close to Welwyn North station? Then outside joints that have cracked can let damp into a rental, so check them between lets.
What to look for in a quote
- What the quote includes, VAT included or not
- Whether all the old sealant comes out first
- The length of the guarantee
- Which colour the sealant will be