Hertfordshire · England
External Mastic in Blackmore End
Problem with your movement joints in Blackmore End? We cut out the old, prepare the joints and apply the right sealant, with a 12-month guarantee.
- Brick movement joints where the sealant has split
- Joints pointed up with cement by mistake
- Gaps where render, brick and frames meet
About our external mastic in Blackmore End
Failed sealant is cut out, the depth set with closed-cell backing rod, and the joint resealed with a low-modulus sealant made for outside. We pick a colour that sits well with the wall. Common call-outs are for water getting into walls through failed joints and failed seals between different materials, on brickwork movement joints, façade panels and render & cladding joints.
Around Blackmore End, older buildings can be stone, brick or timber-framed, many with extensions added later. The joints where those materials meet are the weak points.
A one-year guarantee comes as standard, so you’re not left on your own once the job is done.
- Location
- Blackmore End (AL4)
- Region
- Hertfordshire
- Cover
- A year, on every job
- Quote
- Quoted before any work starts
- Call
- 0345 464 0079
- Page updated
- 26 October 2026
Outside joints around Blackmore End
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. With historic houses such as Shaw's Corner (about 1.6 miles east) nearby, there’s older building stock in the area around Blackmore End as well as newer homes. In older buildings, older walls and rooflines need a sealant that moves with them and suits the material.
If you let a flat or house near Harpenden station, outside joints that have cracked can let damp into a rental, so check them between lets. For site and facilities teams at buildings such as Kimpton Primary School, larger buildings have long runs of movement joint that eventually need resealing. For an older house near a landmark such as St Peter, an extension built onto an older house moves differently, and the joint between them takes the strain.
Blackmore End is about 2 miles north-east of Harpenden, in Hertfordshire. Across the AL4 area, our applicators can take on external mastic. 381 places in Hertfordshire are on our list, from the larger towns to small villages. London is on our list too, around 25 miles away.
All of AL4 is within our area for external mastic. You’ll find streets such as Kimpton Road, Beech Way and The Drive, all of which we cover. We also cover the places around Blackmore End: Porter's End (1 mile north-west), Mackerye End (1 mile south-west), The Folly (1 mile south), Lea Valley (1 mile south-west) and Bower Heath (2 miles west). Further out, we also cover Watford (13 miles south), Hemel Hempstead (9 miles south-west), Stevenage (7 miles north-east), St Albans (6 miles south), Welwyn Garden City (5 miles south-east) and Hitchin (8 miles north).
What’s near Blackmore End
Locally, jobs take us to homes and businesses close to Cross Keys Public House and Sir John Lawes School and the streets around them. The nearest railway station is Harpenden station, roughly 2.6 miles south-west of the centre. Well-known places in the area include St Albans City Hospital (5.6 miles south) and Harpenden Museum (2.9 miles south-west). Open spaces near Blackmore End include Stocking Springs Wood.
What sealant work can’t fix
We will always be straight with you, so here’s what new sealant won’t do:
- Structural movement, which needs a surveyor or engineer
- Failed render or damaged cladding panels
- Walls that need new movement joints cut into them
If in doubt, phone 0345 464 0079 or drop us an email and we’ll say what we think.
Where we work in Blackmore End
- Joints between brick and frames
- Brickwork movement joints
- Render & cladding joints
- Façade panels
- Copings & trims
- Concrete & block
Why it’s worth doing now in Blackmore End
Resealing a run of joints in one go is usually better than patching one at a time. Joints of the same age tend to fail together, and access only has to be arranged once. Around Blackmore End, older homes often have timber in their floors and frames, and water through a failed joint can rot timber if it’s left.
A failed joint rarely gets cheaper to fix by waiting. If you’re not sure whether it needs doing, ring 0345 464 0079 or send us an email and we’ll give you an honest view and a quote.
Why Blackmore End customers choose us
- We are fully insured
- You choose the colour before work begins
- We leave the area clean and tidy
- ARBOMaster approved applicator
The process
At the end, we look at the finished work together and explain the cure time. The work normally takes a few hours to a few days, depending on the length of joint and the access, depending on the size of the job. Somewhere like Blackmore End needs a little journey planning: tell us about narrow lanes, gates, long drives and where we can park. Everything for the job comes with us, so a second trip is avoided where possible.
- We start by checking each joint, agreeing the ones to redo and how we will reach them.
- The old mastic and anything lodged in the joint is removed.
- Both sides are cleaned back to sound material and primed if needed.
- We push in closed-cell backing rod so the sealant is the right depth and bonds to the sides only.
- We gun in a flexible exterior sealant in a colour that suits the wall and tool it neatly.
How to prepare
- Clear the area around the work
- Let us know about parking restrictions
- Mention if the joints have been sealed before
Ask for a quote
Call for a quick quote, or email us if that’s easier. Describe what needs sealing and we’ll talk you through the work before giving you a price.
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Can I do it myself?
Sealing over old silicone is the commonest DIY error. New silicone won’t bond to old, so it lifts and traps water and mould underneath. All the old material has to come out first.
Overfilling a joint is a classic DIY mistake. A thick, smeared bead looks untidy and does not move well, and the joint depth matters as much as the finish.
Looking after it afterwards
Once the sealant has cured, a few habits help it last:
- Keep soil, plants and ivy away from the joints
- Wash walls gently around the sealant
- Fix overflowing gutters above sealed joints
- Ask before painting over external sealant
Have a quick look at the joints each spring. Splits, gaps at the edges or plants in the joint are early warning signs.
If you can see gaps, plants or bare sections in a joint, have it checked. Earlier resealing means a smaller job and a better result.
Blackmore End: your questions answered
Is Blackmore End in your area for external mastic?
Yes, Blackmore End is within the area we cover. Our work covers London and the South East. You can phone 0345 464 0079 or email us and tell us what needs doing, and we will give you a clear price.
Will you come out to Blackmore End?
Yes, smaller places are covered as well as towns. Our work covers London and the South East. Let us know about access, parking and anything unusual about getting there when you book.
How long will you be here?
As a guide, most jobs take a few hours to a few days, depending on the length of joint and the access. Bigger jobs take longer.
What information do you need?
The location, the problem and the rough size of the job. If you can, email a few pictures.
What is a movement joint?
It’s a deliberate gap that lets the wall expand and contract without cracking. It should be filled with flexible sealant, never mortar.
Can you reach joints on upper floors?
It depends on the height and the site. Low-level joints are straightforward; for higher ones, send details and we will confirm the access.
Do you hold any approvals?
Yes. ARBOMaster is the approved applicator scheme for Arbo sealants, and we are part of it.
I’m a tenant. Can you help?
Of course. Ask your landlord first, and we can send them the quote if that helps.
How the job changes by surface
Render joints
Water behind render can lead to blown patches. A sound joint keeps the render dry behind and protects the render itself.
Concrete and blockwork
The face of concrete or block is often dusty, so preparation is key. A primer goes on where the product requires one before the bead.
Quotes for external mastic in Blackmore End
Jobs vary more than people expect. The main factors:
- How much there is to do
- Mould in or behind the old joints
- Whether old sealant or filler has to be cut out
- When we can work, for business premises
Homes and businesses are quoted the same way: the amount of work, the access and the products needed. Ask us for a quote and we’ll explain what it covers.
What we need to know
When you contact us, it’s useful to know:
- How many joints, rooms or windows are involved
- If someone has resealed it before
- How we’d reach the joints
- A few pictures by email, if you can
It all starts with a short chat. Tell us what’s wrong, where it is and how much there is, and we’ll explain how we’d tackle it. When it suits you, phone 0345 464 0079 or email us.