Greater London · England
Harrow External Mastic
Professional external mastic in and around Harrow, with every bit of old sealant taken out first and a 12-month guarantee.
- Joints where the bead has come unstuck
- Cracked joints where brick meets render or frames
- Weathered mastic that no longer stretches
External mastic in Harrow
A movement joint is only as good as its depth. Backing rod controls how deep the sealant goes and keeps it off the back of the joint, leaving it bonded to two faces so it can move with the wall. We are often asked to deal with damp patches traced to an open external joint and movement joints filled with mortar instead of sealant, on brickwork movement joints, joints between brick and frames and façade panels.
Outside work in London starts with access: side passages, upper floors and where to park. In Harrow, joints up high may need ladders or other access, which is worth talking through when you book.
Big or small, you get the same 12-month guarantee in Harrow and across Greater London.
- Location
- Harrow (HA2, HA1, UB5, HA3)
- Region
- Greater London
- Cover
- 12 months on all our work
- Quotes
- Quoted before any work starts
- Telephone
- 0345 464 0079
- Page updated
- 26 October 2026
What the job involves on each surface
Joints between dissimilar materials
Each side of the joint may need its own preparation. We prepare both faces so the sealant bonds properly.
Cladding and façade panels
Panels heat up and cool down quickly, so the joints work hard every day. The sealant is chosen with that in mind.
Should I try it myself first?
A DIY attempt does not rule out a professional job. We remove it and do the joint properly.
Some jobs are best left to a professional: work at height, external movement joints, fire-rated sealant and large areas need the right product, the right access and the right method.
What we do, step by step
We work through the job in order: protection first, then preparation, sealing and a tidy-up. Typically it takes a few hours to a few days, depending on the length of joint and the access. Parking in much of London is by permit or pay-and-display, so it helps to know what’s near you in Harrow. A visitor permit or a nearby bay makes the visit run more smoothly.
- We start by walking round the building and agreeing the joints and the access.
- Every bit of old sealant and any mortar or debris is cut out.
- The faces are brushed clean and primed where the product requires it.
- We fit closed-cell backing rod to control the depth.
- The exterior sealant is applied, tooled and checked along its length.
Before we arrive
- Mention any sign-in or site rules at work premises
- Keep children and pets away from the work area
- For indoor joints, keep the area dry beforehand if you can
Getting a quote
Need a quote? Please call 0345 464 0079 or send us an email with a few details. We will explain what we’d do and send you a clear quote before any work starts.
Read more
Outside joints around Harrow
With historic houses such as Headstone Manor (about 1.4 miles north) nearby, there’s older building stock in the area around Harrow as well as newer homes. In older buildings, older walls and rooflines need a sealant that moves with them and suits the material. In terraces and conversions, gutters and walls are often shared with the neighbours, so check who is responsible for the outside before booking.
For landlords with property near Harrow-on-the-Hill station, split movement joints are easy to miss on a rental until damp shows inside. If you live in an older home near Church of St Peter, older walls, gutters and the joints where extensions meet them need a flexible sealant. Managing a site like Whitmore High School? Then its movement joints, gutters and trims need looking after like any other building’s.
Harrow is about 7 miles south of Watford, in Greater London. Wherever you are in the HA2 postcode area, we cover external mastic for homes and businesses. Our Greater London coverage runs to 764 places, Harrow among them. We also work in London, around 12 miles away.
Our applicators cover all of the HA2, HA1, UB5 and HA3 postcode area for external mastic. If you need external mastic at a property on High Street, London Road and Perceval Square or anywhere close by, our applicators can take on the job. You’ll find Harrow on the B457 and B458, which makes it straightforward to reach with everything needed for external mastic. Close by, we work in Roxeth (1 mile south), South Harrow (1 mile south), North Harrow (1 mile north-west), Harrow on the Hill (1 mile south-east) and Rayners Lane (1 mile west). Larger places within reach include London (12 miles south-east), City of Westminster (9 miles south-east), Marylebone (10 miles south-east), Kensington (8 miles south-east), St Pancras (11 miles east) and Shoreditch (12 miles east).
Around Harrow
We work in homes and buildings around Kingsfield Arms, Grange Primary School and Northwick Park and the streets around them. For rail, the closest station is Harrow-on-the-Hill station, about 0.7 miles north-east of the centre. Other landmarks nearby include St. George's Shopping Centre (0.7 miles north-east) and Clementine Churchill Hospital (1 mile south-east). Not far away are Islip Manor Meadows and Northala Fields.
What we cannot put right
We will not take on work that won’t last, so here’s what sealant cannot put right:
- Movement that’s more than a sealant joint can take
- Rising or penetrating damp from elsewhere in the wall
- Cracked brickwork or render that needs a builder or plasterer
If you’re not sure, just call 0345 464 0079 or send us an email and we will give you a straight answer.
Joints and surfaces we work on
- Façade panels
- Concrete & block
- Render & cladding joints
- Brickwork movement joints
- Copings & trims
- Joints between brick and frames
Also in Harrow
Why it’s worth doing now in Harrow
Sealing over the top of old mastic doesn’t hold for long. New sealant does not bond well to old, weathered sealant, so we always take the old sealant out. In London conversions, one flat’s bathroom is often above another flat’s ceiling, so a small leak from tired sealant can mean a neighbour’s ceiling and a lot of disruption. Putting it right early is much less trouble for homes in Harrow.
Commercial jobs are quoted on the same basis as homes: the amount of work, the access and the products needed, plus any working-time requirements on site. Call for a quick quote and we will explain what it covers.
Why choose Mr Mastic in Harrow
- Any old sealant cut out fully, not covered over
- Sealant colour chosen to suit
- 12-month guarantee on every job
- We agree the colour with you first
See what affects the cost or our reviews.
FAQs for Harrow
Do you work for landlords with property near Harrow-on-the-Hill station?
Yes, rental work is part of what we do. Just call 0345 464 0079 or drop us an email and tell us about the property, and we’ll send you a clear quote.
Is parking an issue for external mastic in Harrow?
Parking in Harrow, as in most of London, takes a little planning: a visitor permit or a nearby bay makes the day run more smoothly, so mention what’s available when you book.
Which products do you use?
It depends on the joint: anti-fungal sanitary silicone in wet areas, and other sealants where joints move or face the weather. We use products from Arbo and Everbuild.
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.
Do you work at height?
That depends on the building and the joints. Where joints can be reached safely, we will do them. Give us some details and we’ll confirm the access.
Why do you put foam in the joint first?
That is backing rod, a closed-cell foam. It stops the sealant going in too deep and bonding to three sides so it can flex with the joint.
Will it make a mess?
No. We protect the surfaces around the joints, and we tidy up before we go.
Can you do several jobs on one visit?
Of course. Doing several areas at once is generally more efficient.
What affects the cost of external mastic
For a quick quote, just ask. What matters most is:
- Working hours on commercial sites
- Whether old sealant or filler has to be cut out
- The number and length of the joints
- The product the joint needs
Combining jobs into one visit generally makes better use of the time than several visits, so it’s worth mentioning everything you’d like done. You’ll get one clear quote for the whole job.
What to tell us for a quote
Before you ask for a quote, have a think about:
- The problem you’ve noticed
- Whether it’s a home or a business
- Any previous sealing or DIY work
- Where the job is, and your postcode
For a quick quote, get us on the phone, or email us if that’s easier. Tell us about the joints that need doing and we will talk you through the work.
Keeping the joints in good condition
After the cure time, these simple habits help:
- Deal with leaking gutters that soak the joints below
- Get split or bare joints resealed while the damage is small
- Never let anyone fill a movement joint with mortar
- Tell anyone working on the walls about the movement joints
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.
Small splits are the time to act. At that stage it’s a simple cut out and reseal.