Greater London · England
St John's Wood External Mastic
Sealing brickwork movement joints, render, cladding and façade joints for homes and businesses in St John's Wood and across Greater London. Fully insured, ARBOMaster approved and guaranteed for a year.
- Damp patches traced to an open external joint
- Sealant pulling away from one side of the joint
- Joints pointed up with cement by mistake
External mastic in St John's Wood, done properly
Short cuts in external joints show up within a season or two. New sealant over old, joints packed with mortar and beads with no backing rarely last long. We strip the joint back and start again. We are often asked to deal with damp patches traced to an open external joint and weathered mastic that no longer stretches, on copings & trims, façade panels and brickwork movement joints.
Modern London flats rely on sealed movement joints and cladding joints. 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.
The work is covered for a year after we finish. If there’s a problem with it, please ring 0345 464 0079 or send us an email.
- Place
- St John's Wood (NW8, NW6, NW3)
- Wider area
- Greater London
- Guarantee
- 12 months on all our work
- Quotes
- A clear quote up front
- Call
- 0345 464 0079
- Page updated
- 26 October 2026
What affects the cost of external mastic
The price is agreed before any work begins. It takes into account:
- How easy the joints are to reach
- Which sealant suits the joint
- Any mould that needs treating
- The amount of work
Every job is a little different, so each one gets its own quote. The number of joints, how easy they are to reach and the sealant needed all play a part, and we’ll explain the quote if you ask.
Helping us quote
There’s no need for technical detail. Just tell us:
- Photos by email, if that’s easy
- How many joints, rooms or windows are involved
- Whether it has been sealed before
- If it’s domestic, commercial or a hotel
For external mastic in St John's Wood, call 0345 464 0079 or email us. Tell us what the problem is and where, and you will get a clear quote before any work starts.
What happens if it’s left?
Sealing over the top of old mastic does not hold for long. The new layer cannot bond properly to old, weathered material, so we always take the old sealant out. Damp from a failed joint spreads quietly in older London brickwork and timber, and by the time it shows on a wall or ceiling in St John's Wood, the repair is bigger than the sealant job would have been.
Some things make a job bigger or smaller: the old material and how easily it comes out, any mould to treat, and whether we need extra access for height. Your quote takes all of that into account.
Why choose Mr Mastic in St John's Wood
- Experience in homes, commercial sites and hotels
- 180+ five-star Google reviews
- A 12-month guarantee on all work
- Old sealant always removed completely
Read how sealant jobs are priced or our reviews.
DIY versus a professional finish
A DIY attempt doesn’t rule out a professional job. We remove it and do the joint properly.
If you are set on DIY, put the effort into preparation: take out all the old sealant, make sure the joint is clean and dry, and give it the full cure time. If it does not work out, we can still put it right.
How the job changes by surface
Copings and trims
A failed coping joint lets water into the top of the wall. Resealing them is a small job that protects a lot.
Brickwork movement joints
People sometimes point up a brick movement joint by mistake. If that’s happened, we rake it out and reseal it properly.
Landlord and commercial work in St John's Wood
Letting agents and landlords in St John's Wood can book us as well as homeowners. Resealing between tenancies is a simple way to keep a property in good order before the next move-in.
Businesses can book us for washrooms, kitchens, offices and more. For commercial premises, we can talk through working times when we quote.
FAQs for St John's Wood
Can parking be tricky for a visit in St John's Wood?
London parking needs a little planning, so let us know about permits, bays or a driveway when you book. The visit is then planned around what’s available.
Do you cover St John's Wood for external mastic?
Yes, St John's Wood is within the area we cover. We work across London and the South East. Just phone 0345 464 0079 or drop us an email with a few details of the job.
Can you seal joints between render and window frames?
Yes. Materials like render, brick, metal and uPVC all move differently, so a flexible sealant is needed where they meet. The sealant is matched to both sides.
What is a movement joint?
A vertical gap in a long brick wall is generally a movement joint. It gives the wall room to move.
Do you take out the old sealant first?
Always. Sealing over the old bead is the commonest cause of early failure, so we never do it.
Do you do commercial work?
Yes. We can plan bigger jobs in stages.
How long does external mastic take?
Typically it takes a few hours for a handful of joints, longer for a whole elevation. Bigger jobs take longer.
Is it OK to fill a movement joint with cement?
No, not where the gap is a movement joint. Pointing it stops it working. A flexible sealant is the right fill.
St John's Wood buildings and the weather
London terraces and converted houses often share gutters, downpipes and walls, so it’s worth agreeing who is responsible before work starts. With historic houses such as Winfield House (about 0.6 miles south-east) nearby, there’s older building stock in the area around St John's Wood as well as newer homes. In older buildings, older walls and rooflines need a sealant that moves with them and suits the material.
For site and facilities teams at buildings such as George Eliot Primary School, larger buildings have long runs of movement joint that eventually need resealing. Homes close to Church of All Souls include some older properties, and older walls, gutters and the joints where extensions meet them need a flexible sealant.
St John's Wood is about 1 mile south of Hampstead, in Greater London. From the centre of St John's Wood to the outskirts, we cover external mastic for every kind of property, from flats and houses to offices and hotels. St John's Wood is one of 764 towns and villages in Greater London where we work.
Postcode districts we cover here for external mastic: NW8, NW6 and NW3. Whether your property is on The Lane, Hall Road and Hill Road or a nearby road, our applicators can take on the job. Being close to the A5205, St John's Wood is simple for us to get to for external mastic. We also cover the places around St John's Wood: South Hampstead (1 mile north), Swiss Cottage (1 mile north), Primrose Hill (1 mile north-east), Lisson Grove (1 mile south) and Haverstock (1 mile north-east). We also work in nearby towns such as London (4 miles south-east), Kensington (3 miles south-west), St Pancras (3 miles east), Shoreditch (4 miles east), Walthamstow (7 miles north-east) and Hoxton (4 miles east).
Local landmarks in St John's Wood
Locally, jobs take us to homes and businesses close to Cecil Sharp House, Primrose Hill and Arnold House School. Nearby too are Quintin Kynaston Community Academy (under half a mile north-west) and Cafe Med pub (under half a mile north-west). There’s a railway station in St John's Wood itself: South Hampstead station. A little further out are Hospital of St John and St Elizabeth (under half a mile south) and Montfichet's Tower (3.5 miles south-east). The countryside around St John's Wood includes The Long Water and Primrose Hill.
How the visit works
There’s no need to move everything out, just the space around the work. We protect the rest, and the work takes a few hours for a handful of joints, longer for a whole elevation in most cases. For a job in St John's Wood, parking and access are the things to plan: let us know about parking, the floor we’ll be working on and any lift or shared entrance.
- 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.
- 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.
- A flexible exterior sealant is applied and tooled in a colour to suit the wall.
Preparing for the visit
- Let us know about earlier repairs to the joints
- Keep children and pets away from the work area
- Tell us about access, such as stairs, a lift or high-level joints
How to get a quote
Want a quote for external mastic? Please phone 0345 464 0079 or email us and tell us about the job. You’ll know what we plan to do, and what it will cost, before any work begins.
Read more
Signs you need external mastic in St John's Wood
Sometimes a movement joint was filled with mortar or a rigid filler by mistake. With the joint locked solid, the movement turns into cracks elsewhere in the wall.
Look out for any of the following:
- Brick movement joints where the sealant has split
- Panel joints letting water in
- Weeds or debris growing in open joints
- Rain getting behind render or cladding at a joint
Rented homes near South Hampstead station change hands often, and failed wall and movement joints let damp in, so they’re worth checking between tenancies.
Comparing quotes
- How much experience the applicator has
- Which colour the sealant will be
- What preparation is included
- The length of the guarantee