Greater London · England
Window & door mastic in St Johns, Greater London
Window mastic across St Johns and nearby, with old sealant always removed completely and a one-year guarantee.
- Mastic that has split away from the wall
- Black, perished sealant on frames
- A bead that does not match the frame
Window mastic in St Johns
Over two decades of sealing frames has taught us which products suit which walls. We’re ARBOMaster approved and fully insured, and we give a one-year guarantee on the work. We work on uPVC windows, patio & French doors and aluminium frames, and common reasons people get in touch are old mastic that has gone black or come away and gaps where the sill meets the wall.
Many London homes are flats, with windows high above the street. In a converted house or block, the lease generally says who is responsible for the windows, so it’s worth checking before booking, and access in St Johns is something to talk through when you book.
We stand behind the work with a 12-month guarantee.
- Place
- St Johns (SE13, SE10, SE8)
- Wider area
- Greater London
- Cover
- 12 months on all our work
- Quote
- A clear quote up front
- Phone
- 0345 464 0079
- Page updated
- 26 October 2026
Different surfaces, different methods
Window sills
Masonry sills can have joints where water gets in, including the one between the sill and the frame above. Those are sealed with a product suited to stone or concrete.
Aluminium frames
Large aluminium doors have long joints down each side and across the head, and the threshold joint takes a lot of water. Each is cleaned, backed where deep and resealed.
Landlord and commercial work in St Johns
We also work for businesses, from offices to washrooms and kitchens. For commercial premises, we can talk through working times when we quote.
Hotels need rooms back in use, so we plan the work room by room around your bookings where we can. We’ve worked in hotels as well as homes and businesses.
What happens on the day
When we get to you, we check the joints match what you told us. If we find something unexpected, we talk it through with you first. Most jobs then take a few hours for a handful of frames, longer for a whole house. Parking in much of London is by permit or pay-and-display, so it helps to know what’s near you in St Johns. It helps if you can point us to a visitor permit or a bay close to the door.
- Each frame is looked over and we agree which ones need resealing.
- The old mastic is cut out and scraped back to clean surfaces.
- Where the gap is deep, backing rod goes in first.
- A flexible exterior sealant is applied in a colour to suit the frame.
- We tool each bead neatly and check every joint is closed.
Before we arrive
- Mention if the joints have been sealed before
- Plan for the work area to be out of bounds to pets and children
- Mention any parking rules near the property
Getting a quote
If you’re in St Johns and need window mastic, call 0345 464 0079 or email us. A short description of the job and where it is helps us quote, and you will get a clear quote before any work starts.
Related advice
Other services in St Johns
When to call us in St Johns
Water runs down the glass onto the sill, so the sill ends and the joint below the frame are common leak points. When that sealant fails, water gets into the wall or under the frame.
These are the problems we’re asked about most:
- Cold air getting in at the frame edge
- Cracked or shrunken mastic around the frame
- Gaps between the frame and the inside reveal
- Cracked sealant at the sill ends
Schools such as Ashmead Primary School are buildings too, and the mastic around its windows and doors needs looking after like any other building’s.
What to look for in a quote
- How the colour is chosen
- What happens if the job turns out different
- What product goes in the joint
- When the joint can take water or movement
Aftercare tips
Aftercare is simple, and it makes a real difference:
- Wash the frames with mild soapy water
- Check the mastic once a year
- Ventilate rooms so frames don’t stay wet inside
- Do not use solvent cleaners on the joint
Try not to touch the new joint while it cures, following the cure time we give you. Do not point a pressure washer at the joints.
On timber windows, water behind the joint can lead to rot. Once wood has rotted, it needs repair before it can be sealed, so early resealing is worth it.
FAQs for St Johns
Is parking a problem for a job in St Johns?
It can be, so it helps to tell us about permits, visitor bays or off-street space when you book. The visit is then planned around what’s available.
Is St Johns in your area for window mastic?
Yes, St Johns is within the area we cover. Our work covers London and the South East. You can call 0345 464 0079 or drop us an email and tell us what needs doing, and we will give you a clear price.
Can I paint over the new sealant?
It depends on the sealant. Silicone is not paintable, but some hybrid sealants are. Mention any painting plans before any work starts.
What should I tell you?
A short description and the location, plus pictures if you have them. We will ask anything else we need to know.
Where is Mr Mastic based?
Our base is in Crawley, and we work across South East England, London and Scotland.
Do you take out the old sealant first?
Yes, always, and completely. New sealant will not bond properly to old, so any old material comes out before anything new goes in.
When is the sealant fully cured?
That depends on the sealant used, so please follow the time we give you when we finish.
What does window mastic cost?
Size, access and the old material all affect the cost. Call for a quick quote.
Quotes for window mastic in St Johns
Some jobs are bigger than they look, and some smaller. The main factors:
- Working hours on commercial sites
- How easy the joints are to reach
- The number and length of the joints
- The old material and how stubborn it is
Commercial jobs are quoted on the same basis as homes: 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
You don’t need to know the technical terms. It helps if you can tell us:
- Access, such as stairs, a lift or height
- The problem you’ve noticed
- Whether it’s a home or a business
- Where the property is
Need a quote? Please call 0345 464 0079 or drop us an email and tell us about the job. You’ll know what we plan to do, and what it will cost, before any work begins.
What window mastic looks like in St Johns
With historic houses such as Queen's House (about 1.1 miles north-east) nearby, there’s older building stock in the area around St Johns as well as newer homes. Where a home is older, older frames set in brick or stone need a sealant that suits them, and sometimes that’s traditional sand mastic. The South East is one of the drier parts of the UK, though outdoor sealant still has to go onto a dry joint, so the forecast decides the day.
As a London address, St Johns can be tricky for parking: tell us about permits, visitor bays or off-street space when you book. If you live in an older home near Church of St John, replacement windows fitted into old openings often leave uneven gaps that need backing and a flexible joint.
St Johns is about 4 miles south-east of London, in Greater London. St Johns gets the same service as everywhere we cover: the old material taken out, joints prepared properly and a 12-month guarantee. St Johns is one of 764 places in Greater London we cover.
Work in SE13, SE10 and SE8 works the same way as everywhere else we cover. That includes homes and businesses on Avon Road, Mill Lane and Clare Road and the roads around them. Main roads around St Johns include the A2210 and A2211, which makes travel easy. Other places close to St Johns include New Cross (1 mile west), Deptford (1 mile north), Lewisham (1 mile south-east), Telegraph Hill (1 mile west) and Ladywell (1 mile south). Within a short drive, we also cover London (4 miles north-west), City of Westminster (6 miles north-west), Marylebone (6 miles north-west), Kensington (8 miles west), St Pancras (6 miles north-west) and Shoreditch (5 miles north-west).
Near you in St Johns
Typical jobs here are in homes near Wickham Arms, Brookmill Park and Addey and Stanhope School and the streets around them. Nearby too are St. Paul's (0.7 miles north) and Laban Centre (0.8 miles north). There’s a railway station in St Johns itself: St Johns station. Well-known places in the area include Fan Museum (0.9 miles north-east) and Lewisham Shopping Centre (0.7 miles south-east). For a walk, there’s Brookmill Road Local Nature Reserve and Tower Hill.
Why act early in St Johns
If you’re painting or rendering, reseal at the same time. Fresh joints finish the job neatly, and it’s easier while the walls are being worked on anyway. In a London flat, a failed joint rarely stays your problem alone: water from a shower or window joint can reach the flat below, and gaps in fire-stopping matter to everyone in the building.
A failed joint rarely gets cheaper to fix by waiting, as water and mould keep getting in. If you’re not sure whether it needs doing, ring 0345 464 0079 or send us an email for an honest opinion and a quote.
Why choose Mr Mastic in St Johns
- Anti-fungal sanitary silicone in wet areas
- 4.9 on Google from 180+ five-star reviews
- Neat joints and a tidy finish
- White, clear, grey or another colour, product permitting
Take a look at our cost guide and our reviews.