Greater London · England
Sand mastic for sash & case windows in Harrow, Greater London
Traditional burnt sand mastic around sash and case windows and doors in stone and brick in Harrow, with a one-year guarantee on every job. Ask us for a quote.
- Mastic worn away on windows facing the weather
- Draughty sash windows at the surround
- Tired mastic found when preparing windows for paint
Sand mastic in Harrow, done properly
For a listed home or one in a conservation area, ask your local planning authority if consent is needed before window work. Like-for-like sand mastic is often the preferred finish. If you’re dealing with draughts and rain around sash boxes and cement or filler used on old window joints that has cracked, we can help.
Much of London’s older housing was built with timber sash windows, some still in place and some long since replaced. Whatever is in the opening now, the joint between frame and wall is what keeps the weather out.
Big or small, you get the same 12-month guarantee.
- Place
- Harrow (HA2, HA1, UB5, HA3)
- Wider area
- Greater London
- Guarantee
- 12 months on all our work
- Price
- A clear price before work begins
- Telephone
- 0345 464 0079
- Last updated
- 26 October 2026
Leave it or put it right?
Gaps round a sash window let in rain and draughts. Water can get to the frame and the masonry so renewing it early protects the window. In a London flat, a failed joint rarely stays your problem alone: water can reach the neighbours below, and gaps in fire-rated joints affect the whole building.
No two jobs are quite the same, so each one gets its own quote. The number of joints, how easy they are to reach and the sealant needed all make a difference.
Why choose Mr Mastic in Harrow
- We are fully insured
- Homes, businesses and hotels
- Anti-fungal silicone where joints get wet
- We leave the area clean and tidy
Take a look at our cost guide and customer reviews.
The process
A visit follows a simple pattern: we check the joints, mask and protect the surrounding surfaces, carry out the job and clear up. It normally takes a few hours for a single window, longer for several. Parking in much of London is by permit or pay-and-display, so it helps to know what’s near you in Harrow. It helps if you can point us to a visitor permit or a bay close to the door.
- We check each window surround and the condition of the joint.
- We rake out the old mastic and clean the joint.
- We fill deep gaps so the new mastic is not too thick.
- We point the joint with sand mastic by hand.
- The finish is dressed neatly.
Before we arrive
- Let us know about parking restrictions
- Clear items away from the joints we are working on
- Let us know about stairs, lifts or work at height
Get a quote
Want a quote for sand mastic? Please ring 0345 464 0079 or send us an email and tell us about the job. You will know what we plan to do, and what it will cost, before any work begins.
Related advice
Quotes for sand mastic in Harrow
We quote each job individually. These are the things that affect it:
- Mould in or behind the old joints
- The product the joint needs
- Access and height
- Any working-time requirements on site
Instead of a standard rate, we look at what your job involves: the amount of work, the old material and the access. The price is agreed before we begin.
Asking for a quote
The more you tell us, the more accurate the quote. When you get in contact, it helps to mention:
- Whether it’s a home or a business
- Access, such as stairs, a lift or height
- Where the job is, and your postcode
- What the problem is, such as mould, gaps, leaks or cracking
Quotes start with a quick conversation. Tell us what’s wrong, where it is and how much there is. When it suits you, call 0345 464 0079 or email us.
Harrow questions
Do you cover Harrow for sand mastic?
Yes. We work across London and the South East, and Harrow is part of that. You can ring 0345 464 0079 or email us and let us know what needs doing.
Can you help a landlord with a rental near Harrow-on-the-Hill station?
Yes. We work for landlords and letting agents as well as householders, and several properties can go on one quote. You can phone 0345 464 0079 or email us with the details for a clear quote.
Do I have to be there?
It depends on the job. For work indoors, someone needs to let us in.
Are you an approved applicator?
Yes. ARBOMaster is the approved applicator scheme for Arbo sealants, and we are part of it.
Is it possible to paint over sand mastic?
It’s traditionally left unpainted, which is what traditional guidance recommends. Talk to us before any painting near it.
Can you seal over the old sealant?
No. New silicone will not bond to old silicone, and anything trapped underneath, including mould and water, stays there. We remove it all first.
Can I do it myself?
It depends on the job. High-level, external movement and fire-rated joints are professional jobs, while a short indoor joint may be within reach.
What is sand mastic?
It’s sand mixed with a linseed-oil-based binder for the joint between timber frames and masonry.
For landlords and businesses
Letting agents and landlords in Harrow can book us as well as householders. Sealant work between tenants is a simple way to keep a property in good order.
From a single let to a portfolio, the work is the same standard: the old material taken out, careful preparation and a 12-month guarantee.
Do you need sand mastic in Harrow?
Timber swells and shrinks with the weather. That movement opens up the mastic joint over time.
Common reasons people contact us are:
- Mastic worn away on windows facing the weather
- Silicone on a period window that looks wrong
- Draughty sash windows at the surround
- Crumbling sand mastic round a window
For site and facilities teams at buildings such as Whitmore High School, its window and door frames need their mastic kept sound, just like a home’s.
Questions to ask before you book
- What insurance they hold
- How the colour is chosen
- How the joint is cleaned and prepared
- Which sealant they’ll use, and why
Keeping the joints in good condition
So the new joints stay in good condition:
- Clear gutters so water doesn’t pour onto the windows
- Avoid heavy scraping on the mastic joint
- Do not paint over the new mastic
- Check with your local planning authority if your home is listed
When preparing windows for paint, take care around the mastic. Hard scraping can loosen it.
Good mastic helps keep the frame dry. Once it fails, water can sit against the timber, which can lead to rot.
Sand mastic in Harrow: local notes
Historic houses like Headstone Manor (about 1.4 miles north) are a reminder that the area around Harrow has older buildings as well as new. 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, yet outdoor joints still have to be sealed dry, so jobs are booked with the weather in mind.
If you live in an older home near Church of St Peter, the windows may be timber, sash or later replacements, and each needs the right sealant against the wall. If you let a flat or house near Harrow-on-the-Hill station, cracked sand mastic around sash windows lets in the draughts tenants complain about.
Harrow is about 7 miles south of Watford, in Greater London. From the centre of Harrow to the outskirts, we cover sand mastic for every kind of property. Across Greater London, we cover 764 places, and Harrow is one of them. London is on our list too, around 12 miles away.
All of HA2, HA1, UB5 and HA3 is covered for sand mastic, from the main roads to the quietest streets. We cover properties on High Street, London Road and Perceval Square. With the B457 and B458 close by, Harrow is simple for us to get to. Other places close to Harrow include 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). Further out, we also cover 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).
Near you in Harrow
Locally, jobs take us to homes and businesses close to Kingsfield Arms, Grange Primary School and Northwick Park and the streets around them. The closest station to Harrow is Harrow-on-the-Hill station, about 0.7 miles north-east of the centre. Well-known places in the area include St. George's Shopping Centre (0.7 miles north-east) and Clementine Churchill Hospital (1 mile south-east). For a walk, there’s Islip Manor Meadows and Northala Fields.
What else we do in Harrow
Should I try it myself first?
Not every job suits DIY: work at height, external movement joints, fire-rated sealant and large areas need the right product, the right access and the right method.
The difference is mostly in the preparation and the product. Joints are cleaned back to sound surfaces and dried, and the sealant is matched to the joint and its conditions, then guaranteed for a year.