Hertfordshire · England
External Mastic in Knebworth
External mastic for Knebworth and the surrounding area: careful preparation, the right product and tidy work. Ask us for a quick quote.
- Plants rooting in failed movement joints
- Weathered mastic that no longer stretches
- Movement joints filled with mortar instead of sealant
External mastic for homes and businesses in Knebworth
Ground-floor and low-level joints are straightforward to reach. For joints higher up, give us some details of the building and we will confirm what’s needed to reach them safely. We work on joints between brick and frames, concrete & block and façade panels, and people most often call us about brick movement joints where the sealant has split and moss and dirt in joints with no sealant left.
Around Knebworth, garages, outbuildings and extensions all add to the joints on the outside of a home. Leaks generally start at a gutter end, a garage door frame or where an extension meets the house.
Our work is backed for twelve months.
- Where
- Knebworth (SG3)
- Wider area
- Hertfordshire
- Guarantee
- One year on every job
- Quote
- A clear quote up front
- Telephone
- 0345 464 0079
- Last updated
- 26 October 2026
What affects the quote in Knebworth
Jobs vary more than people expect. The main factors:
- Mould in or behind the old joints
- When we can work, for business premises
- The product the joint needs
- Whether the work is at height
Instead of a standard rate, we look at what your job involves: the amount of work, the old material and the access. You will know the price before we begin.
Asking for a quote
Photos by email are helpful but not essential. Try to tell us:
- If it’s domestic, commercial or a hotel
- Photos by email, if that’s easy
- What’s wrong with the joints now
- Whether it has been sealed before
Getting a quote is simple: tell us what needs doing and where. Photos by email help us understand the job, and we will come back to you with a quote.
Can I do it myself?
A DIY attempt doesn’t rule out a professional job. We remove it and do the joint properly, so tell us about it when you ask for a quote.
The biggest DIY mistake is sealing over old sealant. Silicone does not stick to old silicone, so it lifts and traps water and mould underneath. All the old material has to come out first.
Looking after it afterwards
After the cure time, these simple habits help:
- Point out the joints to other trades
- Look over the joints every year, ideally after winter
- Check the sealant is paintable before painting near it
- Act on the first splits you see
Keep pressure washers away from the sealant. High-pressure water can lift the edges over time.
A joint that has stopped moving pushes the movement elsewhere, normally as cracking in the brickwork or render nearby. Resealing it early helps prevent that.
Knebworth: your questions answered
Do you cover smaller places like Knebworth?
Yes. Knebworth is within the area we cover, which takes in towns, villages and rural addresses. We work across London and the South East. Just mention lanes, gates or anything else about the route when you book.
Can you help a landlord with a rental near Knebworth station?
Yes. We work for landlords and agents as well as householders. You can ring 0345 464 0079 or email us with the details for a clear quote.
Will the new joint stand out?
The sealant colour is picked to suit the wall so the joint blends in. There’s a wide choice of exterior colours.
Do you use backing rod?
The foam is closed-cell backing rod. It keeps the sealant at the right depth and bonded to the sides only, which would stop it stretching.
Do you do commercial work?
Yes. Commercial and hotel work is a large part of what we do, as well as homes. Working times can be discussed when we quote.
Can you do several jobs on one visit?
Yes. Tell us about everything you’d like done and you will get one quote.
Do I have to be there?
It depends on the job. For work indoors, someone needs to let us in, and for outside work we will agree access with you when we book.
Should I point up the gap in my brickwork?
No, not if it’s a movement joint. Filling it with mortar defeats its purpose. We’ll reseal it with a flexible sealant.
Outside joints around Knebworth
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. With historic houses such as Knebworth House (about 1.3 miles north-west) nearby, there’s older building stock in the area around Knebworth as well as newer homes. In older buildings, older brick and stone need a sealant that suits them, at the right joint depth.
For site and facilities teams at buildings such as Knebworth Primary and Nursery School, its movement joints, gutters and trims need looking after like any other building’s. Letting a property close to Knebworth station? Then failed wall and movement joints let damp in, so they’re worth checking between tenancies.
Knebworth is about 3 miles south of Stevenage, in Hertfordshire. Every kind of property in Knebworth can book us for external mastic, old or new, large or small. Every job comes with our one-year guarantee. Knebworth is one of 381 places in Hertfordshire where we work. We also work in London, around 26 miles away.
We cover every part of SG3 for external mastic and the streets around it. Local streets include London Road, Deanscroft and Lytton Fields, and we cover them all. We also cover the places around Knebworth: Woolmer Green (1 mile south), Bragbury End (1 mile north-east), Oaklands (2 miles south), Datchworth (2 miles south-east) and Shephall (2 miles north). Within a short drive, we also cover Hemel Hempstead (14 miles south-west), Stevenage (3 miles north), St Albans (10 miles south-west), Bishop's Stortford (15 miles east), Welwyn Garden City (5 miles south) and Harpenden (8 miles south-west).
Local landmarks in Knebworth
We work in homes and buildings around Barnwell School and Knebworth Park. Knebworth has its own railway station: Knebworth station. Nearby you’ll also find Benington Castle (3.6 miles north-east) and Datchworth Museum (1.7 miles south-east). Green spaces nearby include Mardley Heath.
Nearby places to Knebworth
Also in Knebworth
What happens if it’s left?
Water gets in through a failed movement joint and can run behind render or cladding. Resealing early keeps the job to the joint itself, instead of the wall and finishes around it. In a smaller place like Knebworth, wherever a wall is exposed, a failed outside joint is a real source of damp. Indoors, a failed seal lets water into floors and panels as it would anywhere.
Leaving a failed joint generally makes the job bigger, not smaller, because water and mould keep working on it. If you are not sure whether it needs doing, ring 0345 464 0079 or drop us an email and we’ll give you an honest view and a quote.
What you get with us
- 12-month guarantee on every job
- Tidy, careful work
- Full insurance on every job
- Sealant colour chosen to suit
Take a look at our cost guide and what customers say.
Signs you need external mastic in Knebworth
Buildings move more than people expect. Brickwork grows a little over the years, blockwork and concrete shrink, and all of it moves with heat and moisture. All of that ends up in the joint sealant.
Common reasons people contact us are:
- Leaking joints around cladding and façade panels
- Old sealant gone brittle and cracked
- Sealant that has lost its bond to the brick or render
- Rain getting behind render or cladding at a joint
Homes close to Church of St Martin include some older properties, and older walls, gutters and the joints where extensions meet them need a flexible sealant.
Before you book anyone
- What preparation is included
- The length of the guarantee
- Who does the work, and how experienced they are
- What happens if the job turns out different
How the visit works
We work through the job in order: protect, prepare, seal, tidy. For most jobs that takes anything from a few hours to several days, depending on the building and we leave things clean and tidy. In a smaller place like Knebworth, parking is seldom the issue; lanes, gates and long drives are worth mentioning. If the job is outside, tell us about side access and how high the joints are.
- Each joint is checked and the colour of the new sealant agreed.
- Failed sealant is removed back to clean, sound faces.
- Closed-cell backing rod is pushed in so the sealant bonds to two sides only.
- We prime the faces where required.
- We gun in low-modulus sealant and tool it neatly.
Getting ready for the visit
- Move anything stored near the joints
- Keep pets and children clear of the area while we work
- Tell us if parking is tricky
Ask for a quote
Every job is quoted individually, so the first step is telling us about yours. Simply ring 0345 464 0079 or email us and describe the job and send photos if you have them.
Advice
Surface by surface
Brickwork movement joints
Brick movement joints are often mistaken for a missing mortar line. Where one has been mortared, we cut it out and reseal it with sealant if the joint can be cleared.
Copings and trims
Coping and trim joints face rain and sun head on. Each is resealed with a weatherproof, flexible sealant.