Hertfordshire · England
Old Knebworth Window Mastic
Mastic around window and door frames, sills and patio doors for homes and businesses in Old Knebworth and across Hertfordshire. ARBOMaster approved applicators, fully insured, with a 12-month guarantee.
- Gaps where the sill meets the wall
- Cold air getting in at the frame edge
- Mastic that has split away from the wall
When it is not the sealant
We’d rather tell you now than do work that will not last, so here’s what sealant cannot put right:
- Penetrating damp from a failed cavity tray or render
- Draughts through the window seals rather than the frame
- Condensation trapped between the panes
If in doubt, call 0345 464 0079 or email us and we will say what we think.
What we seal
- Patio & French doors
- uPVC windows
- Window sills
- Aluminium frames
- Composite doors
- Timber windows & doors
Window mastic for homes and businesses in Old Knebworth
A window frame and the wall around it expand and shrink at different rates. Plastic and aluminium frames move with the temperature, while masonry stays fairly still. So the sealant has to stretch and recover, which is why we use a low-modulus product. Common call-outs are for cracked sealant at the sill ends and draughts coming in around windows and doors, across uPVC windows, window sills and composite doors.
In a village or hamlet like Old Knebworth, older cottages and farmhouses often have timber windows set in stone or brick. Older buildings like these do not always suit modern silicone, so the sealant has to be chosen to suit the building.
Whatever the size of the job, it’s backed by our one-year guarantee.
- Location
- Old Knebworth (SG3)
- Region
- Hertfordshire
- Cover
- 12 months on all our work
- Quotes
- A clear quote up front
- Call
- 0345 464 0079
- Updated
- 26 October 2026
Nearby places to Old Knebworth
Other services in Old Knebworth
Old Knebworth questions
Is Old Knebworth within the area you cover?
Yes, we do. We work across London and the South East, from towns to the smallest places. The journey is planned ahead, so it helps to know about access.
Can you do window mastic in Old Knebworth?
Yes. We work across London and the South East, and Old Knebworth is part of that. Just phone 0345 464 0079 or send us an email with a few details of the job, and we’ll give you a clear price.
Can you work on commercial sites?
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.
What information do you need?
Tell us what needs doing and where. We’ll ask anything else we need to know.
How do I get a quote for window mastic in Old Knebworth?
We do not publish prices, because the cost depends on the job: its size, the old material, access and the product needed. We agree the price before any work begins.
Are you ARBOMaster approved?
Yes. We’re an ARBOMaster approved applicator.
Can you seal the gap between the frame and the plaster indoors?
Yes. Outside is the weather seal; inside closes the gap to the plaster. We do one or both.
Can new sealant go over the old?
No, it won’t last. It can look neat for a few weeks, but the new bead soon lifts. Removing the old sealant completely is the only lasting way.
Window mastic in Old Knebworth: local notes
With historic houses such as Knebworth House (about 0.2 miles north-west) nearby, there’s older building stock in the area around Old Knebworth 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 is 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.
For landlords with property near Knebworth station, renewing tired window mastic between tenants heads off draught and damp complaints. If you look after a building such as Barnwell School, the mastic around its windows and doors needs looking after like any other building’s. In the older streets around Church of St Mary and St Thomas, the joint between an old wall and a newer frame is generally the first place rain gets in.
Old Knebworth is about 3 miles south-west of Stevenage, in Hertfordshire. Old Knebworth gets the same service as everywhere we cover: the old material taken out, joints prepared properly and a 12-month guarantee. Old Knebworth is one of 381 places in Hertfordshire where we work. For nearby work, see our London page.
Here we cover the SG3 postcode area for window mastic. Streets in the area include Old Knebworth Lane and Park Lane, and we cover them all. Neighbouring places we cover for window mastic include Nup End Green (1 mile south-west), Driver's End (1 mile south-west), Rableyheath (1 mile south), Pottersheath (1 mile south) and Langley (2 miles north-west). Larger places within reach include Hemel Hempstead (13 miles south-west), Stevenage (3 miles north-east), St Albans (10 miles south-west), Welwyn Garden City (5 miles south), Harpenden (7 miles south-west) and Hitchin (6 miles north-west).
Local landmarks in Old Knebworth
Around Old Knebworth, our work takes us to homes and buildings near Knebworth Park and Robin Hood And Little John Public House, among other places. The nearest railway station is Knebworth station, roughly 1 mile east of the centre. Other landmarks nearby include Lister Hospital (3.7 miles north) and Stevenage Museum (2.2 miles north). The countryside around Old Knebworth includes Mardley Heath.
How to look after the new joints
Good habits help new joints last longer. A few tips:
- Keep drainage slots and sill grooves clear
- Air rooms to keep condensation down around windows
- Avoid harsh chemicals near the mastic
- Don’t paint over the silicone
Keep sill grooves and drainage slots clear, so water can run away instead of sitting against the joint.
A small gap around a frame only grows. As the old mastic shrinks further, more air gets through, and heat escapes around the frame. A new joint closes it up.
What we do, step by step
There’s no need to move everything out, only the area right around the joints. We protect the rest, and the job takes anything from an hour or two to a full day, depending on how many frames there are for a typical job. Somewhere as small as Old Knebworth, access is often the only tricky part. If the job is outside, mention outbuildings, exposed walls and how to reach them.
- First we look at every frame and agree the list.
- We remove the old mastic and clean the frame and the wall.
- Deep gaps are filled with backing rod to set the depth.
- New exterior sealant goes on in a matching colour and is tooled smooth.
- Finally we tidy up and check each joint.
How to prepare
- Move anything stored near the joints
- Keep pets and children clear of the area while we work
- Mention any leaks or damp you’ve noticed
Ask for a quote
We quote each job on its own. Simply call 0345 464 0079 or drop us an email and describe the job.
Related advice
What happens if it’s left?
Once mastic fails, rain can get between the frame and the wall. Left long enough, it can stain inside walls, spoil plaster and rot wooden frames. Resealing now is a small job by comparison. Where a wall is exposed, as many are in small places like Old Knebworth, wind-driven rain gets into any failed joint, and damp in older walls is slow to dry out.
The cost depends on how much there is to do, how hard the old material is to remove and access to the work. You’ll have a clear quote before any work starts, so you know where you stand.
Why us?
- Rated 4.9 stars on Google
- 12-month guarantee on every job
- White, clear, grey or another colour, product permitting
- 20+ years of sealant experience
Landlords, agents and businesses in Old Knebworth
Businesses can book us for washrooms, kitchens, offices and more. On commercial sites, working times can be discussed when we quote.
We regularly work with letting and managing agents. Send the address, what’s needed and the access arrangements.
How we quote for window mastic
There’s no set price list. What we look at:
- When we can work, for business premises
- Access and height
- Mould in or behind the old joints
- How many rooms, windows or areas are involved
Some things make a job bigger or smaller: how much old sealant there is and how stubborn it is, any mould to treat, and whether we need extra access for height. We take all of that into account in your quote.
What to tell us for a quote
There’s no need for technical detail. It helps if you can tell us:
- Photos by email, if that is easy
- How we’d reach the joints
- What’s wrong with the joints now
- Whether it’s a home or a business
We do not publish a price list, because every job is different. Instead, your job gets its own clear quote. To get started, phone 0345 464 0079 or email us.
What the job involves on each surface
Aluminium frames
Bi-fold and sliding aluminium doors have long frame joints, and the threshold joint takes a lot of water. Each is cleaned, backed where deep and resealed.
Timber windows & doors
On period timber windows, traditional sand mastic is sometimes the better choice. We can talk through the options.