How to Seal Block Paving to Stop Weeds?

Block paving joints are a prime spot for weeds to take hold, and left alone, they can wash out the sand and leave blocks rocking or sinking. Sealing is the most dependable fix: clear the weeds, resand the joints, and apply two thin coats of PU or resin-based sealant on a dry surface to keep a driveway weed-free for 3 to 5 years.

This guide explains why weeds grow in block paving, how to clear them, and how to prepare and seal the surface step by step. It also covers choosing the right sealant, how long it lasts, the mistakes that cause it to fail early, and when it is worth calling a professional.

Why Weeds Grow in Block Paving

Block paving is laid with sand in the joints instead of solid mortar, so it is never completely closed off from its surroundings. Three things tend to work together to let weeds take hold:

Loose, open jointing sand. Kiln-dried sand steadies the blocks but contains nothing to bind it, so it stays loose and welcoming to seeds and water.
Damp joints. Rain, condensation, and shade keep the sand moist for long enough for seeds to sprout and roots to settle in.
Organic debris. Leaves, dust, moss spores and grass clippings gather in the gaps over time, forming a compost-like layer for roots to feed on.

Common Weeds and Growth Found on Block Paving

Not every green patch on a driveway is the same problem, and recognizing what is growing helps in treating it properly before sealing:

Growth	Appearance	Typical Location
Dandelions	Long taproot and yellow flowers, regrows if any root is left	Sunny, open joints
Clover	Low, creeping plant with three-leaf clusters	Sunlit joints and edges
Chickweed	Thick mat with small white flowers	Damp, shaded joints
Moss	Green and cushion-like, with no visible roots	Shaded, north-facing or poorly drained areas
Algae	Dark green or black slime with no leaf structure	Permanently damp or shaded patches

Deep-rooted weeds such as dandelions must be pulled out or killed at the root, not just cut back; otherwise, they will force their way through a new seal within weeks.

What You’ll Need to Seal Block Paving

A stiff outdoor broom and/or a pressure washer with a fan nozzle
A weed killer made for hard surfaces, for any existing growth
Kiln-dried or polymeric jointing sand
A block paving sealant (see the selection guide below)
A roller, pump sprayer or soft-bristle brush for applying it
A squeegee, which is optional but helps spread sealant evenly over larger areas

How to Seal Block Paving to Stop Weeds: Step-by-Step

Step 1: Remove Existing Weeds, Root and All

Pull weeds by hand where possible, and make sure you get the root, because snapping off the top of a dandelion or similar taproot plant lets it return within days. For widespread or stubborn growth, apply a hard-surface weed killer 3 to 5 days before cleaning so it can kill the root system. Never seal over living weeds. The sealant cannot stop a plant that is already rooted, and it will simply grow through the new coating.

Step 2: Deep Clean and Fully Dry the Surface

Pressure wash the whole area with a fan nozzle held at a slight angle, not pointing the jet straight down into the joints. A close, direct jet can blast out too much sand or even wear the joint edges, leaving deeper gaps than before. Work section by section, clearing dirt, moss, algae, and any oil or organic staining as you go.

Drying time matters more than many guides admit. Moisture trapped deep in the joints, and not just on the surface, is one of the most frequent reasons sealant fails, later showing up as patchy whitening or peeling. Allow at least 24 hours in mild weather and up to 48 hours in cool or damp conditions before moving on.

Step 3: Resand the Joints

Sweep fresh sand into every joint until it is packed level, or about 1cm below the block surface, giving the sealant room to bond without pooling on top. There are two sand options to weigh up:

Kiln-dried sand: cheaper and easy to apply, but it stays loose and can still wash out over time.
Polymeric sand: costs more, but contains a binder that hardens when activated with water, creating a semi-rigid joint that resists weeds and washout far better than kiln-dried sand alone.

For weed prevention in particular, polymeric sand gives a clearly better long-term result, especially on driveways that face regular rain and vehicle weight.

Afterwards, brush every trace of loose sand dust off the block surfaces, because leftover dust can cloud or discolour the finished sealant.

Step 4: Choose the Right Sealant

Sealants don’t all behave the same once applied, and the wrong choice for your paving or level of use can cause whitening, poor adhesion, or a glossy finish that offers little real weed protection.

Sealant Type	Weed Resistance	Best Suited To	Typical Coverage
Acrylic (water or solvent-based)	Good	Patios and lightly used paths	5 to 7m² per litre
Polyurethane (PU)	Excellent	Busy driveways	4 to 6m² per litre
Resin-based	Very good	Wider joints and older paving	4 to 6m² per litre

PU and resin-based sealants soak deeper into the joint, where the weed-blocking work really happens. A shiny surface coat achieves little if the joint beneath stays porous.

Step 5: Apply the Sealant Correctly

Use thin, even coats. Two thin coats consistently beat one thick one, because a single heavy application can collect in low spots, oversaturate the sand and cause it to lift instead of harden. Work in manageable sections with a roller, sprayer, or brush, and focus on fully saturating the joints rather than just coating the faces of the blocks. Allow each coat to become touch-dry before applying the next.

Step 6: Respect the Cure Time

Most sealants feel dry to the touch within a few hours, but they need much longer to cure completely:

Foot traffic: usually safe after 12 to 24 hours
Vehicle traffic: usually 48 to 72 hours, sometimes longer in cold or humid weather
Full hardness: up to two weeks with some products

Always check the manufacturer’s datasheet, as cure times differ noticeably between acrylic, PU and resin formulas.

If the job seems too time-consuming or complicated, experienced paving contractors in Enfield can clean, resand, and seal the block paving correctly, providing long-lasting protection against weeds and weather damage.

Common Mistakes That Cause Sealing to Fail

Sealing over damp joints
Applying too much sealant at once
Using the surface too soon
Wrong sealant for the paving or jointing type
Sealing over live weeds

How Long Does Sealing Keep Weeds Away?

A seal applied correctly and maintained well typically lasts 3 to 5 years before it needs redoing, though this depends heavily on traffic, sun exposure, and product quality. Shaded, damp, or heavily used driveways often need resealing closer to the 3-year mark, while lightly used patios can often last 5 years or more.

Signs that it is time to reseal include:

Water no longer beads on the surface
The colour looks dull, faded or patchy
The first few weeds begin reappearing in the joints

Ongoing Tips & Maintenance To Control Weeds from Block Paving

Sweep regularly so leaves, dust and organic debris do not settle in the joints
Rinse off moss, algae and bird droppings quickly instead of letting them sit
Top up the jointing sand wherever it looks low, even on sealed paving
Treat individual joints as soon as weeds reappear, rather than waiting to reseal the whole surface
Avoid aggressive, close-range jet washing straight into the joints during routine cleaning, as it strips sand and sealant too early

DIY vs Professional Sealing

Sealing block paving is a realistic DIY task for most homeowners, provided you properly clean, resand, and fully dry the surface first. The process is not complicated, but it punishes shortcuts. Professional application often makes more sense for larger driveways, surfaces with extensive weed damage, or when you want an even, streak-free finish, because professional equipment and experience reduce the risk of pooling, patchiness, or missed joints.

A Local Recommendation: First 4 Paving

If you are planning to seal, clean, or replace a block paving driveway, First 4 Paving is a professional paving and landscaping company in Enfield, covering the surrounding Hertfordshire area. The expert team installs block paving driveways, cleans and seals driveways, and handles garden fencing and landscaping, so one local team can manage the whole job, from preparing the surface to the finishing touches.

As with any paving contractor, ask about relevant experience, request a written quote, and confirm insurance before agreeing to any work.

Website: https://first4paving.co.uk/
Email: [email protected]
Phone: 020 3953 5878
Address: 252, 204 Baker St, Enfield EN1 3JY, United Kingdom