How To Seal an Exterior Gap in Concrete
How do you seal a long 10mm exterior gap between a concrete path and a brick wall?
Selleys ProSeries Flexiseal is the best product for sealing these gaps. It is a highly flexible polyurethane construction sealant, commonly used in construction and expansion joints.
Two things about the installation matter as much as the product. Use a polyethylene foam backing rod under the sealant to avoid three-sided adhesion, and make sure all the surfaces are clean and free of dirt or crumbly cement so the sealant can get a proper grip. The backing rod lets the sealant flex within the joint more easily.
What three-sided adhesion is, and why it wrecks a joint
This is the single most useful idea in the answer and it is worth spelling out, because it is invisible once the joint is finished.
A sealant in an expansion joint works by stretching. When the path and the wall move apart, the bead has to elongate to bridge the extra distance. That only works if the bead is anchored to the two sides of the joint and is free in the middle.
If the sealant also bonds to the bottom of the joint — the third side — it cannot stretch. Instead of elongating across its width, it is pinned at the base and tears, usually at the bond line on one face. The joint then leaks along a hairline crack that is almost impossible to see and impossible to repair without cutting the whole run out.
A closed-cell polyethylene foam rod pushed into the bottom of the joint does two jobs at once: it stops the sealant reaching the base, and it sets the depth of the bead so you are not filling a 40mm-deep gap with sealant. Selleys' construction sealant data sheets state it directly — sealant must adhere to the sides of the joint only, and not to the back, so that it is free to stretch.
Sizing the joint and the rod
The published joint design guidance for the Selleys construction sealants is consistent:
- Optimum width-to-depth ratio for joint sealing is 2:1.
- Minimum joint width and depth is 6mm.
- For joint widths up to 12mm, use a width-to-depth ratio of 1:1.
- For joint widths greater than 12mm, use 2:1.
- Backing rod should be 25%–50% larger than the joint width, so it grips and stays put.
Your 10mm gap therefore takes a bead roughly 10mm deep at a 1:1 ratio, over a foam rod of about 12.5–15mm diameter compressed into the joint below it.
A wider joint accommodates more total movement than a narrow one, which is worth remembering if you are choosing where to cut a control joint rather than sealing one that already exists.
Published figures for ProSeries Flexiseal
| Property | ProSeries Flexiseal |
|---|---|
| Technology | Non-slumping polyurethane construction sealant |
| Modulus | Low modulus |
| Joint movement capability | ±25% |
| Adhesion | Excellent to common building materials; good adhesion without priming to the materials indicated, with pre-testing recommended |
| Weather | Weatherproof; suitable indoors and outdoors |
| Paintable | Yes, after it has cured |
| Application | Stores and applies better at moderate temperatures, around 20°C, especially in cold weather |
| Clean up | Mineral turps before cure; mechanically once cured |
Indicative data only, not intended for the preparation of specifications.
What ±25% means on a 10mm joint. The bead can absorb about 2.5mm of opening or closing. A concrete path against a brick wall, both in full sun, moves seasonally and daily — that headroom is exactly why a low-modulus polyurethane is specified for the job instead of a rigid filler or a mortar patch.
Low modulus means the sealant is soft enough to stretch without pulling hard on the joint faces. On a friable concrete edge or an old mortar joint, a stiff sealant can pull the substrate apart rather than stretching itself. Low modulus is a deliberate choice for masonry.
Preparing and sealing the run
- Rake the joint out. Remove old sealant, dirt, moss and any crumbly cement — this is the part of the job the answer specifically calls out, and a bead stuck to loose material is only as strong as that material.
- Brush the joint clean. A wire brush is effective on concrete and brick.
- Check that the joint is dry. Applying a polyurethane sealant to substrates holding high moisture can cause bubbling within the bead as it cures.
- Press the foam backing rod in to the depth you want, using a rod 25–50% wider than the joint so it stays compressed in place. Keep the depth consistent along the run.
- Mask both edges if you want a clean line on face brickwork.
- Extrude the sealant into the joint so it is in full contact with both sides, avoiding overlaps that trap air.
- Tool the bead with a spatula, forcing it into the joint for better contact with the sides, and pull the masking tape before it skins.
- Clean tools and spills with mineral turps before the product cures. Once cured it can only be removed mechanically.
Limitations — read before you start
- Do not apply if the air or substrate temperature is below 5°C. The product also stores and applies better at moderate temperatures around 20°C.
- It requires atmospheric moisture to cure.
- Not recommended for continuous submersion or below-waterline use. A path-to-wall joint that ponds after rain is at the edge of this; one that is permanently under water is outside it.
- Do not apply in the presence of curing silicone sealants — the cure of the polyurethane can be affected.
- Avoid contact with alcohol and other solvent cleaners during cure, for the same reason.
- Not for structural or general glazing.
- White sealant may yellow slightly with age and sun exposure, and the product may affect the drying of alkyd or oil-based paints.
- Classified as hazardous — the polyurethane products contain isocyanates. Wear gloves and safety glasses and read the safety data sheet.
Common questions about sealing a path-to-wall joint
Do I really need a backing rod?
On a 10mm joint of any depth, yes. Without it the sealant bonds to the base of the joint and cannot stretch, which is the failure mode this whole answer is designed to avoid.
What if I cannot get backing rod?
Use polyethylene or polyurethane foam rod of the right diameter — that is what the data sheets specify. Do not substitute something the sealant will bond to, because that reintroduces the problem.
How deep should the bead be?
For a 10mm joint, 1:1 — about 10mm deep. For joints wider than 12mm, use a 2:1 width-to-depth ratio.
Can I paint over the finished bead?
ProSeries Flexiseal is paintable after it has cured. Note that it may affect the drying of alkyd or oil-based paints, so a water-based system is the simpler path.
Would a silicone work instead?
Selleys Brick & Concrete Silicone is a neutral cure silicone with ±50% movement capability and superior adhesion to concrete, but it must not be used in below-ground applications and it is not paintable. The polyurethane is what Selleys names for construction and expansion joints of this kind.
The gap runs the length of the house — can I do it in sections?
Yes, but work in lengths you can gun and tool before the sealant skins, and try to avoid stopping mid-run in a visible spot. Joins in a bead are weak points.
What about the crumbly concrete edge?
It has to come off. Sealant bonded to material that is already failing will lift with it. Break the loose edge back to sound concrete before you seal.
Is this the same approach for a gap between a slab and a shed?
Similar, but the products differ — see filling gaps and sealing a shed to a concrete slab, and sealing timber to concrete for a timber-framed junction.
What question does this Selleys 'Ask an Expert' page address?
The page addresses the question: "I need to seal long exterior gaps between a concrete path and brick wall around a house? The gap is 10mm."
What is the topic of this Selleys expert advice page?
The page is titled "How To Seal an Exterior Gap in Concrete" and provides expert advice from Selleys on effectively sealing an exterior gap in concrete to keep surfaces looking great and protected from the elements.
What surface preparation is needed before sealing an exterior concrete gap?
Before applying any sealant, the concrete surface must be clean, dry, and free from oil, grease, release agents, dust, and loose material, as this directly impacts adhesion and the longevity of the seal.
Which Selleys product is recommended for waterproof sealing of exterior concrete joints, and what guarantee does it offer?
Selleys Brick & Concrete Silicone is recommended for weather protection and joint sealing on exterior concrete. It provides superior waterproof sealing with high flexibility and excellent adhesion to concrete surfaces, backed by a 25-year guarantee against cracking, crumbling, or peeling.
What other sealant options exist besides Selleys Brick & Concrete Silicone for sealing exterior concrete gaps?
Pro Trade Joint Sealant LM is designed for weatherproofing joints, offering excellent resistance to aging and weathering with +/-25% joint movement capability. Selleys 630 is a low modulus elastic sealant with +/-35% joint movement capability, suited for expansion joints and construction gaps in concrete.
What joint width-to-depth ratio should be maintained when sealing a concrete gap, and what else is needed?
For optimal performance, a width-to-depth ratio of approximately 2:1 should be maintained, and a suitable backing rod should be installed to control sealant depth and ensure proper free movement of the sealant within the joint.
How should the sealant be applied into the gap between concrete and a brick wall?
The nozzle should be cut at a 45-degree angle, and the sealant extruded into the gap to ensure full contact with both sides of the joint. A spatula should then be used to smooth the sealant and force it into the gap for better adhesion.
How long should sealant cure before it is exposed to water or weather?
The sealant should be allowed to cure fully before being exposed to water or weather stress.