How To Fill Gaps and Seal a Shed to a Concrete Slab
How do you fill gaps and seal a new shed to a concrete slab?
For gaps of up to about 5mm between shed wall sheeting and a concrete slab, Selleys recommends Selleys Brick & Concrete Silicone. It bonds well to all the surfaces involved in this job — concrete, and the metal or fibre cement of the shed. If grey is not the colour you want, use Selleys Roof & Gutter Silicone in translucent instead.
One warning that matters more than the product choice: when sealing the outside base of a shed, do not apply sealant that blocks drainage gaps or holes. Shed bases are designed to let water that gets in find its way out again. Seal those openings shut and you turn a shed that drains into a shed that holds water against the slab.
Why Brick & Concrete Silicone is the first pick
The joint between a shed and a slab has an awkward combination of demands. It sits at ground level, so it gets wet. The slab is porous and alkaline; the shed wall is usually coated steel. And the two move relative to each other as the steel expands and contracts in the sun far more than the concrete does.
Brick & Concrete Silicone is built for exactly that. Its technical data sheet lists superior adhesion to concrete and fibre cement surfaces, and it is a neutral cure, non-corrosive formula — which matters where the sealant sits against galvanised or coated steel. Its published movement capability is ±50% (ISO 9047 Method B), which is unusually high, and it is certified to ISO 11600 Type F Class 25LM.
Published figures for both products
| Property | Brick & Concrete Silicone | Roof & Gutter Silicone |
|---|---|---|
| Technology | Neutral (oxime) cure silicone | Neutral cure, 100% silicone |
| Colour | Grey | Translucent, white and a range of Colorbond-matched colours |
| Pack size | 415g cartridge | 300g / 310g cartridges; 90mL |
| Movement capability | ±50% (ISO 9047) | ±25% |
| Skin time | 70 minutes | 5–10 minutes |
| Tooling time | 35 minutes | 5 minutes |
| Tack-free time | 90 minutes | 30 minutes |
| Cure rate | 2mm / 24 hours | 2mm / 24 hours |
| Elongation at break | 960% (ASTM D412) | 330% (ASTM D412) |
| Temperature resistance | Approx. −40°C to 140°C | −60°C to +150°C |
| Application temperature | −40°C to 140°C | +5°C to +30°C |
| Guarantee | 25 years against cracking, crumbling or peeling | 25 years against cracking and peeling |
Indicative data only, not intended for the preparation of specifications.
What those numbers mean for a shed base
±50% movement on Brick & Concrete Silicone means a 5mm joint can absorb 2.5mm of opening or closing without failing. A steel shed wall on a hot slab moves a great deal over a summer day, and that headroom is why this product suits the job better than a rigid filler.
The long working times are an advantage here. A 70-minute skin time and 35 minutes of tooling time on Brick & Concrete Silicone means you can run a long bead along one wall and come back to tool the whole length rather than working in short sections. Roof & Gutter Silicone skins in 5–10 minutes and has 5 minutes of tooling time, so with that product you must work in sections you can finish before it skins.
2mm per 24 hours is a depth rate. Both products cure at that rate, so a bead pressed 5mm into a gap needs the better part of three days to cure through. Do not hose the shed down the same afternoon.
How to do it
- Check the slab is clean, dry and sound. Both technical data sheets call for surfaces free of oil, dirt and grease; Brick & Concrete specifies cleaning with mineral turps and drying. Sweep out the sand and grit that always collects at the base of a new shed.
- Identify the drainage openings before you start and mark them, so you do not seal them by accident once you get into a rhythm.
- Cut the tip off the cartridge and cut the nozzle at 45 degrees, sized to the gap.
- Hold the gun at 45 degrees and gun the sealant ahead of the nozzle so it is pushed into the joint, not smeared over it. A bead sitting on the surface has almost no depth of bond and will peel.
- Tool with a spatula dipped in soapy water for a smooth finish — within the tooling time for the product you are using.
- Clean up before the sealant skins with a cloth dampened in mineral turps.
- Do the inside edge as well if you want a full seal, and leave the outside drainage points open.
Limitations — read these before you seal the outside
- Do not block drainage gaps or holes. This is the specific caution Selleys gives for this job.
- Brick & Concrete Silicone must not be used in below-ground applications, and is not suitable for below-waterline use. A shed base sitting at grade is fine; a joint being buried is not.
- Roof & Gutter Silicone is not recommended for continuous submersion or below-waterline use either, and should not be applied if the ambient or substrate temperature is below +5°C or above +40°C.
- Neither is paintable. Complete any painting seven days before applying the sealant.
- Neither is for polyethylene, polypropylene or Teflon. Pre-test plastics before use.
- Avoid contact with bituminous materials or high-plasticiser rubbers when using Roof & Gutter Silicone — components can migrate into the sealant and cause staining or reduced performance.
- Both need atmospheric moisture to cure. On non-porous substrates or in low humidity, curing slows.
Common questions about sealing a shed to a slab
Should I seal the inside, the outside, or both?
Both is fine, and it is what the original question described. The distinction is that the outside bead must leave the drainage openings clear.
Why does the drainage matter so much?
Water that finds its way into a shed base needs a path out. Sealing the base continuously traps it against the slab and the bottom of the wall sheet, which is the worst place for it to sit.
My gaps vary from 2mm to 5mm — is that a problem?
No. Both products handle that range. Gun more product into the wider sections and tool the whole run to an even face.
Can I use Roof & Gutter Silicone throughout instead?
Yes, and it is the option Selleys names if grey does not suit — translucent is the colour specified. Note the shorter skin and tooling times, and work in shorter sections.
Will it stick to a Colorbond shed wall?
Roof & Gutter Silicone lists Colorbond sheet and coated metal panels among its suitable substrates, along with aluminium, steel, stainless steel, galvanised steel and powder-coated steel. Brick & Concrete Silicone lists metal among its suitable materials.
How long before rain is safe?
Both skin well before they cure — Roof & Gutter in 5–10 minutes, Brick & Concrete in 70. The skin sheds water; the cure takes 2mm per 24 hours of bead depth.
Can I paint the bead to match the shed?
No. Neither silicone is paintable. If you need a paintable finish, complete the painting first and seal afterwards.
What about the joint where the shed meets a brick wall instead?
Brick & Concrete Silicone is specified for bricks and mortar, fibre cement, concrete, marble and stone, so it covers that too.
Is there a related answer for a garden shed base?
What is the topic of this Selleys expert advice page?
The page addresses how to fill gaps and seal a shed to a concrete slab, including sealing both the outside and inside edges where gaps of up to 5mm may exist.
Which Selleys sealant is recommended for accommodating high movement between a shed and a concrete slab?
Selleys 630 is recommended, as it is a premium low modulus multipurpose elastic joint sealant with excellent adhesion to concrete and ±35% joint movement capability, forming a strong, elastic waterproof seal once cured that is paintable with water-based paints.
What are Proseries Flexiseal and Proseries Flexiseal FC used for?
Proseries Flexiseal and Proseries Flexiseal FC are polyurethane-based sealants designed for weatherproofing joints between building materials including concrete, offering ±25% joint movement capability and excellent resistance to aging and weathering for both indoor and outdoor applications.
What is Pro Trade Joint Sealant FC and what movement capability does it offer?
Pro Trade Joint Sealant FC is a fast-curing polyurethane adhesive and sealant with ±25% joint movement capability, offering excellent adhesion to concrete and weatherproofing properties, suitable for both vertical and horizontal applications.
How does Selleys 650 FC compare to the other recommended sealants for this job?
Selleys 650 FC is suitable for weatherproofing joints and offers ±20% joint movement capability, with excellent adhesion to concrete and resistance to aging and weathering, though its movement capability is lower than Selleys 630 (±35%) and Proseries Flexiseal/Pro Trade Joint Sealant FC (±25%).
What surface preparation steps are needed before sealing a shed to a concrete slab?
Surfaces must be thoroughly cleaned to remove all dirt, grease, dust, loose material, and old or crumbling sealant from both the shed base and concrete slab, and both surfaces must be completely dry before application to ensure proper adhesion and curing.
Is a backing rod needed when sealing gaps between a shed and concrete slab?
For wider gaps, a backing rod should be installed as part of the surface preparation process before applying the sealant.