Best Sealant for the Base of a Garden Shed

What is the best sealant to stop water entering the bottom of a metal garden shed on a paver base?

Use Selleys Brick & Concrete Silicone. It is a highly flexible sealant that withstands building movement and has superior adhesion and resistance to water, and it bonds to both halves of this particular joint — metal on one side and concrete or masonry on the other.

Before you buy anything, though, read the next section. Where you apply it matters more than which product you use.

Seal the inside, not the outside

Most shed installers recommend sealing around the inside of the shed rather than the outside, and there is a good drainage reason for it.

A metal shed sits in a channel at its base. Water running down the outside of the walls is meant to reach that channel and drain away. If you seal the outside, you block the escape route: water collects in the metal channel, has nowhere to go, and eventually overflows back into the shed. You have converted a drainage path into a reservoir.

So if you do choose to seal the outside as well, do not block the joins at the corners, and do not block any drainage holes. Those are the outlets the design depends on.

Why Brick & Concrete Silicone suits this joint

A shed base on pavers is a hard case for a sealant. The steel wall expands and contracts substantially in the sun; the paver base does not. That difference has to go somewhere, and it goes into the sealant bead.

Brick & Concrete Silicone is rated at ±50% movement capability (ISO 9047, Method B) — the highest movement figure in this part of the Selleys range, and double what most gap fillers offer. That is the property that keeps the bead attached through a summer of expansion cycles rather than tearing away from one face.

Skin time	70 minutes (23°C, 50% RH)
Tooling time	35 minutes
Tack-free time	90 minutes
Cure rate	2mm per 24 hours
Full cure	Approximately 72 hours
Movement capability	±50% (ISO 9047, Method B)
Tensile strength	1.2 MPa

Indicative data only, not intended for the preparation of specifications.

The 35 minute tooling time is worth knowing, because a shed perimeter is a long run. You can seal a full wall and then go back and tool it, rather than stopping every metre. Skin forms at 70 minutes and tack-free at 90, so do not leave it much past half an hour.

The 2mm per 24 hour cure rate is the reason not to hose the shed down the same afternoon. Excess sealant can be trimmed with a sharp blade once full cure is reached at about 72 hours.

How to seal the base

1. Clear the joint. Sweep out grit and leaf litter from the inside base channel and remove any old, failed sealant — Selleys Sealant Remover softens the residue if there is an existing bead to strip.
2. Clean both faces so they are free of dirt, dust, grease, oil and loose material. On galvanised or Zincalume steel, remove any loose surface rust.
3. Let everything dry. This is the step most likely to be rushed on a shed floor that has been damp for weeks.
4. Cut the nozzle to the joint width and load a caulking gun.
5. Working from inside, extrude the sealant into the joint between the wall base and the paver, forcing it into the gap so it contacts both faces.
6. Tool the bead with a spatula within 35 minutes, leaving a smooth face angled so water runs off rather than sits on it.
7. If you seal outside as well, leave the corner joins and any drainage holes open.
8. Allow around 72 hours for full cure before washing the shed down or exposing the joint to heavy water.

Limitations

• Not paintable. Complete any painting seven days before applying the sealant.
• Not suitable for below waterline applications.
• Not for use in below ground applications. If the shed base sits below the surrounding ground level and water is arriving from beneath, sealing the joint is not the fix — the drainage is.
• Not for use on polyethylene, polypropylene or Teflon. Pre-test other plastics before use.

Where the water is really coming from

A sealant fixes a gap. It does not fix a slope. If water is pooling against the shed because the paver base sits level with or below the surrounding ground, sealing the base will hold that water against the steel rather than letting it drain, and you will trade a wet floor for a rust problem. Check that the base sheds water away from the shed before you seal it in.

Safety: working from inside the shed changes the ventilation question

Brick & Concrete Silicone carries the signal word Warning: flammable liquids Category 4 (H227, combustible liquid), skin irritation Category 2 (H315) and skin sensitisation Category 1 (H317, may cause an allergic skin reaction). It is not Dangerous Goods, but it is classified a C1 combustible liquid for storage and handling under AS 1940.

The instruction that defines this job also defines the exposure. This page tells you to seal from the inside, and for good drainage reasons. But a metal garden shed is a small sealed box, and the two things it usually contains are a mower and a fuel container. H227 is the mildest flammability rung there is — combustible, not flammable — and that is still worth respecting in a shed with petrol in it. Take the fuel and the mower out, prop the door open, and do not smoke inside while you work.

H317 and the 35-minute tooling window. A long run and a generous window is what tempts a bare wet finger. Skin sensitisation is not irritation: permanent once acquired, then triggered by far smaller exposures, and built by repeated casual contact. Use a spatula or a gloved finger.

Step 1 is where the serious classification comes in. If there is an old bead to strip, Selleys Sealant Remover is classified Danger, Poison Schedule S5, and is over 60% petroleum naphtha: H304 may be fatal if swallowed and enters airways, H226 flammable liquid and vapour — Dangerous Goods Class 3, UN 1133, Packing Group II — H336 may cause drowsiness or dizziness, and H315.

That combination in a closed metal shed is the worst pairing on this page. Class 3 Packing Group II is a genuine flammability rating, several steps above the sealant's H227, and hydrocarbon vapour is heavier than air, so it settles into the base channel you are working along rather than rising out of the door. A shed in the sun is also hot, which drives evaporation. Before any solvent comes out, get the fuel, the mower and anything with a pilot or an element out of the shed, open the door and any window, and take breaks in the open air.

H304 and sheds go badly together. Never decant the paste into a drink bottle or an unlabelled jar — sheds are where that happens. Lid on between applications, stored high, out of reach of children. If any is swallowed, do not induce vomiting: rinse the mouth, give a glass of water and seek medical advice immediately. Cut and scrape the bulk out first; the remover is meant for a residual film under 5mm.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions

Should I seal inside or outside?

Inside, as most shed installers recommend. Sealing the outside can trap water running down the walls in the metal channel, where it overflows back into the shed.

If I seal outside, what must I leave open?

The joins at the corners and any drainage holes.

Will it stick to a metal shed?

Yes. Metal is among its listed substrates, along with bricks and mortar, fibre cement, ceramics, porcelain, concrete, marble and stone, wood and plastics.

Will it stick to pavers?

Yes — concrete, masonry, brick and stone are all listed.

Can I paint over it to match the shed?

No. It is not paintable. Any painting must be completed seven days before the sealant is applied.

How long before it is fully cured?

Around 72 hours, curing at about 2mm per 24 hours.

How long do I have to tool it?

35 minutes. Skin time is 70 minutes.

What if water is coming up from under the pavers?

That is a below ground problem, and this product is not for below ground applications. Fix the drainage or the levels first.

How do I trim excess once it has set?

After full cure, trim it with a sharp blade.

Is there a paintable alternative?

Not among the silicones — none of them can be painted. If the bead has to be painted, No More Gaps Exterior is paintable, but its own data sheet directs you to a silicone sealant where excessive joint movement occurs or the area sees frequent water immersion, which describes a shed base.

Is there a related answer for sealing a shed to a slab?

Yes — how to fill gaps and seal a shed to a concrete slab.