Is tanking better than injection?

They answer different questions, so neither is better in general. Injection seals a defined path — a crack, a joint, a tie hole — and does nothing for water arriving through the whole face. Tanking treats the face as a whole and does nothing durable about a live crack passing through it, because it has no movement tolerance. On a real basement the two are frequently used together: inject the defined paths, then tank the face for what remains. A contractor recommending one without establishing which of those two conditions applies has not done the diagnosis.

Why is excavating and waterproofing the outside not always recommended if it is the best method?

Because the waterproofing is rarely the expensive part. Excavating to a below-ground wall means removing the driveway, path or garden above it, battering or shoring the excavation, managing groundwater in the hole, and reinstating everything afterwards. On a boundary wall, under a neighbouring structure, or where the water table is close to the surface, it may not be available at any price. Where it is available it is the most durable answer, and the honest comparison is between its total cost and the accepted compromise of a dry-side method.

Does a drained cavity system mean the basement is always slightly wet?

The structure behind the membrane may well be, and the occupied space is not. That is the design intent rather than a shortcoming: instead of trying to defeat hydrostatic pressure, the system gives water a controlled route to a drain and a sump, and the internal finish is built in front of it. The trade-off is real though — it consumes internal space, and it introduces a mechanical dependency, because a sump pump needs power, needs servicing, and will eventually fail. The backup arrangement belongs in the design.

Can crystalline waterproofing be applied over a painted wall?

Not effectively. Crystalline systems work by reacting with moisture and free lime in the concrete to grow crystals within its pore structure, which requires contact with a sound, clean, absorbent substrate. A paint film, a previous coating, oil contamination or a laitance layer all prevent that reaction. Preparation is therefore not a nicety on this method, it is the method — and a quote that does not describe how the substrate will be prepared has left out the part that determines whether it works.

Which of the four is most likely to move the leak somewhere else?

All three dry-side methods can, and injection most visibly, because it closes one specific path and leaves the rest of the element as it was. The water arriving behind the structure has not gone anywhere; it now has one fewer route through and will find the next weakest point, which may be behind a wall lining, into a lift shaft, or into an adjoining property. That is not a reason to avoid dry-side work. It is the reason the risk belongs in writing before the work rather than in an argument afterwards.

Can these be staged over several budget years?

Often yes, and for a strata building it is frequently the only realistic path. A defensible staging runs drainage first, then injection of the defined paths, then face treatment or a cavity system if the first two do not resolve it — which has the useful property that each stage is worth doing on its own and each informs the next. What does not work is staging in the opposite direction: a face treatment applied before the drainage is fixed is treating a symptom that the next stage was going to remove.

Guide

Tanking, injection, drained cavity or excavation: which one a wet basement needs

Updated 25 August 2026

A wet basement, undercroft or below-ground store generally attracts one quote per contractor type, each recommending the thing that contractor does. That is not necessarily dishonest — the four approaches genuinely do different things — but it leaves the owner comparing answers to different questions.

There are four options in general use. Injection seals a defined path through the element. Crystalline or cementitious tanking treats the face of the element. A drained cavity system accepts that water will arrive and manages it. Excavation and an external membrane stops water reaching the structure at all.

What follows is what each one physically does, the conditions that make it right or wrong, relative cost and life, and the two questions that narrow four options down to one or two before anybody quotes.

The two questions that eliminate most of the options

Before comparing methods, two findings do most of the work of choosing between them.

First: does the water arrive along a defined path, or through the whole face? A crack, a construction joint, a tie-bolt hole or a kicker joint is a defined path, and injection exists precisely to seal those. Water pushing through the body of porous or honeycombed concrete across a broad area is not a path, and there is nothing for injection to fill or key into. That single distinction removes injection from consideration on a large share of enquiries, and it is established by observation rather than by opinion — see the seven things water coming through concrete can be.

Second: can anybody excavate? An external membrane is the most durable answer available and it is frequently impossible — a wall on a boundary, a structure under a driveway or a neighbouring building, mature landscaping, or a water table close enough to the surface that the excavation floods. Where excavation is genuinely off the table, the field narrows to the three dry-side options and the conversation becomes about which compromise is best understood.

The four options side by side

	Crack injection	Crystalline tanking	Drained cavity	Excavate & membrane
What it physically does	Fills a defined path through the element and blocks it	Reacts with moisture and free lime to grow crystals in the pore structure, densifying the concrete	Accepts water at the face, channels it behind a dimpled membrane to a drain and a sump	Puts a barrier on the outside face so water never reaches the structure
Requires a defined path	Yes — this is the whole premise	No; treats the face as a whole	No	No
Needs excavation	No	No	No	Yes, which is usually what decides it
Finished appearance	Patched packer holes; making good is a separate line	A cementitious coating; can be rendered or painted once cured	Loses internal space; the membrane is covered by a lining	Unchanged inside
Relative cost	Lowest, priced per linear metre of path	Moderate, priced per square metre of face	Moderate to high; includes a pump that needs power and maintenance	Highest by a wide margin once excavation, disposal and reinstatement are counted
Main failure mode	Water redirects to the next weakest point	Fails on a moving crack or where the substrate is unsound	Pump failure, or the drainage silting up	Damage during backfill, or defects at penetrations and terminations
Ongoing maintenance	None beyond inspection	None if the substrate stays sound	Yes — the pump and the drainage line need servicing	None realistically accessible

Relative comparison. Costs are indicative orders of magnitude for South East Queensland, not quotes.

Injection: right when there is a path, wrong when there is not

Resin injection is the cheapest of the four by a wide margin and the least disruptive, which is exactly why it gets recommended for situations it does not suit.

It is the correct answer for cracks, construction and cold joints, kicker joints, tie-bolt holes and honeycombed zones with a definable extent. On those it is not a compromise at all — a properly executed injection through mechanical packers, sealed within the body of the element, is a durable repair.

It is the wrong answer where water comes through the whole face, where the element is still moving, where the real cause is drainage, or where the membrane above has reached the end of its life. Those are covered in detail in the fifteen situations where injection is the wrong repair, and between them they account for a large share of enquiries that arrive asking for injection by name.

Crystalline and cementitious tanking: treating the face

Crystalline waterproofing is applied as a cementitious slurry to the inside face. Its active chemistry reacts with moisture and free lime in the concrete to grow crystals within the pore structure, densifying the concrete near the surface so that it resists water passing through it. Cementitious tanking systems work on a related principle, forming a bonded barrier coat.

The strength of the approach is that it does not need a defined path — it treats an entire face, which makes it the natural answer to diffuse seepage through porous concrete. It also becomes part of the substrate rather than sitting on it, so unlike a paint-on coating it is not simply pushed off by pressure behind it.

The limitations are specific. It needs a sound, clean, absorbent substrate to react with, so it does not work over a contaminated, oily or previously coated face without preparation. It has no movement tolerance, so a live crack passing through a tanked area will telegraph straight through it. And it does nothing about hydrostatic pressure — it makes the concrete resist water, but the water is still there and still pushing.

In practice tanking and injection are often used together rather than as alternatives: inject the defined paths first, then tank the face to deal with what is left.

Drained cavity: managing water instead of fighting it

A drained cavity system takes the opposite philosophical position from the other three. Rather than trying to stop water reaching the inside face, it accepts that it will, and gives it somewhere to go.

A dimpled membrane is fixed to the wall and floor, creating a void behind the finished surface. Water arriving at the structure runs down that void into a perimeter drainage channel, from there to a sump, and out via a pump. The internal finish is built in front of the membrane and stays dry regardless of what the wall behind it is doing.

It is the most reliable of the dry-side options in the hardest conditions, because it does not depend on defeating hydrostatic pressure — it works with it. That makes it a genuine answer where the water table is permanently high, which across the coastal sand strip is common.

Two costs are easy to underestimate. It consumes internal space, which in a small basement or store matters. And it introduces a mechanical dependency: a sump pump needs power, needs servicing, and will eventually fail. A pump failure in a system designed around pumping is not a leak, it is a flood, so the backup arrangement is part of the design rather than an optional extra.

Excavation and an external membrane: the durable answer nobody can always have

Waterproofing from the outside is the technically correct approach and everyone in the trade knows it. The membrane sits on the face the water arrives at, so hydrostatic pressure presses it against the structure rather than trying to push it off, and the concrete behind it stays dry rather than merely resisting.

It is also, frequently, impossible or disproportionate. Excavating to a below-ground wall means removing whatever is above it — a driveway, a garden, a path, sometimes a neighbour’s access — battering or shoring the excavation, dealing with groundwater in the hole, and reinstating everything afterwards. On a boundary wall or under an adjacent structure it may not be legally or physically available at all.

Where it is available, the questions that decide whether it lasts are about detailing rather than the membrane itself: protection during backfill, how upstands and terminations are formed, how penetrations are detailed, and whether the drainage behind the wall is restored or installed as part of the same scope. A new membrane over an old drainage arrangement is an expensive way to buy another decade of the same problem.

The order to consider them in

The cheapest and most permanent options are not the same thing, and neither is usually the one that gets quoted first. A defensible sequence works down this list.

Fix the drainage and the surface water firstBlocked or absent subsoil drains, weep holes rendered over during a repaint, a driveway falling toward the building, a downpipe discharging at the footing. This is behind a very large share of below-ground water problems, it is the cheapest thing on the list, and it sometimes removes the need for any of the four options.
Redirect the source where something upstream is loading the structureA downpipe extension, a re-graded path, a new fall or a diverter is cheaper, more reliable and more permanent than anything that can be injected or coated.
Seal defined paths by injectionWhere the water arrives along cracks, joints or penetrations, this is the targeted answer and the least disruptive. It is also where a competent scope names what is being sealed and what is not.
Treat the face where seepage is diffuseCrystalline or cementitious tanking, on a sound and properly prepared substrate. Frequently in combination with the previous step rather than instead of it.
Manage the water where it cannot be stoppedA drained cavity system where the head is permanent and defeating it is unrealistic — a structure below the water table, or coastal sand where groundwater moves with the tide.
Go outside where the value justifies it and access allowsExcavation and an external membrane, with the drainage restored as part of the same scope. The most durable answer, and the one whose cost is dominated by everything other than the waterproofing.

The compliance dimension, briefly

One factor sits outside the technical comparison and changes what should be written down. Working from the dry side is the technically compromised option, and the Australian standards for waterproofing largely address the positive side. That does not make negative-side work non-compliant, but it does mean the reasoning has to be documented rather than assumed.

In Queensland the statutory warranties implied into domestic building contracts cannot be excluded by a term on a quote, so a contractor writing "no warranty on negative side work" has written something void. What does carry weight is written advice given before the work — setting out that sealing one path may bring water out at another, and what the compliant alternative costs — followed by the owner instructing the work to proceed on that basis.

That structure is set out in full in where negative side waterproofing sits under Australian standards, and it is the reason the comparison above ends with a written recommendation rather than just a price.

Common questions

Is tanking better than injection?: They answer different questions, so neither is better in general. Injection seals a defined path — a crack, a joint, a tie hole — and does nothing for water arriving through the whole face. Tanking treats the face as a whole and does nothing durable about a live crack passing through it, because it has no movement tolerance. On a real basement the two are frequently used together: inject the defined paths, then tank the face for what remains. A contractor recommending one without establishing which of those two conditions applies has not done the diagnosis.
Why is excavating and waterproofing the outside not always recommended if it is the best method?: Because the waterproofing is rarely the expensive part. Excavating to a below-ground wall means removing the driveway, path or garden above it, battering or shoring the excavation, managing groundwater in the hole, and reinstating everything afterwards. On a boundary wall, under a neighbouring structure, or where the water table is close to the surface, it may not be available at any price. Where it is available it is the most durable answer, and the honest comparison is between its total cost and the accepted compromise of a dry-side method.
Does a drained cavity system mean the basement is always slightly wet?: The structure behind the membrane may well be, and the occupied space is not. That is the design intent rather than a shortcoming: instead of trying to defeat hydrostatic pressure, the system gives water a controlled route to a drain and a sump, and the internal finish is built in front of it. The trade-off is real though — it consumes internal space, and it introduces a mechanical dependency, because a sump pump needs power, needs servicing, and will eventually fail. The backup arrangement belongs in the design.
Can crystalline waterproofing be applied over a painted wall?: Not effectively. Crystalline systems work by reacting with moisture and free lime in the concrete to grow crystals within its pore structure, which requires contact with a sound, clean, absorbent substrate. A paint film, a previous coating, oil contamination or a laitance layer all prevent that reaction. Preparation is therefore not a nicety on this method, it is the method — and a quote that does not describe how the substrate will be prepared has left out the part that determines whether it works.
Which of the four is most likely to move the leak somewhere else?: All three dry-side methods can, and injection most visibly, because it closes one specific path and leaves the rest of the element as it was. The water arriving behind the structure has not gone anywhere; it now has one fewer route through and will find the next weakest point, which may be behind a wall lining, into a lift shaft, or into an adjoining property. That is not a reason to avoid dry-side work. It is the reason the risk belongs in writing before the work rather than in an argument afterwards.
Can these be staged over several budget years?: Often yes, and for a strata building it is frequently the only realistic path. A defensible staging runs drainage first, then injection of the defined paths, then face treatment or a cavity system if the first two do not resolve it — which has the useful property that each stage is worth doing on its own and each informs the next. What does not work is staging in the opposite direction: a face treatment applied before the drainage is fixed is treating a symptom that the next stage was going to remove.