Water Around a Waffle Pod Slab

Water around a waffle pod slab on a reactive clay site is a problem, and it’s not an easy fix once the house is finished.

That’s why AS 2870, the Australian Standard for residential slabs and footings, requires surface drainage on a building site to be controlled from the start of site preparation to the end of construction. It is far easier to get good drainage around a waffle slab house while it is being built than after you have moved in.

Check out our detailed video on why waffle slabs are so affected by site drainage, and what to do about it.

Adding Water to Clay Makes It Swell

When water soaks into the ground, reactive clay absorbs it and swells. As the clay swells, the ground surface lifts.

On a reactive clay site, classified Class M, H1, H2 or E in your soil test, the swelling forces are big enough to lift a house noticeably.

Water Around a Slab

When water sits only around the perimeter of the slab, the ground around the perimeter swells and lifts, and the footings and slab directly above lift with it.

The dry ground under the middle of the house doesn’t swell, so the slab height there doesn’t change.

High edges and a low interior is called a dished slab, or edge heave.

If the ground is only wet on one side of the house, only that side swells and the slab looks as if it has tipped slightly.

If the whole area under the slab were saturated during construction, the whole slab would lift more or less evenly.

Water Lying Around a Waffle Pod Slab

Water sitting against a waffle slab isn’t just sitting there. It can, and probably is, seeping in under the house. That is especially likely near a patio or alfresco area, which is set lower into the ground than the rest of the house.

Now the clay under the slab swells and lifts too.

But the lift is still uneven. The ground where water soaks in rises, and the ground that stays dry doesn’t.

Now Let’s Build

The slab has now deflected because the soil under it has swollen.

The carpenter arrives to frame the walls, and the bricklayer arrives to lay the bricks.

Both take great care to build walls that are straight and plumb. They notice variations in the slab height and assume the concreter didn’t pour a dead flat slab. So they adjust their work to suit, and build nice, straight, true walls on a deflected slab.

The walls get plastered and painted, and you move in.

And the Movement Starts

You know not to water around the house on a Class H1 or H2 site, because you read the Cornell Engineers website (ps, yay you!). So you take great care not to water around the house, not to plant trees against it, and to keep the site well drained.

Over the next twelve months, the ground around the house dries out, and the slab surface drops back towards its original height.

Your brickwork cracks, your plasterboard cracks, and you panic.

You’ve done everything right, and you still have cracks in your house.

The builder sends an engineer out to check the slab levels, and they look pretty good. The slab is nice and flat, but there is damage everywhere.

This is why slab levels taken after the event don’t tell the whole story. A slab that is flat today may have been well out of level when the walls were built on it.

Why Me?

Did the builder do a good job?

No.

Site drainage has to be maintained from the start of site preparation, all through construction, and until the end of time.

• Don’t let water sit against the slab of your newly poured house.
• Don’t let water sit against the slab of your newly bricked-up house.
• Don’t let water sit against your finished, ready-to-live-in house.

If you see water ponding against your slab during construction, speak to your builder. Point out the engineer’s requirement for site drainage to be maintained, and ask for it to be fixed straight away.

The effects of water sitting against a house during construction may not be permanent. Until they are.

If You’re a Builder

If you’re a builder reading this (well, welcome!), yes, some sites are hard to keep drained. But do you really want to keep going back to a house with slab heave?

The National Construction Code requires the ground around a slab to be graded so surface water drains away from the building, with a fall of at least 50 mm over the first metre. That applies whether the site is flat or slopes towards the building.

I’m sorry if you think this is optional. It is not.

If you can, raise the pad with compacted clayey fill so the whole site sheds water off and away from the building pad. Then:

• Use spoon drains on the uphill side to direct stormwater around the house.
• Keep an eye on your plumber. Plumbing trenches under the building backfilled with sand become conduits for water under the slab. Get the plumber to follow the AS 2870 requirements your engineer has specified, very carefully.
• Keep an eye on your contractors. Don’t let bog holes form and stay unfilled; fill them with clayey fill. Turn off taps. Connect downpipes as soon as possible.
• Find a way to drain water from under slab set-downs like verandahs and patios. Talk to your engineer about this, because it is a major weakness of waffle pod slabs.
• Read the engineer’s plans for site maintenance requirements, and pass them on to your subcontractors and then the owners.
• Articulate pipes where they need to be, so they can move without breaking.
• Keep subsurface drains away from buildings. Badly placed ag drains feed water to the footings.
• Seal the joints between paths and the house with Abelflex and flexible sealant.

Waffle Slabs on Piers

Say your waffle slab is on piers that go down to firm ground or rock. Things change slightly, but it depends on how the piers were built.

If the piers were poured first and the waffle slab poured on top without being connected to them, swelling soil can lift the slab off the piers. If dirt gets between the piers and the slab, the house can’t settle back down when the soil dries out. It may stabilise, but permanently out of level. Bad.

If the piers are reinforced and connected to the slab, the heave is partly resisted by friction between the piers and the ground, especially if the piers are deep. The slab movement is moderated. Good.

Water Around a Waffle Pod Slab: The Bottom Line

Waffle pod slabs let water get under a house. If water is sitting next to a waffle slab, it is probably draining under it. Don’t let water pond next to a waffle slab.

It’s just not swell.

For more on why waffle slabs are so sensitive to this, see beware waffle slabs, waffle slabs on H2 class soil and the inevitable failure of waffle slabs.

Cracks in a New Waffle Slab House?

If your new house is cracking and the slab levels “look fine”, the cause still needs to be properly investigated. Cornell Engineers investigates cracking and movement in waffle slab houses, including the role of site drainage during and after construction. Where there is a dispute, we also prepare independent expert reports. See our forensic structural engineering page for more.

Send us the property address, photographs of the cracking and of the ground around the house (including any construction-stage photos you took), a short description of what has happened, and your soil report and engineering drawings if you have them. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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