Ad stacking

Ad stacking is a fraud scheme in which several ads are layered on top of each other in the same slot on a page or in an app. The viewer sees only the top ad, but every ad underneath still records an impression and gets billed.

What is ad stacking?

Google lists ad stacking under “fraudulent ad placements,” one type of invalid traffic, and describes it as a publisher layering multiple ads so only the top ad is visible (Google Ads Help, About invalid traffic, accessed October 2026).

The Media Rating Council groups it with hidden, stacked, covered and transparent ad serving, gives “Z-order stacking” as an example, and classifies it as sophisticated invalid traffic (SIVT). This category takes deeper analysis to detect (MRC, IVT Standards Addendum, June 2020).

How does ad stacking work?

Browsers draw page elements in layer order (z-index). A fraudulent publisher places five 300×250 ad frames at the same coordinates. Each time the page loads, all five frames request an ad and load the creative, so the ad server counts five impressions. The reader sees only the top frame.

Apps work the same way, with several ad views stacked or hidden under content. Google Play bans apps from rendering ads that users cannot see (Google Play Console Help, Ad Fraud, accessed October 2026). AdSense also bans implementing ad code so that ads cover content or content covers ads (Google AdSense Help, Modifications of AdSense ad code, accessed October 2026).

Hypothetical example

A news site gets 20,000 page views a day, and each view has five ads stacked in one slot. The system records 100,000 impressions. At a CPM of 30,000 VND, the advertiser pays 3 million VND a day. 80,000 of those impressions sit in the lower layers, so 2.4 million VND pays for ads nobody saw.

Warning signs

• One placement has many impressions but a click-through rate close to zero.
• That placement’s viewability is far lower than the rest of the campaign.
• Several impressions are logged at the same moment from the same page view.

Google counts a display ad as viewable when at least 50% of its area is on screen for at least one second, and a video ad when 50% is on screen for at least 2 seconds of playback (Google Ads Help, Active View, accessed October 2026).

How to prevent ad stacking

1. Buy on viewable impressions. Average viewable CPM in Google Ads only counts impressions that meet the standard above.
2. Add the Invalid clicks column to your reports to see what Google has filtered. Invalid activity caught before billing is not charged, and activity caught afterward is credited where appropriate.
3. If something looks off, request an invalid traffic investigation for activity in the last 60 days.
4. Exclude suspicious placements and favor publishers you know.

How it differs from similar fraud types

The first two often go together: pixel stuffing squeezes ads into tiny frames, and ad stacking piles many frames into one spot. Both belong to the web and programmatic branch of ad fraud.

Frequently asked questions

Does ad stacking distort metrics beyond cost?

Yes. Inflated impressions push reported frequency and reach above reality and drag click-through rate down. The campaign looks like it has weak creative when the real problem is the placement.

Do I need to handle ad stacking myself when running Google Ads?

Google filters most of it and credits what it catches late. Your job is to review placement reports, exclude odd placements and buy on viewable impressions when running display.

If you need a second look at the placement reports of your display campaigns, you can talk to Adtek through our contact page.