Mind the GAP! What’s up with all these metal aggregates, and how are they used?

Mind the GAP! What’s up with all these metal aggregates, and how are they used?

You know you need it - that essential but unseen layer beneath the beautiful pebbles, JakMat and concrete - but what do GAPs, SAPs and even WAPs actually mean and what do we use them for? This year, post floods, it seems everyone needs aggregates, so here’s a useful guide to their functions.

GAP Metals are the perfect base layer

GAP actually stands for General All Passing through a screen. So GAP metal is screened and sorted into different sizes. Central Landscapes has GAP starting at GAP 7 (7mm) all the way up to GAP 65. Each grade of metal will contain a mixture of sizes - from very fine rock dust (known as fines), up to the maximum size of the screen. Having fines in the mix is important, as it helps bind the larger stones together. The bigger the grade of GAP the deeper you can lay it which is helpful for very muddy areas. 

 GAP 7

A very fine metal, GAP 7 is used by landscapers and builders as a compacted base under pavers or as a base layer before laying concrete. GAP 7 may be used on top of GAP40 and GAP 20 to help smooth out the gaps (excuse the pun) before laying the perfect top surface of pavers, pebbles or JakMat.

 GAP 20

GAP 20 is our most popular, used when creating a sub-base layer for driveways, paths or as a base for other hard landscaping because it compacts well and creates a strong foundation for subsequent layers. Trade professionals typically add a finer layer of compacted GAP 7 on top of the GAP 20 or GAP 40 to help with leveling before laying pavers or a product like JakMat.

 GAP 40

GAP 40 and GAP 65 are heavier grades of metal. GAP 40 is commonly used as a sub-base for driveways, whereas GAP 65 is a heavy-duty product for roading or sites where soft ground may be an issue, particularly during the erosion created by the rains of 2023.

SAPs function for drainage projects - Yay!  


SAP 7 stands for ‘Scoria All Passing’ 7mm. This small grade is used for bedding around pipes and ducting. As SAP7 can tend to compact down we don’t recommend using it as standalone drainage. 

 Scoria 25/7

Scoria 25/7 is the grade of drainage metal that is screened to particles between 7mm and 25mm. You can use this for drainage as its composition ensures the scoria particles stay open rather than compacting together. Commonly used for drainage beds and behind retaining walls.

 Scoria 50/20

Scoria 50/20 is a larger size than 25/7 and the agregates range from 50mm to 20mm. This grade is good for retaining walls and soak pits.  

WAP - Chip aggregate sub-surface for roads & paths

This stands for Winstones All Passing’ and refers to  products introduced by Winstones quarries, certified for specific uses by Councils.  

 WAP 7

WAP 7 is a 7mm screened aggregate that has been washed to remove the fines, offering greater permeability. WAP 7 is specified to lay as a base for certain permeable paving products offered in the NZ market. 

 WAP 12

WAP 12 is a larger grade of aggregate used for the same purposes. 

Hoggin - semi-permeable top layer for paths & patios etc


Hoggin is a fine grade of crushed limestone that compacts really well. Hoggin is an economical alternative to concrete that looks great on paths and in patio areas. Semi-permeable, it limits puddling and doesn’t create water run-off like concrete.

Whatever your drainage or pebble project, Central Landscapes has the right aggregate to fill the gaps.

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