Rubble & Dry Stack Rock Kneewalls

A kneewall is very similar to a retaining wall, and both are common elements in site hardscaping construction. In some cases retaining walls can also be knee walls, especially where they continue above the adjacent upper grade of land.

In the photo below the sandstone is laid in a dry stack configuration. The dry stack actually has a mortar but is intended to look like a traditional wall built without dependency on a mortar or binder.   This wall is a kneewall but is used as a plinth base for a fence. The fence divides between property lines as a parcel boundary.

kneewall (1)

Dry stack sandstone kneewall separating properties at the parcel boundary as a fence plinth.

The stone wall is topped with a similar thin sandstone capstone. In this case that capstone is continuously laid and embedded in a unifying mortar.  When you look closer at the wall in the adjacent photo, you can see the randomized jigsaw puzzle-like layout, made of fitting one stone to the next, as the stones are collected with great variety in nature.   The stones are then individually placed to fit and match the adjacent stones.  However, unlike ashlar masonry, this type of drystack is only only slightly modified to fit.  This type of masonry work requires a somewhat complex site setup which includes staging, sorting, culling, and stockpiling stones by shape and size.

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kneewall (4)

Close up view of a drystack kneewall shows the somewhat random stone layout with varying size and shape stones.

Dry stack masonry is essentially relies on the shape of the stone, so the  stones together without the use of a mortar. This history of this type of construction goes back millennia.    The pyramids are even built with dry stack stone. The big difference between this type of sandstone wall and something like the great pyramids is that this type of wall is a non-ashlar masonry. Ashlar masonry is masonry that has been cut to rectilinear type shapes. In this case though the sandstone is collected and only roughly shaped at best and then stacked and laid together without cutting these stones into rectilinear form.  Dry stack stone, as a retaining wall, is considered. A naturally draining type of wall.

In contrast to ashlar masonry, the major alternative is rubble masonry.  By comparison, rubble masonry is made from stones laid together without cutting and this fits that description except that the natural shape of the sandstone is that the sandstone is relatively flat and thin.  The classification of rubble walls is generally used more often to describe walls built with small uncut boulders.

Like a dry stack wall, a rubble wall is also built with undressed (uncut) or relatively unmodified stones.  However the rubble wall is not made with the dry stack type of flat laying sedimentary stone and cannot be built with a simple dry stack method.  In fact, rubble-work is rarely self supporting without a binding mortar and or a significantly tapered base.


mortared kneewall (3)

Sandstone is a silicate-rich rock, hence the “sand” in the name sandstone.  Limestone is also used ubiquitously in historic building construction but the main difference between sandstone and limestone is that sandstone does not contain as much carbonate which is a main sub-component in limestone.  Both types of stone are sedimentary rock.

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In this article we talked about the following terminology and  concepts, follow the links below for more related information from the IDS website:

  • mortared kneewall (2)Knee wall
  • Retaining wall
  • Hardscaping
  • Dry stack
  • Plinth base
  • Parcel boundary
  • Cap stone
  • Sorting natural stone
  • Culling natural stone
  • Stockpiling masonry elements
  • Rectilinear building elements
  •  Ashlar masonry
  • Naturally draining site elements
  • Rubble masonry
  • Carbonate
  • Sedimentary stone

These concepts are part of the fundamentals of stone masonry, site hardscaping / hardscape, civil construction, historic masonry restoration, and building repair.

The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the historic masonry of your building in Washington DC, fill out the web form below and drop us a line.  We will be in touch if we can help.