page looks at the laying of riven stone flags, and the special problems
they present, compared to concrete flags and other types of paving.
"Riven" refers to the surface of the flags, which is a natural bedding
plane within the rock from which the flagstone was quarried. Riven
surfaces tend to be uneven, reflecting the natural origin of the rock,
and there is no set standard for what would be considered too uneven,
or not uneven enough.
Stone flags come in a variety of forms, but, from a laying point of
view, there are two categories - those flags that are sawn or specially
worked to ensure a regular thickness, which we refer to as calibrated, and the more commonly encountered riven flags that come in variable thicknesses.
Thin or thick sliced?
Much of the imported sandstone flags,
and more or less all reclaimed stone flags come in varying thicknesses.
Newly-quarried British/Irish stone and much of the imported stone is
selected to be of "similar" thickness: for most residential purposes,
this is 20-40mm, although stone flags for driveway and commercial use
can be much thicker. The native-sourced stone for residential use tends
to be slightly thicker (generally 40-55mm) than the imported stone,
which aims for greater economy, and is usually somewhat thinner, around
20-40mm. Reclaimed flags, because they tend to come from a number of
different sources, can vary in thickness quite considerably, being
anything from a worryingly-thin 30mm to a back-breaking 150mm! This
degree of 'tolerance' in flag thickness means that any individual flag
can easily be twice as thick as its neighbour.
Contrasting thickness of an imported sandstone
Reclaimed Yorkstone illustrating the thickness variation
This variation in thickness means that, when it comes to laying riven stone flags, screed bedding is not feasible, and we have to rely on individual bedding. The remainder of this page considers this method.
Which way is up?
With reclaimed stone, identifying the top surface is usually pretty
straightforward - it has a worn, polished, smoother surface, possibly
with the odd stain or dollop of chewing gum to make it even more
obvious. The edges tend to be rounded through years of foot traffic,
and the colour is usually darkened with grime, algae or general wear
and tear. Examination of the underside may show marks or stains from
the previous bedding materials, along with a much coarser, unpolished
texture and the unmistakable rough-and-ready edges.
Top face is more polished, has straight, rounded edges
and tell-tale chewing gum stains!
Underside is coarser, unpolished
and has uneven edges
On newly-quarried stone, other than the sawn or textured products, the
top surface can be less obvious to those not familiar with paving
stone, as both top and underside are equally clean and equally riven.
Thankfully, there are a couple of pretty obvious visual clues that help
distinguish just which is the right way up.
Firstly, the top surface should always have, neat, square edges.
These are usually hand-trimmed, and are known as 'fettled', which
refers to the slightly-less-than-perfect nature of the edges,
emphasising the natural origin of the stone and distinguishing it from
the dead-straight, machine cut or moulded accuracy of manufactured
products. Conversely, the underside has untidy, poorly-defined edges,
as can be seen in these photographs.
Top surface - definite neat edges
Underside - rough, poorly-defined edges
Secondly, the sides of the flags are usually tapered inwards, so that
the top surface of the flag is slightly larger than the underside. This
helps ensure accurate fitting, allowing joints to maintained at a
standard width, and also ensures that any mortar jointing is firmly
held in place.
Side of flag tapers inward, from top to underside
Not only are these imported sandstone flags very badly laid,
they're not even the right way up!
Getting on with it...
Working to plan
Once the top surface
has been identified, laying can proceed. When working with modular
paving, the usefulness of a plan cannot be over-emphasised: it saves an
enormous amount of head-scratching, trying to figure out which flag
goes where, and whether it's one of those or one of the others to be
laid next. Plans for Random Layouts
are indispensible, but pre-planning is also a good idea when laying in
courses, or even to a regular pattern, as it helps the flag-layer
identify just where they are up to.
What follows assumes that the area being paved is
a simple patio or other light-use area. For areas that will be
trafficked by vehicles, a sub-base is to be recommended. If using flags of less than 60mm thickness for a driveway, a full concrete bed
(CBM4) is the best solution, combining sub-base and bedding in one, but for thicker flags, a 100-150mm
granular sub-base and the usual bedding material should be satisfactory.
The materials should be brought into the working area, and
stockpiled so that they are easily accessible but not in the way. The
flags themselves should be stacked just outside the working area, with
the different sizes stacked in separate piles.
| Note that stacking
flags as shown opposite, with one flag on edge supporting a stack of
other flags at 90° is DANGEROUS and should not be done. We've all seen
flags stacked this way on site, but it only takes some eejit to
accidentally knock the support flag, and the whole lot fall over,
potentially causing serious injury. Several years ago, a small girl
playing on a building site 'after hours' was crushed to death when she
(or her playmates) bumped into the support flag and a stack of heavy,
concrete 3x2s (as they were back then) collapsed on top of her. Don't
VERY, VERY NAUGHTY!!!
Bedding and Buttering
The bedding material
can be mixed elsewhere and brought into the working area in a barrow,
or it can be mixed 'in-situ' and distributed as required. Mortar for buttering and pointing is best mixed in advance, and stored in a barrow from which it can be taken as needed.
The flag to be laid is checked and its thickness ascertained. The
bedding material is spread over an area at least 100-150mm larger than
the next flag to be laid, and levelled out with a spade or a trowel to
a depth that will accommodate the flag and allow it to be compacted
down to the correct level.
Preparing the bed
Ideally, the bedding should be 25-50mm thick. Less than 25mm renders it
almost impossible to settle the flag, while more than 50mm makes for
uncertain compaction. Adjust the level of the sub-layers rather than
use less than 25mm or more than 50mm of bedding.
The depth of the uncompacted bedding below the surface layer varies
with type of bedding, thickness of bedding, thickness of flag, moisture
content (and quite probably which day of the week it is!) so it's not
possible to give exact figures for how much a bed will consolidate,
but, in general, the flag should be 5-10mm 'proud' when first laid onto
the bedding, ie, before it is tapped down to level.
When using mortar joints, the receiving edges of the previous flags are 'buttered' with the jointing mortar, and the flag is placed into position, as shown on the Laying Flags page. Other jointing options are considered on the
Flag Laying Basics page.
Once in position, the flag should be consolidated. This is best done
with a Pavior's Mall (as shown below), but, when laying on a full
mortar bed (rather than a semi-dry bedding mix) smaller flags can be
tapped down using a small rubber mallet.
A spirit level can be used to check for level and alignment. Take care
not to stain the surface of the flags with the bedding material -
always ensure the 'face' of the mallet is clean.
Stop the rock
Check for rocking this way....
....then the other way.
Once consolidated, the flag should be checked for 'rocking'. With
larger flags (600x600 or bigger) this can be done by straddling the
flag diagonally, and shifting ones weight to check for any movement,
before switching to the other diagonal and checking again.
Any slight movement during the 'rocking test' can sometimes be
eliminated by packing the low corner of the flag with additional
bedding material, which is rammed in using the shaft of the mall.
However, any significant movement, more than 3mm or so, will need the
flag to be lifted and the bedding adjusted, either by addition or
removal of bedding material.
Remember, regardless of which bedding material is being used,
the aim is a "Full Bed", that is one that supports every part of the
flag. Excessive packing of one corner, or 'hollowing-out' the centre of
the bed to ease consolidation, results in voids beneath the flag, which
can lead to all sorts of problems, not least of which is flag breakage.
This is bad practice and should be avoided at all times.
Packing with extra bedding can alleviate slight rocking movements....
....over-doing it leads to voids beneath the flag, and possible breakages.
Once satisfied that the flag is firm, the level should be checked against the guide line and/or with a spirit level.
It's never easy to check riven flags for level accuracy, as the uneven
surface never gives the same reading twice. This is why it's so
important to continually check the work with a long straight-edged
timber, which spans 3 or 4 flags at a time, and gives a good indication
of overall level accuracy. Some parts or the flags might be slightly
low, and some will be slightly proud, but, the overall appearance
should be that the flags are more-or-less level with their neighbours,
that there are no lips between adjacent flags, that the flags tie-in
with the required level, and the fall is correct.
Creating regular joint widths
Many riven flagstones and their concrete counterparts exhibit a problem
regarding even, regular joint widths when laid to random layouts. While
this problem does not always occur, it is sufficiently complex to
warrant a page of its own.
On projects where joint width is irregular or causing a problem with the finished look, the
Regulating Joint Width page offers a simple solution.
When a suitable area has been laid, checked for
solidity and level compliance, and any problems with joint width
satisfactorily resolved, then the surface can be brushed off to remove
any surplus bedding and/or mortar snots (Yes! They are really called that!).
Pointing with mortar
is best done immediately, while the butter mortar is still relatively
fresh and plastic, so that it all sets as one. However, if pointing
cannot be completed there and then, it should be done at the first
Alternative jointing methods are considered on other pages. It's worth noting that the use of good quality
polymeric mortars is an increasingly popular choice for jointing imported flagstones for patios and pathways and there are also
stronger resin mortars
suitable for driveway projects. These materials are well worth
considering if there is any doubt at all regarding cement mortar
And that's about it! Try to keep foot traffic off the paving
for 24 hours, while the bedding begins to set and the jointing mortar
has a chance to begin its curing process. After 3-7 days, it should be
fine for regular foot traffic, or even light vehicular traffic, as long
as the pavement has been designed as a driveway or vehicle over-run
Once laid and consolidated, each section of flagging can be checked with a straight-edge and spirit level.
The last job is the pointing - choosing the right mortar makes a massive difference to the finished appearance.