The Brew Cabin

I would be very grateful for help on the best method for re-laying existing, loose flagstones on a 1.20 meter wide concrete stairway, which rises some 3 meters from street level to a patio at house level, whilst avoiding the water seepage that affects the plaster on the wall adjacent to the stairs.

To complete the problem description (sorry it is a bit long but I’m guessing that a full description will be better than an outline):

At the bottom of the stairs is a level area comprising several flagstones, which are strongly bonded to the underlying concrete. Several steps take us to the middle ‘plate’ and several more take us to the upper ‘plate’ (I’m not sure of the correct terminology), each ‘plate’ comprising several flagstones and with overall dimensions of approx. 1.25 x 1.25 meters.

The horizontal ‘stepping’ flagstones are rectangular, they are cut and ground on all surfaces to 123 cm x 25 cm x 3 cm. The same stone is used for the vertical pieces between the steps – they are also cut and ground to a uniform rectangular shape (121 cm x 13.5 cm x 2 cm). The flagstones are a natural stone, grey/black speckled in colour, I don’t know the stone type.

The right side of the stairs, when looking upwards, is bounded by a wall, which forms the left side of the driveway to the garage. The left side of the stairs is bounded by a number of reconstituted stone slabs (18 cm x 12 cm cross-section and unknown length), which are positioned vertically and form a barrier between the stairs and the garden, which slopes upwards from these slabs. There is a narrow air gap between these vertical slabs and the stairs.

The vertical stones of the steps were originally cemented to the concrete stairs; they are now mostly loose. The horizontal stones are resting on a layer (about 40 mm thick) of what appears to be a loose sand/cement mix, rather like that described in the ‘Laying Flags/Slabs’ section of the website. The mortar between the stones comprising the ‘plates’ is badly degraded, as is the mortar between the horizontal and vertical stones.

The effect of the degraded mortar is that the sand/cement mix supporting the steps remains very moist. This moisture is transmitted to the adjacent wall, causing unsightly discolouration and degradation of the plaster in the area of the wall that is immediately adjacent to the stone steps. My aim is to prevent this happening.

The second issue is whether or not one should have any ‘fall’ on the horizontal steps to help clear rainwater (and, in winter, to minimise ice formation) – and if so, how much?

I would be very grateful for any help with these questions.

Hi Ray,

what you call 'plates' are actually termed 'landings' - it's one of those things that you actually know, but that you can't call to mind on the only occasion in your life you'll ever need the information!

The loose flags can be re-bedded using a good quality, Class II mortar, with an added waterproofing and bonding agent - I'd be tempted to use SBR (Styrene Butadiene Resin), as it imparts excellent adhesive, water-proofing and strengthening properties to a standard mortar. It can be bought from most good BMs for around 20-30 quid for 5 litres, but it's money well spent for a job such as this.

Remove all the loose paving and any other stonework that needs to be re-seated. If in doubt, take it out - it's much easier to chisel out an 'iffy' piece of stone now, rather than wait and hope that it doesn't work itself loose next winter.

Clean off the old, degraded moratr and expose the main structure of the steps as far as possible. Don't leave any soft, degraded or suspect material in place.

Starting at the top of the steps, and working your way down, re-set the treads on a 25-45mm full bed of SBR-enhanced mortar. Spread the mortar with a trowel, ripple the bed to allow seating, then press the flags into the mortar and tap down to the required level using a small rubber hammer. Make sure they flags are well and truly held by the mortar, and point up any joints immediately. You need a fall of around 1:40, that's 50mm over the 1250mm depth of each tread, to ensure there's no standing water or chance of ice formation, so use a spirit level to guide your levels. There's no need for a cross fall if you have sufficient end fall.

Any vertical stonework that needs re-fixing can be stuck on using the same mortar, although you'll need a thinner 'bed'. Again, work from the highest step downwards, so that you're not having to stand on newly-finished work to proceed.

SBR hastens the initial cure of the mortar, and you'll have only 20 minutes or so of working time, so don't mix too much at once. Follow the guidance on the container for dosing quantities, and don't be tempted to overdo it, or add a bit extra just to be safe - it's a waste and will only reduce the working time without improving the end quality of the mortar.

A building sand is fine for making the mortar, and you can use an ordinary portland cement - the SBR acts as its own plasticiser, so no need to add any or to buy pre-plasticised cement. Keep your measures fairly accurate - X ml of SBR per litre of gauguing water, 4 empty paint tins of building sand to one empty paint tin of cement, that sort of thing, and then don't mix the mortar too wet or it starts dribbling everywhere. Keep it stiff but workable, and don't rush!

Good luck!

Many thanks for your prompt response Tony – and for a wonderfully informative and well laid-out website.

In the meantime I have found your “Steps� page, among many others, and so I am now up to speed on the terminology for my question!

Sorry to be a pest but I have a three subsidiary questions:-

1. Concerning the ‘fall’ for each tread, you advise 50 mm for a 1250 mm tread depth. I’m guessing that this refers only to the landings, which are 1250mm x 1250 mm. The depth of each regular tread is 25 cm (they are 1230 mm wide) and a 40:1 fall equates, I think, to 6,25 mm per tread. Bottom Line: I’m guessing that we want the water to flow down from step to step rather than tipping it over the side of each tread into the garage drive. Is that right or is tipping it into the drive what you had in mind?

2. Would you recommend using a flexible (elastomeric, such as acryl or silicone or whatever) pointing material for the gaps between (a) the treads and the stone plates that cover the riser area and (b) the gaps between the treads/risers and the (whoops, I’ve run out of terminology – they’d be called skirting boards inside the house) side bits. (I’m thinking minus 15o C in winter in this part of Germany).

3. Would you recommend using a urethane sealant on top of the Class II mortar that you propose for bedding the flags and pointing of the joints? (To minimise, maybe, wash-out of the pointing when using a high pressure washer for occasional cleaning of the stone?) Or would it offer no improvement?

Looking forward to hearing from you,

Q1 - for the steps, use the same sort of fall, 6-10mm per step will be plenty, and yes: we do prefer the water to run down the steps than over the edges, but, as long as the water gets away, it doesn't really matter. The reason why the fall is most commonly arranged to send surface water cascading down the steps rather than over the edge is that, if you slope a flight of steps from one side to t'other, it looks odd to the eye when you're at the foot of the steps. It's an aesthetic thing, rather than anything structural. :)

Q2 - for the joints between treads and risers, I'd use the same mortar, but, for the joint between the top landing and the side of the house, an elastomeric sealant would be used as long as there no direct tie-in of the step structure to the house structure, i.e., they are two, separate entities.

Q3 - If you use a SBR-improved mortar for the joints, there's no way a power-washer is going to loosen that mortar! We've successfully used SBR mortar to poiint setts and stone pavings in city centres that are subjected to very high-pressure jet-washing to remove chewing gum, and to the little motorised street cleaning vehicles that use vacuums to collect up any litter and detritus, and the mortar stays put!

Leave out the selant at this stage - I'm pretty confident that you'll never need it. :)