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Welcome to the Flooring Section: here we install a slate floor
 
How to install a slate floor
Laying a new slate floor is a 'big-ish' job that to do properly requires a fair amount of time and patience. Some types of slate are very uniform (eg. Brazillian black slate) and are fiarly easy to lay, while others such as this 'green / grey' slate from China as we have shown here is heavily riven and every individual slab different, in terms of thickness, texture, colour, shape (some are curved)… so you'll need have patience to find the right combination of slabs, which work well along side each other.

You'll also need to allow a few days for drying of adhesive, applying undercoat sealer, satin polish, grout and so on.

Is it worth the trouble, effort and expence? You bet it is! If the slate is properly installed, it looks fantastic, can increase the value of a home by thousands, increases its saleability, and its so hard wearing there is no reason why it shouldn't last for a good few hundred years or more!

Skill Level Required: If you've never tackled any type of tiling job before I wouldn't recommend you attempt working with riven slate. That said, if you've got a few tricky tiling jobs under your belt and know what its like to deal with warped walls (or floors) and tiles that aren't exactly the same shape or size, then you should be able to install a slate floor given careful preparation. We hope you find our feature useful.

Good luck

Todd O'Neill (editor of www.TheHomeDr.com)

 
Installing a slate floor: basic stages
1) Subfloor suitability: concrete is ideal, wood may require strengthening
2) Prepair subfloor: clean and make ready for the slate
3) Layout: ensure slabs line-up with focal point of room
4) Laying slate: riven slate varies in thickness and is not always flat so choose each slab individually
5) Cutting slate slabs: make a paper template, measure twice, cut once
6) Apply stone 'sealer' as an undercoat: reducing water absorption of slate
7) Apply stone 'polish': easy to apply
8) Apply grout: easy provided the slate is sealed before hand!
9) Apply stone tile polish (again): hard wearing protective top coat
 
1. Subfloor suitability
 
 

Before installing any stone flooring it's imperative that the subfloor is completely rigid or there may be future difficulties with slab cracking or lifting.

Note: This is our finished floor which was installed onto a concrete base

     
  Concrete subfloor: Concrete is an ideal base to work with and requires very little preparation, but you will need to ensure it has a damp proof membrane installed underneath, and if the floor is fairly new, make sure it is fully cured and completely dry. If the floor is really rough or very uneven you might consider using some floor levelling mix, but we usually just lay the slate onto a slightly thicker bed of adhesive to compensate. .
     
  Wooden subfloor: If you intend on laying a slate on to a wooden floor you'll probably need to carryout some strengthening works to ensure the floor is completely rigid and does not flex or spring, and is strong enough to cope with the added weight of the slate. Flexing wooden floors can often be rectified by adding more struts between the floor joists, strengthening the joists, or screwing down the floorboards, but it may be advisable to get advice from a professional builder if you are concerned about the structure. You might also consider gluing then screwing a layer of exterior plywood (18mm or thicker!) as a base, with screws at no more than 200mm centres..
     
 

Underfloor heating: Although we didn't use underfloor heating on this job, underfloor heating does work REALLY well with stone flooring. Choose a good quality under floor heating system and ensure its working 100% before laying your stone floor… you wouldn't want to have to lift the stone floor up once its been laid!

     
2. Prepair subfloor
     
 

Concrete subfloor: Sweep clean the floor, and then use a slightly damp mop to collect the smaller particles. To ensure good adhesion we used a garden sprayer filled with a diluted PVA adhesive (10 parts water to 1 part PVA) sprayed the floor. When dry you can begin the layout stage.

     
    Wood / Plywood subfloor: preparation of a wood subfloor may vary depending on which type of stone adhesive you intend on using. Check instructions as described by the adhesive manufacturer
     
3. Layout: ensure slabs line-up with focal point of room
     
 

Consider the room's main focal point: Before you lay any of the slate slabs consider the room as a whole. The slab layout should be in line with, and help draw the eye to the main focal point of the room (in this case the fireplace). Also consider the shape of the room and try to ensure the slabs are as square as possible to the walls (not always easy with older houses when the walls aren't square).

     
 

To help with the layout process position lines of slabs onto the floor (don't use any adhesive yet) and use tile spacers. Here I'm using off-cuts of wood, which are about 10mm thick. Since the slate is not always 100% square, allow for fairly think grout gaps as they are much more forgiving to work with. Ideally, when planning for the edge sections (i.e. those along the walls), try to leave more than half a tile. Any less, and the grout lines could emphasise that fact that the room may not be square.

     
  If you are continuing the slate into neighbouring rooms, you'll need to factor that in as well to ensure everything is in one continuous line. Once the layout has been determined, draw guidelines (or snap a chalk line) onto the floor. Some people fix a long wooden batten to the floor as a guide, but this may the restrict excess adhesive from oozing out when pressing the slab into place.
     
4. Laying slate: riven slate varies in thickness
     

Floor level? If your subfloor is not 100% level you will need account for that before laying the first slab. The first slab will determine the height of the finished floor, if it's a thin slab installed onto a thin layer of grout or into a depression on the floor, it will lead to problems because the neighbouring slabs will be sitting proud, not good!

Many recommend starting with the thickest slab and then just use a thicker bed of adhesive for the other thinner slabs. A thick bed of adhesive provides a healthy margin for any variations in slab thickness.

     
  Since riven slate varies in colour, texture, thickness and they are not always flat, so it's important to choose each slab individually and see how it fits along side its neighbouring slabs. Here we find a slab which bends up at the corner which does not fit well along side the others.
     
  It will help to organise the slab pile. Put the good side facing forwards, thick ones together, thin ones together, odd shaped ones, broken ones (cut and used for small sections?) and so on.
     
  Choosing slabs: before the adhesive is mixed, lay several slabs down on to the section of floor to check that edges and corners will work well together and there won't be any protruding bits that people could trip on. You might need to do a lot of swapping around before you find the right combination of slabs, which work well against each other.
     
  Mix the adhesive. Use a good quality adhesive designed for use with stone flooring. Its usually comes in bag form and is easy to mix in a sturdy bucket (just add water).
     
  Refer to the floor layout lines and spread the adhesive evenly onto the floor using a large adhesive spreader. To aid adhesion, wet the underside of the slate tile before laying it into position (a water filled garden sprayer is ideal).
     
  Begin to lay the slabs into place working from the centre of the room outwards. Since the slabs are not exactly square, the tendency is that the more slabs you lay, the more the layout will be 'out of 'square'. The beauty of using relatively large spacers is that they provide a healthy margin, allowing you to close or widen the space slightly to keep the layout square.
     
  Its important to ensure the entire slab is sitting on a good layer of adhesive (no voids). Completely pack the slab edges and corners with adhesive, as these areas are prone to breaking if anything like a large cooking pot falls onto it. Have a bucket of water and sponge handy so you can immediately wipe off any adhesive if it comes into contact with the top part of the slab!
     
  Make sure those corners and edges are well packed with adhesive!
     
  Continue to lay rows of slate and make use of the spirit level and a straight edge to assist with levelling.
     
  The slabs along side the wall need to be cut to size, so they can be done last, using the slabs that are more secondary! For now the edge of the wall is a exit pathway.
     
  Continue laying the slate and allow at least 24 hours (or more) for the adhesive to set before walking on the slate slabs.
     
5. Cutting slate slabs: make a template, measure twice, cut once
     
  Once the main slabs are laid into position and the adhesive has dried enough that they can be walked on, its time to cut some slabs for the floor edges and around any tricky objects.
     

  We recommend using templates. They are easy to make, cheep and if used properly reduce the risk of cutting errors, which can be costly. Here's how it works.
     
  Cut a thick piece of paper (like lining paper) the same size & shape as a full slab. Use the spacers to position it to the neighbouring slab. Using scissors roughly cut strips so the paper template can fit around the object.
     
  With the template laying flat onto the floor, begin to make the cut line.
     
  Fold the paper back and draw a cut line onto the template. Here we want a standard grout space between the pedestal so we are using one of our grout spacers to help draw a cut line.
     
  Cut the template and ensure it fits correctly. It's usually worthwhile to label the template 'face' or 'back' to avoid cutting errors.
     
  Use the template to mark the cut line onto the slab with a pencil. Cutting slate kicks out clouds of dust, so work outside and tell the neighbours to bring in their laundry! Remember to wear protective gear (i.e. goggles, mask, gloves…)!
     
  Using an angle grinder (a 4.5" should do nicely) and a suitable stone grinding disk, begin to cut the top of the slate. For thicker slate slabs (or where curves are required) it might be necessary to cut half way through the face of the slate, turn it over and cut continue cutting on the back side.
     
  You should have a nice clean cut ready for installation. Keep the off-cuts, they can come in handy.
     
  Hey, it fits!.
     
  Edge slabs: do not butt the slab edge tight to a wall as this would have a tendency to draw in moisture from the brickwork (on external walls), and would not allow for any expansion. Leave a small gap, which will be covered by the skirting (or baseboard).
     
  For cutting straight lines, its usually easier to use a 9" angle grinder, but treat them with respect, these things can bite back!
     
6. Apply stone 'sealer' as an undercoat: reducing water absorption of slate
     

Reducing water absorption of slate: Slate floors are very porous and will first require a coat of a solvent-based 'sealer'. Its important that the sealer be applied before the grouting stage, if not the grout will stick like crazy to the top surface of the slate and not wipe off cleanly during the grouting process (a nightmare when using riven slate).

The specialist stone 'sealer' reduces the slates ability to absorb water and prepares the surface to accept the topcoat of polish. Its important to choose a proper stone 'sealer' as it does not contain silicones and will allow the slate to 'breathe'.

     
  Before applying the 'sealer' coat, clean the slate well using a slightly damp mop. Allow plenty of time for the slate to fully dry before sealing.
     
  Apply the stone 'sealer' with a lint free cloth ensuring the sealer goes on evenly. Put just enough on so it lightly covers the slate, but not so much as the sealer 'pools'. A thick coat of sealer will have a tendency to peel, so it's better to apply 2 thin coats of sealer (of course let the 1st coat dry before applying the second coat).
     
7. Apply stone 'polish': easy to apply
     
  The satin polish is a water-based coating which protects the floor from becoming stained. Apply the stone 'satin polish' in the same way as the 'stone sealer' with a lint free cloth ensuring the polish goes on evenly and not too thick as to leave 'pools' or a glossy appearance. Other coats of polish will be applied after the grouting stage.
     
    Put just enough on so it lightly covers the slate, but not so much as the sealer 'pools'. A thick coat of sealer will have a tendency to peel, so it's better to apply 2 thin coats of sealer (of course let the 1st coat dry before applying the second coat). Once dry we can move onto grouting.
     
8. Apply grout: easy provided the slate is sealed before hand!
     
  The photo on the left is what you don't want! Instead choose a good quality grout (obviously suitable for use with stone flooring), and ensure its colour compliments the slate. On our floor we are using a 'green-grey' slate so we've chosen a grey grout (this is what the finished product lookes like ). As a general rule, if there is a strong colour contrast between the grout and the slab, the more noticeable any imperfections will be, as shown in this photo.
     
  Mix the grout (as per the instruction in a sturdy bucket). Fill another bucket with clean water and have a sponge handy.
     
  Use a slightly damp sponge to remove any dust from the joint.
     
  Using small trowels, pack the grout into the deep joints.
     
  Use your finger (or a rounded finishing tool) to finish the joint and using a damp sponge and immediately wipe-clean any access grout from the surface of the slate.
     
  You'll find that the excess grout wipes off easily because you've applied the stone 'undercoat' and 'polish' at an earlier stage!
     
  Ahhh... that joint looks lovely, now to do about 400 more! Once the grouting has dried, you can then think about polish.
     
9. Apply stone tile polish (again): hard wearing protective top coat
     
  Apply two more thin coats of stone 'satin polish' in the same way as the you've done before (i.e. with a lint free cloth ensuring the polish goes on evenly and not too thick as to leave 'pools' or a glossy appearance) but this time you'll be coating the new grouting as well as the slab. Once dry, you can begin to use and enjoy your new super-dooper slate floor.
     
     
   
The good bits of slate / stone floor
     
   
The bad bits of installing a slate / stone flooring
     
 
 
 
 

 

Our slate floor cost £
 
 
Living with slate flooring: Slate is incredibly hardwearing and will outlast just about every other type of flooring (apart from granite that is). Riven slate does hide dirt pretty well, but every 3 years or so, you'll need to remove the old polish (using a liquid 'tile and stone renovator') then apply additional coats of the protective 'satin polish'.
 
 
 
 
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