Written on April 12, 2010 at 8:59 am by Power Tool Team

Compound Mitre Saw

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Compound Mitre Saw
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Ceiling Coving & Cornice – tips for installation

Tools needed

Saw – a medium tooth, cross-cut saw is best. Choose one with a reasonably stiff blade - if it is too flexible it may bend when cutting the coving, resulting in a curved edge making it difficult to obtain a good fit with the adjoining coving length.

Mitre box – ensure that it is big enough for the coving you are cutting. Remember that the coving is propped against the back plate of the mitre box – a common mistake is to lie the coving “flat” and cut as if it was architrave or a picture frame. The coving is always put in the mitre box “upside down” - i.e the edge that will be fixed to the ceiling should be on the base (horizontal section) of the box.

Adhesives – Decofix Pro (or Hydrofix if walls have been recently plastered) for mounting and filling. You will also need Extra Fix cement for all joins (a specialist adhesive that will prevent any cracks appearing later on). Ensure the room temperature is above 10°C otherwise the adhesives won't bond correctly.

Other tools required will be: Pencil, ruler, spirit level, standard caulking/cartridge gun, hammer and small nails, sponge & water, filling knife, trimming knife, paintbrush (to use when blending in the filler).

Cutting – general advice (Note: to view this article with diagrams, please see www.decorative-coving.co.uk)

Use a firm, positive stroke of the saw. Don't try and force the cut or cut too quickly. Hold the coving securely with your non-cutting hand to prevent it slipping or moving in the mitre box. If possible, try and support/raise the other end to the same level throughout so that the cove isn't sloping along it's length. Make sure you position the adjustable stops in the mitre box to the correct measurement (the amount that the coving/cornice protrudes across the ceiling). Measure, don't guess!

Internal corners - When positioned in the mitre box, the main section of coving that will be hung on the left wall should be to your right, and similarly the main section of coving should be on your left for the piece that will be hung on the right hand wall.

External corners - The same theory applies to external corners – except this time you will be cutting using the opposite 45° guides. A stop end is cut the same way as an external corner using a short surplus length.

Hanging

If you have any external corners (such as a chimney breast), start with them. Otherwise start at the first corner you see when entering the room (it is always easier to get the 1st corner of the installation correct, so make sure it is the most noticeable corner in the room).

Establish the drop down the wall from the ceiling that the coving will be fixed at, and ensure this is clean and sound. Draw a line around the room at this position (or mark at various points). Ensure the line is horizontal – do not be tempted to follow the ceiling which may not be level (if there is any subsequent gap, this can be filled later). At every 500mm or so, tap a small pin nail into the wall as a temporary supporting guide. (The small nail holes will eventually be covered by the adhesive/filling compound). For extra adhesion you can score the area between line and the ceiling. Optionally, repeat this process on the ceiling.

Now, carefully measure along the wall the length you need – then measure it again! Transfer these measurements to the coving, and if it helps, also mark a line in the direction that the cut will roughly be, so that when put in the mitre box you can be assured you are cutting in the correct direction.

Make the cut, then offer up to check you are happy that the 2 sections meet. Don't worry if they do not meet perfectly – not many rooms have corners that are exactly 90°, so a perfect join is rare. That's where careful use of the filler comes in later!

Using Decofix Pro or Decofix Hydro, apply along the entire length of the back edges of the coving where it will meet the wall and ceiling. As a rough guide to coverage, a cartridge should be sufficient for 6-8m.

Using the line on the wall as a guide and the nails as a lever, push the coving firmly into place, adjusting as necessary. Now apply a thin layer of Extra Fix to the end of the coving where it will meet the next piece. This will give an ultra high strength join and prevent any cracks appearing in the future (if applied correctly, the join will be stronger than the actual coving piece itself).

Repeat the above process around the rest of the room.

Remove any excess adhesive and use it to fill in any gaps - between joins and between coving and wall/ceiling. Use your finger, a filling knife and a damp paintbrush to smooth over, followed by a wet sponge to wipe off any remaining adhesive. Finally after allowing 3-4 hours for the adhesive to dry, remove the nails and fill the holes as above.

Painting

The coving is pre-finished with an undercoat/primer already applied, so it will be ready to paint after 24 hours. Although any paint is suitable, remember that emulsion is water based. This means that it may suffer hairline cracks if there is excessive movement or vibration in the ceiling. Acrylic/oil based based paints have more “elasticity” in them so are less prone to these fine cracks.

For further info, please see www.decorative-coving.co.uk

About the Author

David Warner is the owner & MD of Davuka GRP Ltd - the UK's leading on-line supplier of decorative home mouldings - specialising in ceiling coving and cornice

where can i find plans to build a jig for cutting crown moulding on a compound miter saw?

The jig is already on your saw. It's called the degree scale. DeWalt saws have arrows at 33.9 degrees, etc. (don't do residential anymore, so I can't remember the numbers). You hold the crown vertically against the fence, put the miter on 45 degrees and the bevel on whichever arrow/tit works for that size molding. That's if your going to cope it, of course, which is the right way to do it.

The other way is to 'bed' the molding on the miter saw base and fence, upside down, and set both angles to 45. The base is the ceiling and the fence is the wall. Cut it so that the point of the miter cut is on the back of the molding, like an inside corner, then use the profile as a guide to cope it back the other way. You can clamp a board to the base so that every piece sits the same way when you cut it.

The reason that coping is better is because wood contracts longitudinally. If you just 45 both pieces, both of them will contract away from each other in winter. Coping allows one board to pass behind the other, so when it contracts, it doesn't leave any gap. That reduces the contraction gap by half. (only one board shrinks away from the other)

Edit: When bedding the molding, it may not sit equally on the base and the fence. It just depends on the style of molding. Some sit more on the wall than the ceiling. Just cut a one foot piece off and hold it on the saw and see how the flats on the back sit. They should both be sitting flat on their surfaces. (it's the 1/2" or so flat part that touches the wall or ceiling).

Use that same one foot piece to mark a straight line on the wall, too. Set it in one corner like it's going to go and mark the bottom of it. Do the same at the other end and snap a chalk line (use blue chalk, not red). The surfaces are never straight or flat, but you want to make your molding as straight as possible (you don't want it to follow humps and dips in the ceiling.) Nail it to the wall first (a few feet at a time), and then nail it to the ceiling. Don't force it into any 'low' spots (valleys), or you will see a wave when its done. The whole point of crown molding is to hide these valleys, so dont accentuate them by pushing it up into them. Just caulk it and you'll never see it.

Festool Kapex KS-120 Sliding Compound Miter Saw