Replacing loose or dislodged ridge tiles
Safety: Working on a roof involves working at height (even on a bungalow), so always use scaffolding or other stable platform, never use just ladders.
Ridge tiles will often be found loose or dislodged following high winds etc, this is often due to crumbling mortar resulting from the breakdown of the original mortar after exposure to the elements. Water will find its way into any small crack, and if then subjected to a frost, the cracks will be forced wider.
If you need to purchase replacements for damaged ridge tiles, take an existing ridge tile (or a large piece of a broken one) along to the builders merchant to make sure you get the correct shape and colour. If you have problems finding an appropriate replacement, don't forget Reclamation and Salvage Yards where you may find what you are looking for.
Replacing ridge tiles
Having safely gained access to the ridge, check all the ridge tiles for any other loose ones, or ones with obvious cracks in the mortar joints. Remove any ridge tiles which are loose and need replacing - if a tile is firmly attached but the mortar joints are cracked or crumbling, see below. Any ridge tiles which are cracked or disintegrating should be removed even if they are firmly attached to the roof.
Use a hammer and chisel to remove any cracked or crumbling mortar from the top of the roof and the tiles on both sides of the ridge. Try to get off as much original mortar as possible to allow room for a good thickness of new mortar. Carefully remove any mortar from the ends of any ridge tiles which are to be joined on to.
If the ridge tiles are to be reused, remove any mortar from the undersides and around the edges. Set each ridge tile in place before re-fixing them to make sure that it sits below the line of any undisturbed tiles - this will ensure that there is a gap underneath which can be filled with mortar and that the replaced tile will not stand proud.
Use a stiff 3:1 (sharp sand:cement) mortar to bed the tiles - make sure that it is wet enough to bond to the tiles, but not so wet that it sag or runs down the roof.
Brush away any loose dust from the roof and tiles before starting to replace the tiles.
Soak the tiles in water before fixing them and dampen the top of the roof before applying the mortar - you may find that the sun or wind will quickly dry this off, so dampen the roof as you work.
Put a bed of mortar along each side of the ridge on the top roof tiles, put a solid bedding of mortar under each end of each tile so that it will fill the gap between each ridge tile - use a piece of tile slip or slate to support this bed across the gap between the slates/tiles on either side of the roof.
Set the ridge tile onto the mortar beds and push it firmly down - remove any mortar which is squeezed out along the lower edges using a trowel and smooth the joint - take care not to smear the mortar on to the tiles. Ensure that the gap between the ends of each ridge tile is filled - smooth the mortar off at each joint as you work along the ridge.
Cracked mortar joints
If the ridge tiles are firmly fixed but the mortar joints are cracked and/or crumbling, use a metal spike to rake out the mortar along the joints.
If the resulting spaces are the full width of the joints, mix up some 3:1 (sharp sand:cement) and apply using a small trowel - dampen the gaps before apply the mortar to ensure a good bond.
Alternatively, if the cracks etc are fairly narrow, apply a non-setting sealant into the joints to fill any gaps and seal the joints. Various colour sealants are available so choose one to match, as near as possible, the colour of the existing mortar - then the repair won't stand out too much.