Replacing loose or dislodged roof ridge tiles
Safety: When working on a roof, always work from a stable scaffolding tower or other stable platform, never try to work on a roof using just ladders.
Ridge tiles often become loose or dislodged after high winds etc as a result of the breakdown of the original mortar after years of exposure to the elements - especially rain and frosts. Rain will enter any small crack, and if then subjected to a frost, the water will expand causing more cracking and the eventual crumbling of the mortar.
Should the original ridge tile be damaged and you need to purchase a replacement, take an existing ridge tile (or a large part of a broken one) along to the builders merchant to ensure that the one you purchase is of the correct colour and profile. If you have trouble finding an appropriate shaped/coloured replacement, consider any local Reclamation and Salvage Yards where you might find a suitable ridge tile.
Replacing ridge tiles
Having erected a stable scaffolding tower or other stable platform to gain access to a roof, and using a roof ladder to get to the ridge, check all the ridge tiles to find all the loose ones, and to locate any mortar joints which have obvious cracks. Remove all the ridge tiles which are loose - see below if a ridge tile is firmly attached but the mortar joints around it are cracked or crumbling. Any ridge tile which is cracked or damaged should be removed even if when firmly attached to the ridge of the roof.
Using a hammer and cold chisel, remove any cracked or crumbling mortar from the ridge of the roof. Try to remove as much of the original mortar as possible, this will allow a good thickness of new mortar to be applied under the tiles when they are replaced. Carefully remove any mortar stuck to the ends of any ridge tiles which are to be joined on to.
If a ridge tile is to be reused, remove as much mortar as possible from around the edges and on the underside. Position each ridge tile in position along the ridge to make sure that it sits below the line of any undisturbed tiles before re-fixing them - this check should ensure that there is sufficient gap underneath each tile for a good thickness of mortar and that, after fixing, the replaced tiles will not stand proud.
Before starting to replace the tiles, make sure that all the removed tiles are ready to refix and brush away any loose dust from the roof and tiles.
Use a stiff 3:1 (sharp sand:cement) mortar to bed the tiles onto the ridge - it needs to be wet enough to bond to the tiles, but not so wet that it sags or runs out from under the tiles and joints.
Before fixing the tiles, dip them in water to dampen the underside and dampen the ridge of the roof before applying the mortar - on a hot/sunny day, you will find that the water will dry off fairly quickly, so it is best to dampen an area the roof as you are about to work on it.
Lay a bed of mortar along each side of the ridge, put a solid bedding of mortar across the ridge under each end of each tile so that mortar will fill the gap between each ridge tile - lay a broken piece of tile or slate across the ridge to support this mortar across the gap between the slates/tiles on each side of the ridge.
Position the ridge tile onto the beds of mortar and push it down - use a small trowel to remove any mortar squeezed out along the lower edges and smooth the joint - be careful not to smear the mortar on to the slates/tiles. Make sure that the joint between the ends of each ridge tile is filled with mortar - use the small trowel to smooth off each joint as you work along the ridge.
Cracked mortar joints
Where the ridge tiles are firmly attached to the roof but the mortar joints at the ends or bottom of the tiles are crumbling or cracked, rake out the mortar along the joints using a metal spike - there's no proper tool here for the diy'er, a suitable spike can be made from an old, thin screwdriver; the end of the blade can be ground into a spike and then the blade bent into a right angle.
If the gaps in the mortar are fairly wide (i.e. nearing the full width of the joints), mix up a fairly stiff 3:1 (sharp sand:cement) mix and fill the gaps using a small trowel - before applying the mortar, use a wetted brush to dampen the sides of the gaps so that the mortar can make a good bond with the existing mortar.
Where the gaps are fairly narrow, it is better to use a non-setting sealant rather than a mortar mix to fill and seal them. Various coloured sealants are available, one of which may closely match the colour of the existing mortar.