How to read a slate roof from the ground

A slate roof is the most expensive element of most châteaux and the first place a survey goes. You cannot judge one from the ground — but you can learn to see what is worth asking about.

Look at the lines, not the slates

Slate is laid in courses, and courses are laid to a line. A roof that has been relaid competently reads as a grid: horizontal courses parallel to the eaves, vertical joints stepped evenly. Where a roof has been patched over decades, that grid breaks up — a band of slightly different colour, a course that drifts out of parallel, a patch laid to a different gauge.

None of that is a defect in itself. Patching is normal maintenance. What it tells you is how often the roof has needed attention, and that is a different question from whether it is sound today.

Ridge, valley, penetration

Water enters a roof where the plane is interrupted. Three interruptions matter most:

• The ridge. In France, ridges are commonly finished with mortar-bedded terracotta or with lead. Mortar ridging fails visibly — you see gaps, missing sections, vegetation.
• The valleys, where two roof planes meet. These carry more water than any other part of the roof and are usually lined in zinc or lead. From the ground you can rarely see the lining, but you can see staining on the wall below.
• Penetrations: chimneys, dormers, finials. Look at the junction between masonry and roof. Vegetation growing at that junction means something has been holding moisture for a long time.

The wall below tells you about the roof above

Where a roof has been leaking, the evidence is usually on the elevation: a vertical stain running down from a valley, moss concentrated in one band of stone, surface loss limited to a single bay. On tuffeau and other soft limestones, prolonged wetting shows as flaking — the stone crumbles back and the joint stands proud.

What you cannot see

The covering is only half the question; the structure beneath it is the other half. A perfectly laid slate roof can sit on a charpente with beetle damage, split tie beams or spread rafters. Nothing observable from the ground will tell you about the timber. The same goes for the fixings — nail sickness, where the fixings corrode while the slate itself is still sound, is invisible until slates start sliding.

That is the honest limit of a ground-level reading. It tells you what to ask, where a surveyor should spend their time, and which parts of a building have a history of water. It does not tell you the condition of a roof, and no photograph, listing or directory entry can.

What to do with it

If you are looking seriously at a building, the sequence that saves money is: read the elevations yourself, then commission an inspection of the roof structure and covering before anything else, then price the work. A roof survey costs a small fraction of what it costs to discover the problem after purchase.

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General guidance on how historic roofs are built and how they fail. It is not advice on any specific building, and nothing in this directory constitutes an assessment of the condition of a listed property.