Ridge, ridge board and ridge beam

Frequently asked questions

What is the difference between a ridge board and a ridge beam?

Whether it carries anything, and the answer decides what the walls below have to do. A ridge BOARD is a thin member — often no deeper than the rafter cut requires — that the opposing rafters bear against. It carries essentially no vertical load: the rafters push against each other across it, and the resulting outward thrust at their feet is taken by ceiling joists or collars acting as ties. A ridge BEAM carries the top of each rafter as a genuine support, so there is no opposing thrust and the rafters behave as simple spans — but the beam then has a real reaction at each end that must land on something able to take it, which usually means a post and a foundation. That is why removing a ceiling to make a vaulted space is a structural project: it removes the tie, which means the ridge board must become a ridge beam, which means new bearings.

Why is a mortar-bedded ridge now unusual?

Because it fails predictably and the failure is above people's heads. Ridge tiles bedded in mortar rely on that mortar staying intact through decades of thermal movement, wind and frost at the most exposed line on the building — and it does not: the bed cracks, water gets behind it, the tiles loosen, and eventually one moves. British practice moved to DRY RIDGE systems, which fix each ridge unit mechanically to a batten with a ventilated profiled filler under it, and standards now effectively require mechanical fixing of ridge and hip units rather than relying on mortar alone. The secondary gain is ventilation: a dry ridge is a ready-made exhaust for the roof space, which a mortar bed sealed shut.

How is the ridge the exhaust of a roof's ventilation?

Because warm air rises to the highest point, and a cold roof works by moving air from low to high. Air enters at the eaves, passes over the insulation picking up moisture, and has to leave at the top — so the ridge is where the exhaust belongs, and the ridge vent or the ventilated dry ridge provides it. The system only works BALANCED: exhaust without intake pulls air from inside the house through every ceiling penetration instead, which is worse than no ventilation, and intake without exhaust achieves nothing. That is why American codes state the intake and exhaust proportions explicitly and why the commonest British loft-damp finding — insulation blocking the eaves — disables a ridge vent that is working perfectly.