A summer beam is the heavy main beam in a traditional timber floor, spanning the room in the short direction and carrying the lighter joists laid across it. In the Netherlands it is called the moerbalk, from “mother”: the large beam on which the smaller “child beams” rest. Together they form the mother-and-child floor found in virtually every historic Dutch house.
The logic is straightforward. One heavy beam every two or three metres handles a large span, while the much slimmer joists between them only have to bridge short distances. That way not every beam had to come from a heavy, expensive log.
Build-up and sizes
Summer beams are usually oak, and in later periods pine or spruce. They are noticeably substantial: sections of the order of 250 × 300 to 350 × 400 mm are common, with the larger dimension vertical for maximum capacity. Spacing is often two to three metres.
The joists run across them, typically at 400 to 600 mm centres, and are far lighter — around 120 × 140 mm. They are worked into the beam with a mortice and tenon or a dovetail housing, or simply laid over the top.
Beneath the beam end there is often a bracket block (Dutch sleutelstuk): a moulded timber block that widens the bearing and takes the moment at the beam end. Below that may sit a brace or corbel. In grander houses these are the parts that are richly moulded — they were, after all, on show.
Where things go wrong
- Beam end decay. The most vulnerable point is the beam end built into the external wall. Rising or penetrating damp plus poor ventilation causes rot. This is by far the most common defect
- Deflection. A visibly sagging summer beam is not necessarily an emergency — centuries-old beams often carry a permanent set — but increasing deflection is a warning sign
- Later notches and holes. Openings cut in afterwards for services or a stair opening weaken the beam badly, especially in the underside where tensile stress is highest
- Beetle attack. Mainly in softwoods; oak is less susceptible but not immune
- Removed joists. During alterations joists are sometimes taken out without the remaining span being checked
If in any doubt, get a structural engineer or conservation specialist in before demolishing, insulating or cutting through. The summer beam is a primary structural member.
Related terms
- Common joist
- Floor structure
- Beam end
- Mortice and tenon joint
- Corbel
- Wet rot
- Summer beam repair
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