Wood rot in Dutch houses: why it happens and how to fix it

Three things combine to make the Netherlands a wood-rot country. First, the climate: mild, wet winters, frequent rain from the south-west and long periods of high humidity mean timber rarely dries out fully between October and April. Second, the housing stock: millions of homes built between 1930 and 1980 have timber window frames, doors and fascias, often in softwoods such as pine or meranti that need a sound coating to survive. Third, maintenance culture: exterior painting is easy to postpone, and a coating that is two or three years overdue is all the fungus needs.

Wood-rot fungi need moisture content above roughly twenty percent to grow. In a well-painted frame the timber stays below that threshold. Once the paint film cracks at a joint, or the sealant around the glass shrinks, rainwater enters through end grain, is trapped by the intact paint on either side, and the moisture level climbs. The fungus then digests the cellulose that gives wood its strength, which is why rotten timber crumbles rather than splinters.

Certain house types are especially prone: 1930s homes with large bay windows and deep sills, 1960s and 1970s terraced houses with wide meranti frames and floor-to-ceiling glazing, and any house where the ground floor frames sit close to a paved terrace that splashes rain back onto the wood.

Rot rarely announces itself. The visible clues are usually in the paint: a patch that has lifted or blistered, a hairline crack along a joint, or a section that looks slightly darker or duller than the wood around it. Paint that flakes off in a single sheet rather than small chips is a strong signal that the wood beneath is damp. Sealant that has pulled away from the glass or the masonry is an entry point even before any rot is present.

Look first at the places water lingers: the bottom corners of every window frame, the sill (onderdorpel) especially where it meets the side jambs, the bottom rail of doors and garage doors, and the ends of fascia boards. Ground-floor frames facing south-west are the usual first casualties. Check upstairs too; rot in a first-floor frame is easy to miss until it becomes serious.

Inside the house, a musty smell near a window, condensation that seems to come from the frame rather than the glass, or paint bubbling on the inside face of a sill can all mean water is travelling through the timber from outside.

The quickest diagnostic tool is a small flat screwdriver or an awl. Press the tip firmly into the suspect area. Sound timber resists and the tool leaves only a small dent. Rotten timber gives way with little pressure; the tip sinks in, the surface feels spongy, and the wood underneath may be brown and crumbly or, in advanced cases, wet and stringy. Test at several points around the suspect area to find the boundary between sound and decayed wood, because rot usually extends further than the paint damage suggests.

Professionals use the same test alongside a moisture meter, which shows whether adjacent timber is wet enough to be at risk even where it still feels firm. That matters for deciding how much to cut out. Our painters do this on every frame during a free inspection and record the readings on a photo report so you can see exactly what was found.

If the screwdriver sinks in more than a few millimetres, stop probing and get the frame assessed. Continued poking will not help and can widen the entry point for water.

For most localised rot, repair is better than replacement. The affected wood is cut or routed out to sound timber, the cavity and surrounding wood are treated with a liquid epoxy consolidant that soaks in and hardens the fibres, and the void is filled with a two-component epoxy repair paste that bonds permanently to the timber. Once cured it is shaped, sanded and painted like wood. The repair does not shrink, does not absorb water and moves with the frame through the seasons. Kivits uses professional-grade systems designed for this purpose and backs the repair with a 5-year warranty.

Replacement of a section, such as a new sill let into the existing frame, makes sense when the rot runs along a large length of timber or the piece is structural. Full frame replacement is the last resort, reserved for frames where more than a third of the timber is affected or the joints have failed. Because replacement means new glazing, plastering and often planning considerations in historic areas, a proper epoxy repair is nearly always faster and less disruptive. Learn more on our wood rot repair page.

Cheap fillers and paint-over repairs fail within a season or two because they do not remove the decayed wood or address why water got in. If you have had a frame "fixed" that is bubbling again, that is almost certainly what happened.

Prevention is mostly about the coating and the details. Repaint exterior timber every five to seven years, sooner on sun-facing elevations; our guide on how often to paint window frames explains the intervals. Renew the sealant around glazing and between frame and masonry whenever it cracks. Keep drip grooves under sills clear so water falls away from the wall, and make sure gutters are not overflowing onto woodwork.

If you rent and the frames are rotting, the repair is generally the landlord's responsibility under Dutch law; see who pays for exterior painting. If you own, a written maintenance plan with photos means you know which elevations to watch and when.

Kivits Vastgoedonderhoud offers free inspections across Noord-Brabant, including 's-Hertogenbosch, Eindhoven, Tilburg and Breda, with a written quote within 48 hours. Leroy Kivits and the team speak English, so you will get a clear explanation of what was found and what it will take to fix. Contact us to book.