Repairing wood rot in window frames and exterior woodwork: cause, repair and who does it

Wood rot is the result of one thing: wood that stays wet for too long. Fungi that break down wood need moisture, and as long as a window frame, sill or fascia board is properly sealed and can dry out, they do not get a chance. As soon as water soaks into the wood and can no longer get out, the process begins. The nasty part is that the paint layer on the outside can still look reasonable while the wood underneath is already soft. This article is about detecting and repairing wood rot in window frames, sills, fascia boards, bargeboards and timber cladding, and about the question of who does it: the carpenter or the painter. The painting itself, which follows the repair, is covered in Exterior painting: when to paint, what weather and what the painter does.

How does wood rot develop?

There is almost always a spot where water can get into the wood and cannot get out again quickly enough. The most common causes:

• Damaged or ageing paint. Small cracks, bare patches and flaking paint let rainwater reach the wood directly. Wood that gets wet behind a paint layer dries poorly, because the paint also keeps the moisture in.
• Open seams and joints. The corner joints of frames and windows, the seam between the frame and the glazing bead and the junction between the frame and the brickwork move as they shrink and expand. A gap of a fraction of a millimetre is enough to let water soak into the wood via the end grain. End grain absorbs water much faster than wood along its length.
• Standing water on sills. A bottom sill must shed water. A sill with too little fall, a frame that has sunk or a sill where the drip groove on the underside is missing or has been painted over holds water.
• Contact with the ground or with wet material. Timber cladding, door frames and shed walls that run down to ground level, or that have soil, leaves or planters against them, stay wet.
• Poor ventilation. Air must be able to circulate behind cladding and in the box of a fascia, otherwise moisture does not dry out. Moisture from the inside, such as condensation on cold frames when ventilation is insufficient, can also attack the wood on the inside.
• Leaks from elsewhere. A leaking gutter, an overflowing box gutter or a poor connection with the roof sends water along fascias and frames. Fix that cause first, otherwise the wood rot will come back; how to tackle a leaking gutter is explained in Cleaning or replacing a gutter.

Detecting wood rot: where and how to look

Wood rot almost always occurs in predictable places: where water stays put or can soak into end grain. Look in particular at:

• the bottom sill of windows and doors, and especially the corners where the sill and the jambs meet;
• the bottom of the jambs of window frames and door frames, where they stand on the sill or on the brickwork;
• the bottom of windows and opening parts, where water runs against the bottom rail;
• the underside and the seams of fascia boards and bargeboards, especially near the gutter and at the joints between two boards;
• the lowest boards of timber cladding and the places where boards meet.

The simplest test is probing. Press an awl or a small screwdriver into the wood. Healthy wood offers resistance and the tip hardly penetrates it; with wood rot, the tip sinks in effortlessly and the wood feels soft or fibrous. Other signs are flaking paint in one spot with dark wood underneath, discolouration of the paint or wood, paint that bulges, sealant joints that come loose, and wood that sounds dull when tapped. Do not only probe where you see something: under the paint, rot often extends further than the visible damage.

An annual round of the house, preferably in spring, is enough to discover wood rot early. When to schedule that in the year is explained in the Maintenance calendar for your home.

Wet rot or dry rot?

What is called wood rot in practice is in most cases wet rot: decay by fungi that only grow in wood that is wet for a long time. Broadly speaking, there are two types. With brown rot, the wood turns brown, shrinks and breaks into cubes as it dries. With white rot, the wood becomes lighter in colour, soft and fibrous. For you as the owner, the type makes little difference: the cause is moisture, and the approach is removing the affected wood and eliminating the source of moisture. As soon as the wood stays dry permanently, wet rot comes to a standstill; the affected wood does not become strong again as a result.

A different story is true dry rot (Serpula lacrymans), called echte huiszwam in Dutch. This fungus occurs mainly inside homes, in damp and poorly ventilated spaces such as crawl spaces, under timber floors and behind panelling, and rarely in exterior frames that can dry out well. Dry rot causes a form of brown rot, but once established it can also spread via strands across brickwork and through joints to wood further away. Characteristics can be white to grey cotton-wool-like or thread-like fungal growth, rust-brown spore dust and a musty smell.

If you suspect dry rot, do not bring in a wood rot filler. Dry rot calls for a specialist who determines its extent and a broader approach: removing affected wood to well beyond the visible damage, treating the surrounding brickwork and above all structurally solving the moisture problem. See also Recognising damp and mould in your home for tracking down the source of moisture.

Repair: wood rot filler, insert or replacement

Which method is suitable depends mainly on the extent of the damage and on whether the component is still strong enough. There are three levels:

• Routing out and filling. With small, local spots, the tradesperson routs or chisels away the rotten wood down to sound, dry wood. The wood is then pre-treated if necessary and the cavity is filled with a two-component wood rot filler, often epoxy-based. After curing, the repair is sanded, shaped and painted along with the rest. This method is suitable for spots that are not load-bearing and where most of the wood is still sound. A filler is not a structural material: you will not make a sill or jamb that is rotten over a great length strong again with it.
• Insert or splice. If the rot is larger, for example a whole corner of a frame or the lower part of a jamb, the carpenter saws out the affected part and fits a new piece of wood, which is glued and, if necessary, secured with a joint. This is also called splicing. The new piece gets the same profile as the existing frame, so that it is barely noticeable after painting. The existing frame stays in place and the glass usually does not have to come out.
• Replacing the component. A bottom sill that is affected over its entire length is replaced in its entirety with a new sill, often made of durable hardwood, cast stone or composite. If the whole frame is affected, or if there are several spots with severe rot, a new frame may be the more sensible choice. With fascias and bargeboards, an affected board is usually replaced in its entirety.

A good tradesperson always removes all the affected wood. A repair on rotten wood will not last: the filler or the insert does not bond to soft wood and the rot continues underneath. It is also important that the wood is dry before it is filled or glued, and that the repair is primed and painted immediately afterwards. If you are torn between repair and replacing the whole frame, read Replacing window frames: wood, uPVC or aluminium, which also covers the permit rules and choice of material.

Fascia boards, bargeboards and timber cladding

Fascia boards, the boards along the roof edge below or behind the gutter, and bargeboards, the boards along the sloping side of a pitched roof, are high up and are therefore often looked at late. Wood rot here is usually on the underside, at the seams between boards and near the gutter. A commonly used solution is to clad the fascia with a plastic or sheet material that no longer requires painting. Be careful: anyone who fits cladding over rotten wood is only hiding the problem. On the front of the home or on a listed building, such cladding may require a permit because it changes the appearance. Have affected wood replaced first and make sure ventilation remains behind the cladding.

With timber cladding, rot is often in the lowest boards and at the butt joints. Cladding must stay clear of the ground and of paving at the bottom, and needs a ventilated cavity at the back so that moisture can dry out. Loose affected boards are usually replaced; if the rot has occurred because the construction is not properly ventilated, that must be improved during the repair.

After the repair: painting and prevention

Wood rot repair is only finished once the wood is sealed again. Immediately after the repair, bare wood is primed, followed by a full build-up of undercoat and topcoat. Which steps the painter follows and in what weather this can be done is explained in Exterior painting. Preferably have the repair and the painting carried out in one job and in the dry season: bare wood or an unpainted repair left out in the rain for weeks absorbs moisture again.

Prevention is easier than repair. The most important measures:

• A fixed maintenance cycle. Wash the exterior paintwork regularly, touch up damage as soon as you see it and have the paintwork renewed in good time. A small bare patch that is touched up straight away prevents an insert later.
• Keeping sealant joints in good order. Have loose or cracked sealant joints between the frame and brickwork and at the glazing beads replaced, with a sealant suited to the application.
• Drainage of sills. Make sure sills have a fall, that the drip groove on the underside is open and that no water stands on the sill. An aluminium or plastic cover profile on a sill can also help, provided the wood underneath is sound and dry.
• Distance from the ground. Keep planters, soil and leaves away from frames and cladding, and let water from the patio and paving run away from the facade.
• Ventilation. Ventilate sufficiently indoors to limit condensation on frames, and keep ventilation openings behind cladding and fascias open.

Listed buildings and permits

Repairing wood rot with the same material and in the same shape is ordinary maintenance. For an ordinary home you do not need an omgevingsvergunning (environmental permit) for this, as long as detailing, profiling and design do not change. For a rijksmonument (national monument), necessary regular maintenance aimed at preserving the monumental values is permit-free, as long as detailing, profiling, design, type of material and colour do not change (article 13.11 of the Besluit activiteiten leefomgeving, the Dutch Activities Decree). An insert or a new sill in the same type of wood, with the same profile and in the same colour, generally falls under this. A plastic fascia, a cast stone sill instead of a wooden one, or a different profile may require a permit for a listed building. For municipal monuments, the rules are in the municipality's omgevingsplan (local zoning plan) or heritage by-law, and extra rules may also apply in a protected townscape or village view. If in doubt, consult the municipality in advance and choose a tradesperson who is used to restoration work, because with listed buildings as much original wood as possible is preserved and repaired locally.

VvE and rented homes

If you live in an apartment, the frames in the external facade are often a common part and wood rot repair is a matter for the vereniging van eigenaren (VvE, owners' association). According to the notaries' model regulations, the frames in the facades are common property, but your own deed of division and regulations may deviate from this. The repair is then usually included in the long-term maintenance plan and is often combined with the exterior painting. Report wood rot to the board and let the repair go through the VvE. More on this in Renovating in a VvE apartment.

If you rent, wood rot repair to frames and exterior woodwork is generally a task for the landlord. The annex to the Besluit kleine herstellingen (Minor Repairs Decree), which lists which minor repairs are the tenant's responsibility, does not mention wood rot; for frames, the annex only mentions keeping them clean, insofar as you can reach them. Painting interior woodwork and filling and stopping small holes are listed there, but the repair of rotten exterior wood is not covered by it. So report wood rot to your landlord in writing.

Carpenter or painter: who does what?

• Filling and painting small spots: a painter. Routing out and filling small spots of wood rot with a two-component filler is standard work for many painters, which they include in the preparation for the paintwork.
• Inserts, new sills, fascias and bargeboards: a carpenter. Structural repair, making an insert with the correct profile and replacing components is carpentry work.
• A completely new frame: a carpenter or window frame specialist.
• Suspected dry rot: a specialist in timber decay or a building consultant, before anything is repaired.

Many painting firms do both: they employ a carpenter or work regularly with one, so that the repair and the painting fall within one job and one schedule. In that case, agree who is responsible for the end result. Painting firms that are members of the trade association OnderhoudNL and have completed that organisation's quality programme may carry the Vakwerk Plusgarantie quality mark (formerly OnderhoudNL Garantie); they work with their own consumer terms and conditions with a disputes procedure and a performance guarantee via the Geschillencommissie (Dutch Disputes Committee). Note that a guarantee on paintwork usually does not apply to wood rot that was already present before the painting was done.

What belongs in the quote?

Wood rot is difficult to estimate precisely in advance, because its extent only becomes clear once the wood is exposed. A good quote therefore makes clear what is included and how extra work is charged. Check that it states:

• which components and spots will be repaired, per frame or per facade;
• which method is applied per spot: filling, insert or replacement;
• which type of wood or material is used for inserts, sills and fascias, and whether the profile stays the same;
• how extra work is calculated if more rot turns out to be present when it is opened up, and that you will be informed about it first;
• whether priming and painting of the repairs is included, and if so with which system;
• replacing sealant joints and improving the drainage of sills;
• scaffolding or an aerial work platform for fascias and bargeboards;
• the guarantee on the repair and its conditions.

How to compare quotes is explained in Comparing quotes: how to read a building quote.

In summary: probe bottom sills, the bottom of jambs and the seams of fascias every year, and have soft wood repaired quickly before the damage gets bigger. Small spots are routed out and filled, larger damage gets an insert, and a sill or frame that is affected over a great length is replaced. Always eliminate the source of moisture, have the repair painted immediately, and if you suspect dry rot, call in a specialist first. On VerbouwPro you post your job free of charge, and carpenters and painters from your region respond.