Best Wood for Window Sills: Durable Hardwoods for Exterior and Interior Sills

The sill is the hardest-working part of any timber window. It sits at the bottom of the frame, catches all the rain that runs down the glass and must shed it away from the wall, year after year. It is also the part that rots first on poorly designed or badly finished windows. Inside, window boards take sunlight, condensation, plant pots and the occasional cat. Choosing the best wood for window sills means picking a species that is durable outdoors, stable, hard enough to resist knocks and easy to finish, then detailing it so water drains away.

In this guide we compare the hardwoods window makers use for sills, explain why many factories use a different species for sills than for the rest of the frame, cover sizes and profiles, finishing and maintenance, and work through an example for a window manufacturer. At Global Timber Trade Hub we supply sill stock in durable African hardwoods to joinery manufacturers.

Why sills fail first

Water runs down the glass, collects on the sill and, if the detailing is poor, sits against joints, end grain and the bottom of the frame. Paint or stain on horizontal surfaces breaks down faster in sun, and once moisture gets into the wood, decay can start in less durable species. Joints between the sill and frame are especially vulnerable. That is why window designers use the most durable timber they can for sills, and why detailing such as weathered slopes, drip grooves and sealed end grain is critical.

Sill woods at a glance

Iroko: the standard sill timber

Iroko has long been one of the most popular sill timbers in the UK and Europe. It is naturally durable, very stable, available in thick, long sections and less expensive than doussie or afrormosia. It takes paint, stain and oil well with proper preparation, and its golden-brown colour suits stained windows. See our iroko wood price guide.

Moabi for hard-wearing sills

Moabi is dense, hard and durable, with a fine texture that finishes beautifully. It resists knocks and wear, which suits sills in busy buildings and on doors that see heavy traffic. See moabi wood price.

Doussie and afrormosia for premium windows

Doussie and afrormosia are among the most stable and durable hardwoods, ideal for premium windows, doors and conservatories. Both are CITES Appendix II listed, so allow for permits and longer lead times. See our doussie wood price and afrormosia wood price guides.

Why not sapele sills?

Sapele is an excellent frame and sash timber, but it is only moderately durable. Many manufacturers therefore use sapele or sipo for frames and sashes and a more durable species for sills, which take the most water. This combination gives long life at a sensible cost, and when the window is painted or stained, the slight colour difference is hardly noticeable. See our sipo wood price guide.

Sizes and profiles

• Thickness: commonly 65, 75, 80 or 100 mm for exterior sills.
• Width: 120–200 mm depending on wall depth and design.
• Length: random or fixed lengths to suit window widths.
• Profile: sloped top (weathering), drip groove underneath, grooves for frame and seals.

Thick sill stock needs good logs and careful drying, so it costs more per cubic metre than thinner boards.

Detailing for long life

• Slope the top surface so water drains outwards, typically at least 8–10 degrees.
• Cut a drip groove under the front edge so water falls clear of the wall.
• Seal all end grain and joints carefully before assembly.
• Avoid flat ledges where water can sit.
• Use a quality microporous paint or stain system and maintain it.

Interior window boards

Inside, window boards do not face rain, but they do face strong sun, condensation and wear. Sapele, sipo, moabi, movingui and oak-coloured species all work well, often chosen to match floors or joinery. Seal the underside and ends to reduce movement, and choose a finish that resists water rings from plant pots.

Moisture content

For exterior sills, order kiln-dried stock at about 12–16%, matching typical outdoor conditions. For interior window boards, 8–12% is better. See kiln dried hardwood.

A worked example for a window manufacturer

Consider a manufacturer producing timber windows for new homes and refurbishment projects. Its frames and sashes are sipo, but it had complaints about sills decaying at the joints on some older windows made entirely of a less durable wood. It now orders 80 × 160 mm iroko sill stock, kiln dried to 14%, alongside its sipo frame stock in the same container. Sills are machined with a 10-degree weathering, a deep drip groove and pre-sealed end grain before assembly, and the whole window gets a factory-applied microporous paint system. The manufacturer offers a longer guarantee on its windows as a result. The iroko adds only a small amount to the timber cost per window, but it removes the most common cause of early failure.

Maintenance of timber sills

Even the best sill benefits from simple maintenance. Once a year, clean the sill and frame, check the paint or stain for cracks, especially at joints and on the horizontal surface, and touch up any damaged areas before water can get in. Every few years, depending on exposure, apply a fresh maintenance coat over the whole window. South-facing and west-facing windows in sunny, exposed positions need attention more often than sheltered ones. Clear any debris that blocks the drip groove or drainage slots. If you spot soft or dark areas in the wood, investigate early: small repairs with a resin filler and a new coat of finish can save a sill that would otherwise need replacing. Homeowners and building managers who follow this routine often get many decades from hardwood windows, far longer than many alternatives.

Heritage and conservation windows

On historic buildings, replacement sills often need to match the profile and appearance of the originals. Conservation officers may ask for traditional detailing and species. Iroko, moabi and similar durable hardwoods are commonly used for new sills on old windows, sometimes spliced into existing frames. Share drawings and requirements early so we can supply suitable sections.

Lead times and payment

Kiln-dried iroko and moabi sill stock is usually ready to load in 5–8 weeks; CITES species take longer. For first orders, a letter of credit or a deposit with balance against copy documents protects both sides. See our letter of credit for timber guide.

Finishing timber sills

The finish on a sill does more work than on any other part of a window, because it faces rain, standing water and direct sun on a horizontal surface. A good system starts in the factory: sand the sill smooth, round sharp edges slightly so the coating can build up a proper film, seal the end grain with a dedicated end-grain sealer and apply a primer or base coat to all faces before assembly. Then apply the top coats, either a microporous paint or a translucent stain, building up the recommended film thickness. Microporous systems let moisture vapour escape from the wood while keeping liquid water out, which reduces blistering and peeling. Oily species such as iroko may need a solvent wipe or a specific primer, so follow the coating maker’s advice. For stained windows, choose a darker tone on south-facing elevations to protect against UV. A well-applied factory finish is one of the best investments a window maker can make, and it supports longer guarantees.

Sill extensions and sub-sills

Deep walls sometimes need a sill extension or a separate sub-sill to carry water clear of the brickwork. These parts face the same conditions as the main sill, so use the same durable species and detailing, and seal the joint between them carefully.

Ordering sill stock

When you send an enquiry, tell us the species, sill thickness and width, lengths, moisture content and volume, and whether you also need frame and sash timber in the same container. Ordering sills and frame stock together saves freight and keeps your production supplied with matching parts. We can supply random lengths for cutting in your factory, or fixed lengths that match your standard window widths to reduce waste on the saw.

Tell us your destination port, too.

Common mistakes

• Using less durable species for exterior sills: they rot first.
• Flat sill tops: water sits and penetrates joints.
• Missing drip grooves: water runs back under the sill.
• Unsealed end grain: the main entry point for moisture.

Our honest advice

Use a durable, stable species such as iroko, moabi, doussie or afrormosia for exterior sills, even if the rest of the window is sapele or sipo. Detail sills to shed water, seal end grain and maintain the finish. That combination gives windows that last for decades.

Sources

• The Wood Database for durability data.
• British Woodworking Federation

Related guides

• Best wood for window frames
• Best wood for exterior doors
• Iroko vs sapele
• Furniture and joinery hardwoods
• Hardwood timber supplier UK

Frequently asked questions about the best wood for window sills

What is the best wood for exterior window sills?

Durable, stable hardwoods such as iroko, moabi, doussie and afrormosia.

Is sapele good for window sills?

It is better for frames and sashes; use a more durable species for sills.

How thick should a timber window sill be?

Commonly 65–100 mm for exterior sills.

How do I stop timber sills rotting?

Use durable wood, slope the top, add a drip groove, seal end grain and maintain the finish.

Get a quote for sill timber

Send us your species, sill sizes and quantities through our quote form. We will reply with a firm price and lead time.