Thermally Modified Ash Decking: Pros, Cons & Lifespan

Thermally modified ash is real hardwood that has been changed using controlled heat and steam. The process improves its resistance to moisture, decay, swelling, and shrinking without using traditional chemical preservatives.

It also gives ash a rich brown color that can look similar to tropical hardwoods.

But thermal modification does not make the wood perfect. The process can make ash more brittle, and the boards still need correct installation, ventilation, cleaning, and UV protection.

So, is thermally modified ash a good decking material?

Short Answer

Thermally modified ash is a good choice for homeowners who want the appearance of premium hardwood with better dimensional stability and no plastic content.

Quality thermally modified ash decking may provide a service life of around 20 to 25 years or longer, depending on the product, installation, climate, and maintenance.

Its main advantages are:

• Real hardwood appearance
• Improved decay resistance
• Better dimensional stability
• No chemical wood preservatives
• Lower moisture absorption
• Comfortable natural-wood feel

Its main disadvantages are:

• Higher cost than common wood decking
• Greater brittleness than untreated ash
• Possible splitting around fasteners
• Color fading without UV protection
• Limited availability
• Product quality varies by manufacturer

This is a research-based evaluation using manufacturer documents, wood-science research, technical information, and common homeowner concerns. We have not personally tested the products discussed.

What Is Thermally Modified Ash?

Thermally modified ash starts as ordinary ash lumber. The wood is placed inside a controlled chamber and heated to a high temperature.

The exact temperature and process vary by manufacturer. Heat, steam, low oxygen, or a vacuum may be used to prevent the wood from burning.

This treatment changes the wood throughout the board. It is not simply a surface coating.

The process changes some of the components that normally attract moisture and support fungal growth. As a result, the finished wood absorbs less water and becomes more resistant to decay.

Thermal modification also darkens the ash. Pale untreated wood becomes a deep brown color similar to some tropical hardwoods.

No conventional chemical preservatives need to be added during the thermal process. However, the installed boards may still receive surface oil or other finishing products.

Is Thermally Modified Ash the Same as Heat-Treated Wood?

The terms are sometimes used as if they mean the same thing, but this can be misleading.

Wood may be heated for several reasons. Kiln drying, for example, removes moisture so that lumber can be used more reliably. Kiln drying alone does not create thermally modified wood.

Proper thermal modification uses higher temperatures and a carefully controlled process to change the wood’s physical and chemical properties.

Ask the supplier for:

• The name of the modification process
• Treatment temperature
• Durability classification
• Intended use
• Warranty
• Installation instructions
• Third-party testing

A dark brown color alone does not prove that the wood has received a reliable exterior-grade thermal treatment.

How Long Does Thermally Modified Ash Decking Last?

Quality thermally modified ash decking is commonly expected to last approximately 20 to 25 years or longer in an above-ground exterior application.

Some manufacturers support their ash decking with a 25-year warranty or a 25-plus-year rot-resistance rating. The wording and coverage vary by product.

A warranty is not the same as a guaranteed service life. It may cover decay while excluding surface checking, color changes, poor installation, trapped moisture, or normal weathering.

The actual lifespan depends on:

• Quality of the original ash
• Thermal modification process
• Board thickness and profile
• Moisture exposure
• Ground clearance
• Airflow below the deck
• Joist spacing
• Fastener placement
• End treatment
• Local climate
• Cleaning and maintenance
• Condition of the supporting frame

Thermally modified ash should not normally be placed in direct contact with soil. It also should not be treated as a substitute for structural framing unless the manufacturer specifically approves that use.

Can it last longer than 25 years?

It may.

A well-designed deck with good drainage and ventilation can continue performing beyond its stated durability period. However, it is safer to use the manufacturer’s tested rating rather than assuming a 40- or 50-year lifespan.

The deck boards may also outlast parts of a pressure-treated wood frame. Inspect the joists, beams, posts, ledger, flashing, and fasteners regularly.

Advantages of Thermally Modified Ash Decking

1. It has the appearance of premium hardwood

Untreated ash is usually pale. Thermal modification changes it to a warm brown color.

Depending on the manufacturer and treatment level, the finished boards may have:

• Medium brown tones
• Dark chocolate shades
• Visible hardwood grain
• Natural color variation
• A smooth, refined appearance

This can give a deck the character of real hardwood without using Ipe or another tropical species.

The color normally extends through the wood rather than sitting only on the surface. If a board receives a light scratch, the exposed area may be less noticeable than it would be on a surface-stained pale board.

Deeper scratches and fresh cuts can still show color differences.

2. It is more dimensionally stable than untreated ash

Ordinary wood absorbs and releases moisture as humidity changes. This causes expansion, contraction, cupping, and warping.

Thermal modification reduces the wood’s ability to absorb moisture. This improves dimensional stability.

Properly made thermal ash generally experiences less:

• Swelling
• Shrinking
• Cupping
• Warping
• Movement between wet and dry seasons

It is still real wood, though. It will continue to respond to moisture and temperature.

Correct board spacing, ventilation, storage, and fastening remain necessary.

3. It offers better decay resistance

Untreated ash does not have the natural outdoor durability of Ipe, cedar, or white oak heartwood.

Thermal modification significantly improves its resistance to decay. The process makes the wood less suitable for the fungi that cause rot.

This allows ash to be used for applications such as:

• Decking
• Exterior cladding
• Porches
• Outdoor walls
• Screens
• Some outdoor furniture

Improved decay resistance does not make the board waterproof. Water can still collect around fasteners, ends, damaged areas, and poorly ventilated spaces.

4. It contains no plastic

Thermally modified ash remains solid wood.

This can appeal to homeowners who want a natural deck surface but do not want composite or PVC boards.

It offers:

• Natural grain
• Real wood texture
• No plastic core
• No wood-plastic mixture
• A surface that can weather naturally

It can still include surface oil or coatings, depending on how it is finished.

5. It does not rely on traditional chemical preservatives

The durability comes mainly from heat modification rather than pressure-injected chemical preservatives.

This does not automatically make the complete product chemical-free. Finishes, cleaners, adhesives, fasteners, and manufacturing processes must also be considered.

Still, thermal modification provides an alternative for homeowners who do not want conventional pressure-treated decking as the finished walking surface.

6. It is usually lighter than dense tropical hardwood

Ipe and Cumaru are extremely dense and heavy. Thermally modified ash is generally easier to lift, carry, cut, and position.

This may reduce installation difficulty, particularly on:

• Rooftop decks
• Balconies
• Sites with limited access
• Upper-story decks
• Projects using long boards

Lower weight does not mean the board is approved for wider joist spacing. Follow the span and support requirements for the exact profile.

7. It can be sourced from temperate forests

Some thermally modified ash products use North American or European ash instead of tropical hardwood.

This can reduce the need to source a tropical species. However, sustainability still depends on legal harvesting, forest management, manufacturing energy, transportation, and service life.

Look for clear sourcing information and independent forest certification when available.

Also read:

Composite vs Aluminum Decking: Which Is Better?

Disadvantages of Thermally Modified Ash Decking

1. The boards can be brittle

This is one of the most important disadvantages.

Thermal modification improves stability and decay resistance, but high heat can reduce some mechanical properties. The wood may become more brittle than untreated ash.

Brittleness can lead to:

• Splitting near board ends
• Damage from poorly placed screws
• Cracking if fasteners are overtightened
• Broken edges during handling
• Damage from heavy impact

This is why the manufacturer’s fastening instructions matter. Predrilling or approved hidden clips may be required.

Do not treat thermal ash exactly like ordinary pressure-treated decking.

2. It costs more than common wood decking

Thermally modified ash is a premium decking product.

It normally costs more than:

• Pressure-treated pine
• Standard cedar in many markets
• Some entry-level composite boards
• Locally available untreated lumber

The higher price reflects the ash lumber, controlled modification process, grading, milling, and specialized distribution.

Compare the complete installed cost, including:

• Deck boards
• Clips or screws
• Joist tape
• Fascia
• End treatment
• Surface finish
• Delivery
• Labor
• Waste
• Replacement boards

Its price may still be competitive with premium composite decking and tropical hardwood.

3. The brown color will fade

The heat-created brown color is not permanent when exposed to sunlight.

Without UV protection, thermally modified ash will gradually weather to a silver-gray patina. This is a normal reaction to ultraviolet light and does not automatically mean the boards are failing.

Weathering may appear uneven at first. Covered areas, shaded sections, and spaces under furniture may change color more slowly.

If you want to preserve the brown appearance, use a finish approved by the decking manufacturer. It will need to be reapplied as required.

4. It can develop checks and cracks

Thermal ash is more dimensionally stable, but it is not immune to weathering.

Small surface checks, end cracks, and grain changes can develop because of:

• Strong sunlight
• Rapid wetting and drying
• Poor end treatment
• Incorrect fasteners
• Inadequate board spacing
• Low ventilation
• Rough handling

Minor checking can be normal for exterior wood. Large cracks, loose boards, sharp edges, or movement around fasteners deserve closer inspection.

5. Availability may be limited

Pressure-treated wood and composite decking are available through many home-improvement stores. Thermally modified ash is more likely to come from a specialist supplier.

Limited availability can create problems with:

• Delivery cost
• Lead time
• Board-length selection
• Replacement boards
• Color matching
• Finding an experienced installer

Ask whether the supplier expects to carry the same profile and color in the future. Keeping a few correctly stored spare boards may be useful.

6. Not all thermally modified wood is equal

Performance depends heavily on the original lumber and the modification process.

A low-quality product may have:

• Inconsistent treatment
• Weak or brittle boards
• Poor milling
• Mixed moisture levels
• Inadequate testing
• No clear exterior rating
• Limited technical support

Do not buy only by comparing color and price. Look for third-party durability data, installation documents, and a clearly written warranty.

Does Thermally Modified Ash Need Maintenance?

Yes, but its maintenance needs are relatively simple.

It does not normally require a coating to maintain the thermal modification itself. The treatment goes through the board and does not wash away like a surface-applied preservative.

Maintenance depends mainly on the appearance you want.

If you want the deck to turn gray

Allow the boards to weather naturally.

Clean the deck when needed and keep gaps free from leaves and dirt. The color will slowly change from brown to gray.

The change may take longer in shaded or covered areas.

If you want to preserve the brown color

Apply a manufacturer-approved exterior wood oil with UV protection.

The finish may need periodic reapplication. The schedule depends on sun exposure, climate, foot traffic, and the product used.

An exposed south-facing deck may need attention sooner than a covered porch.

Basic maintenance checklist

• Sweep away leaves and debris.
• Keep board gaps and drainage paths open.
• Clean spills promptly.
• Wash with an approved cleaner.
• Inspect fasteners and board ends.
• Watch for loose or damaged boards.
• Reapply UV protection when needed.
• Inspect the supporting frame and flashing.

Avoid harsh chemicals and aggressive pressure washing unless the manufacturer allows them.

Does Thermally Modified Ash Get Hot?

Like any decking material, thermal ash can become hot in direct sunlight.

Its dark brown color may absorb more heat than pale untreated wood. However, wood generally does not transfer heat in the same way as metal.

Surface temperature depends on:

• Board color
• Sun exposure
• Air temperature
• Wind
• Shade
• Surface finish
• Time of day

If heat is a concern, request a sample and place it outside in direct sunlight. Test it under the same conditions as other decking samples.

This is especially useful for pool decks, homes in hot climates, and areas used by children or pets.

Is Thermally Modified Ash Slippery?

A clean wood surface can provide reasonable traction, but no deck is slip-proof.

Thermal ash may become slippery when covered with:

• Algae
• Mold
• Wet leaves
• Dirt
• Grease
• Ice
• Sunscreen
• Standing water

Board texture also affects grip. Smooth boards may feel different from brushed or grooved surfaces.

For a pool deck, ramp, stair, or frequently wet area, ask for wet-slip test results for the exact product and profile.

Thermally Modified Ash vs Ipe

Ipe is naturally dense, hard, and decay-resistant. Thermally modified ash gains its outdoor durability through heat treatment.

Ipe is generally better for maximum hardness and long-term durability.

Thermally modified ash may be better if you want a lighter, temperate hardwood with improved stability and no chemical preservatives.

Thermally Modified Ash vs Composite Decking

Composite decking combines wood fibers and plastic. Thermal ash is real solid wood.

Composite usually offers stronger resistance to staining and does not need oil to maintain a manufactured color. Thermal ash provides genuine wood grain and a more natural feel.

Choose thermal ash if real wood is important. Choose composite if color retention and minimal appearance maintenance matter more.

Installation Points You Should Not Ignore

Correct installation has a major effect on lifespan.

Important points include:

Use the approved fasteners

Use the stainless-steel screws, clips, or fastening system required by the manufacturer. Other metals may stain the wood or fail outdoors.

Avoid overtightening

Overtightened fasteners can crush or split thermally modified boards. The fastener should hold the board securely without damaging it.

Support board ends

Board ends and joints need correct framing support. Unsupported ends are more vulnerable to splitting and movement.

Follow the specified joist spacing

Do not assume that thermal ash can use the same spacing as a different wood or composite product. Profile thickness and board orientation affect allowable spacing.

Maintain ventilation

Airflow below the deck helps the boards dry evenly. Poor ventilation can create uneven moisture conditions and shorten the life of the deck system.

Allow proper gaps

Board spacing allows drainage and normal movement. Use the current manufacturer instructions rather than a general spacing rule.

Protect the frame

The decking may resist decay better than the supporting wood. Proper ledger flashing, joist protection, and drainage can help the frame last longer.

Always check local building requirements. Deck structure, stairs, guards, railings, beams, joists, and footings must meet the rules that apply to your location.

Is Thermally Modified Ash Decking Worth It?

Thermally modified ash is worth considering if you want real wood with a premium brown appearance and better outdoor stability than untreated ash.

It is best suited to homeowners who:

• Prefer natural wood over plastic decking
• Want an alternative to tropical hardwood
• Accept that the color will weather without oil
• Can follow a careful maintenance routine
• Have access to a trained installer
• Are willing to pay more than pressure-treated wood costs
• Can buy from a supplier with reliable technical documents

It may not be the right choice if:

• You want the lowest-cost decking.
• You want a completely uniform color.
• You do not want any checks or natural wood movement.
• The deck will be in ground contact.
• You cannot find replacement boards locally.
• The installer is unfamiliar with thermally modified wood.

Final Verdict

Thermally modified ash is a durable and attractive decking material, but its real value comes from the combination of natural appearance and improved stability.

It can provide approximately 20 to 25 years or more of service when the product is well made and the deck is correctly designed.

The wood does not need staining or sealing for decay protection in the same way as ordinary untreated ash. However, a UV-protective oil is needed if you want to maintain its rich brown color.

Its biggest weakness is brittleness. Careless handling, incorrect fasteners, or overtightening can damage the boards.

Before buying, ask for the product’s durability classification, warranty, installation guide, approved fasteners, and maintenance instructions. Also confirm that the supporting structure and deck design meet local building requirements.

For homeowners who want a real hardwood deck without choosing a tropical species, thermally modified ash can be a strong and attractive option.

Frequently Asked Questions

Is thermally modified ash waterproof?

No. It absorbs less moisture than untreated ash, but it is not waterproof. Proper board spacing, drainage, ventilation, and flashing are still required.

Does thermally modified ash rot?

It is much more resistant to decay than untreated ash. However, prolonged moisture exposure, ground contact, poor ventilation, or incorrect installation can still create problems.

Does thermal ash need to be sealed?

It generally does not need a sealer to preserve the thermal modification. Apply an approved UV-protective oil if you want to slow the change from brown to silver-gray.

Does thermally modified ash turn gray?

Yes. Sunlight will gradually change the brown surface to a silver-gray patina unless UV protection is maintained.

Is thermally modified ash stronger than normal ash?

Not in every way. Thermal modification improves dimensional stability and decay resistance, but it may reduce some bending and impact properties. The treated wood can be more brittle.

Can thermal ash be used for structural framing?

Do not use it for joists, beams, posts, or other structural parts unless the product has been specifically graded and approved for that purpose. Most thermal ash decking is intended as a finished surface.

Can hidden fasteners be used?

Yes, when the board profile and manufacturer support an approved hidden fastening system. Follow the required joist spacing, clip placement, and edge support instructions.

Is thermally modified ash good around pools?

It can be used around pools when approved by the manufacturer. Check its wet-slip performance, surface temperature, drainage requirements, and compatibility with pool chemicals.

Is thermal ash better than cedar?

Thermally modified ash is generally harder and more dimensionally stable than cedar. Cedar is lighter, easier to source, and may cost less. Both will weather gray without UV protection.

Sources

1. USDA Forest Service – Field Performance of Thermally Modified Wood
2. American Hardwood Export Council – Thermally Modified American Ash
3. Thermory – Understanding Climate Performance
4. Thermory – Durability and Long-Term Performance
5. Thermory – Warranty vs Durability Rating
6. Thermory – Choosing Thermally Modified Wood
7. Thermal Modification of Wood: A Review
8. Moisture-Dependent Strength Properties of Thermally Modified Wood