Rot & Insect Resistance in Exterior Engineer Types

Choosing the right type of wood is crucial for any exterior application. When exposed to the elements, wood is susceptible to rot, decay, insect damage and other problems that can shorten its lifespan. As an engineer specifying exterior woods or a homeowner planning a deck, fence or other project, understanding wood’s natural resistance and how to work with or improve it is key to success.

This guide will examine:

Common Issues with Exterior Woods

Moisture and insects are wood’s biggest enemies when used outside. Even woods with good natural resistance can develop issues if proper steps aren’t taken. We’ll overview the most common problems and what causes them:

Rot & Decay

Wood rot starts when fungi break down the cell walls. It often begins where wood contacts soil. The biggest contributors are moisture and poor drainage. Types of rot include:

– Dry rot – breaks down cellulose and lignin

– Wet/brown rot – removes cellulose, leaving a brown, cube-like residue

Extended moisture exposure from rain, sprinklers, etc. can also cause decay in above-ground wood if it can’t dry quickly enough.

Insect Damage

Termites, carpenter ants, powderpost beetles and other wood-boring insects are common pests for exterior woods. Usually they attack wood directly in contact with soil, but can sometimes infest above-ground members. Signs include holes, frass and wood debris.


Over time, sun exposure and moisture take their toll on wood. As the surface erodes, cracking, cupping and discoloration occur. This doesn’t structurally compromise wood but impacts appearance.


Seasonal wood movement leads to shallow surface checks/cracks as moisture content changes. Deeper checks may indicate more serious drying defects.

Mold & Mildew

Like rot, mold grows on wood in consistently damp, shaded areas. It can discolor or corrode surfaces but doesn’t structurally damage most woods.

Solutions for Improving Rot & Insect Resistance

The good news is there are many ways engineers can fight back against moisture and insects to extend exterior wood’s service life:

Specify Durable Species

Some wood species have natural resistance to rot and insects – an important consideration for engineers. Compare species’ decay resistance and durability:

Exceptional – Black locust, red mulberry, white oak, cedars

Good – Teak, black walnut, catalpa

Moderate – Cherry, cypress, redwood, elm, black cherry

Poor – Pine, fir, poplar, aspen, birch

Also assess termite resistance. Borate treatments can be applied for better protection.

Design for Fast Drainage

Allowing moisture to puddle on horizontal surfaces or collect on vertical members invites decay. Strategies include:

– Slope flat elements ≥ 1/4″ per foot

– Avoid intricate, moisture-trapping details

– Ensure proper roof drainage

Also design assemblies to promote airflow and quick drying.

Apply Protective Coatings

Sealants regulate moisture absorption and deflect UV rays:

Film-formers like varnish and polyurethane provide water-resistant coating while allowing some moisture exchange. Can slow weathering.

Penetrating oils like tung oil soak into wood, protecting surface with little film build-up. Allow natural movement/checking.

Water repellents minimize moisture absorption through hydrophobic barrier. Help prevent distortion but allow internal drying.

Preservatives contain fungicides, insecticides to prevent attack from fungi, termites, etc if applied correctly. Common types are copper naphthenate and zinc borate.

Use Heartwood Whenever Possible

The dead inner heartwood is more decay resistant than sapwood. Specify heartwood, or use sapwood with extra protection.

Combine Protection Methods

A multi-pronged approach is best. For example, use a durable wood, slope horizontal surfaces and apply a penetrating sealer to regulate moisture.

Tips for Homeowners

Homeowners can take similar steps to protect their exterior wood projects:

Inspect Frequently

Check for early signs of moisture issues, wood damage or pests before they escalate:

– Discoloration/mold

– Surface deterioration

– Insect frass

– Cracks, checking, bulging

Address right away through drying, repairs, pest management, etc.

Address Moisture Sources

Improve drainage, slope flat surfaces away from structures, extend overhangs and relocate sprinklers to keep wood drier.

Maintain Protective Coatings

As sealants wear down over time, moisture and UV penetration increases. Follow manufacturer recoating schedules.

Clean Overhangs, Vertical Surfaces

Gutters, downspouts, siding and other “high and dry” areas can still accumulate moss, mold and moisture. Clean frequently.

Allow Proper Airflow

Vegetation, debris and poor spacing can trap moisture on wood’s surface. Keep surrounding areas open to encourage airflow.

Preventative Measures

Both engineers and homeowners should also incorporate fundamentals into project planning:

Use Quality Materials from Reputable Sources

Defect-free wood is less vulnerable to insects, moisture, fungal attack and other issues. Don’t compromise quality to save money.

Design for Proper Wood Movement

Accommodate dimensional changes through design details and wood selection. Restricting natural movement can cause checking and cracks.

Avoid Ground Contact Whenever Possible

Foundations, posts, lattice and other members touching soil are extremely vulnerable. Use masonry plinths or metal standoff bases.

Allow Acclimatization Before Installation

Pre-installation conditioning reduces significant dimensional change after assembly. Stabilize moisture content beforehand.


The strategies above help engineers specify and homeowners select exterior wood projects that stand up to weather, moisture and wood’s other natural enemies. No material can resist decay and insects forever, but an intentional, multi-pronged approach can maximize its resilience and service life.

By choosing durable species, addressing vulnerability to moisture and employing secondary protection measures, both engineers and DIYers can have greater confidence in exterior wood’s performance.

Frequently Asked Questions

What is the most rot and insect resistant wood?

Black locust has exceptional natural durability, along with resistance to both rot and insects. Other great options are red mulberry, white oak, Atlantic white cedar, redwood and Western red cedar.

How do you treat wood for weather resistance?

Top treatments for weather resistance are penetrating oils and water repellents. These regulate moisture without trapping it underneath a thick film. Some contain UV blockers. Be sure to prepare surfaces thoroughly and apply products per manufacturer instructions.

Should engineered wood touch concrete?

In general it’s best to avoid direct contact between engineered wood and concrete. The moisture transfer can cause fungal and termite issues. Use sill sealer, pressure treated sleepers or other spacers to allow airflow if proximity is unavoidable.

What is the best wood for outdoor furniture?

Teak is the gold standard for outdoor furniture thanks to its natural oil content, but redwood, cedar, white oak and shorea are quality alternatives. Apply protective sealants regularly since furniture takes a lot of abuse.

How long does pressure treated wood last outside?

Properly installed and maintained pressure treated lumber can last 15-20 years before replacement is needed. Higher quality treatments, like copper azole, can extend this lifespan. Regular cleaning and reapplication of water repellents is recommended.

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