How Mortise & Tenon Joints Hold or Fail in Post & Beam Frames

Many structural issues in historic homes develop slowly and can go unnoticed for years. Moisture, natural movement, and the passage of time often cause problems where heavy timbers connect, placing stress on the mortise and tenon joints that hold the frame together. The good news is that these issues usually show warning signs if you know where to look.

Colonial Restorations has worked on historic timber frames since 1981, and one thing we see again and again is this: the joinery usually tells the story before the structure does, if you know what to look for. 

In this article, read about how these joints function, why they fail, and what that means for your old or historic New England home. 

The mortise & tenon joint explained

A mortise & tenon joint is the classic “lock-and-key” of timber framing:

• The mortise is a pocket (a rectangular hole) cut into one timber (often a post or girder).
• The tenon is a tongue cut on the end of the mating timber (often a beam) that fits into that pocket.
• The joint is often secured with a wooden peg (also called a trunnel or tree nail), which ties the members together without modern fasteners. 

In historic post & beam construction, these joints are common throughout older New England homes because they were designed to be strong and long-lasting. The good news is that many of these joints can be repaired when they are kept dry and properly supported. When damage is more severe, replacement joinery may be recommended by a structural restoration expert like Colonial Restorations.

How mortise & tenon joints fail and what causes it

1) Rot in the contact points

Also known as the “bearing zones”, this occurs even if the joint is perfectly cut; it can still fail if the wood around it softens. This is concerning because these contact points carry load-bearing zones that experience the most compression and stress over time.

Rot commonly begins where:

• A post meets a sill
• A sill meets a foundation
• A beam end is embedded in masonry or trapped near damp sheathing

Once rot reduces the wood’s density, the joint stops acting like a tight mechanical lock and starts acting like a loose fit. That’s when you get sagging, bounce, and drift.

Why it happens: moisture trapped at the foundation line, poor drainage, bulk water intrusion, or long-term high humidity in crawlspaces/basements.

2) Peg problems in the joint

Pegs are strong, but they’re still wood. Over time, they can:

• Loosen as the frame dries and shrinks
• Deform or shear if joints are racked repeatedly
• Rot if moisture is persistent

And once the peg loses its grip, the joint can start to “pump” with seasonal movement, slowly enlarging the peg hole and worsening the looseness.

3) Shrinkage and seasonal movement

Old frames weren’t kiln-dried like modern lumber. Timbers can shrink and move over decades, and they also expand/contract seasonally.

That movement can create:

• Small gaps at the shoulders
• Slight rotation at beam ends
• Separation at brace connections

A little movement is normal. The problem is when movement becomes progressive (it keeps getting worse).

4) Racking forces from wind or from an altered building

When a frame racks, mortise & tenon joints experience stress they weren’t meant to carry alone.

Common triggers:

• Removed walls or braces during renovations
• Undersized or missing braces
• Roof load changes (new roofing materials, snow load patterns, structural alterations)

5) When settlement causes joints to shift

If part of the foundation settles more than another, the frame tries to follow. Traditional joinery can only handle some of that for so long.

Some historic homes were often set on stone foundations without modern waterproofing, and that, when one element fails (like a sill), the whole frame can shift. 

When settlement is significant, you may see:

• A post no longer bearing fully
• A tenon pulling partially out
• Crushing at the shoulders
• Diagonal cracks in plaster, sloped floors, and sticking doors/windows

Learn more about settling foundations in our blog: “How to Repair a Settling Foundation in Historic Homes: A Guide to Repair and Restoration.”

6) Past “repairs” that fight the frame

We often see well-meaning repairs that create new problems, like:

• Steel brackets slapped on without addressing rot
• Sistering that transfers loads oddly
• Modern fasteners are placed where they split old-growth timber
• Boxing in timbers so moisture can’t escape

Learn more about your post & beam in our blog: “5 Things You Should Know About Post and Beam Construction.”