A Dowelled Timber Wall Is Only as Good as Its Tested Connection

There is a quotation on your desk that says "no glue", and nothing on it tells you what that phrase is worth. One kind of dowelled timber wall is built from dried softwood boards laid in crossing layers and pinned one layer to the next with grooved aluminium dowels. No adhesive holds it together. The connection does. So the evidence you need sits somewhere other than where you might look for it, and each stage of a project asks a different question of that connection and the documents behind it.

We design and manufacture solid timber houses in Latvia in both MHM and CLT. We sell both, and we would rather you chose the right one than the one with the better slogan. What follows is the order in which a project runs into these questions, what gets decided at each point, and where buyers tend to misread it.

Shortlisting: "no glue" tells you how the layers are joined, and nothing more

At this stage you are deciding which wall systems stay on the list. A glue-free label describes the joining method. It says nothing yet about strength, warmth or price.

Not every dowelled wall is joined the same way: some systems use timber dowels, others metal ones, and the documents behind one kind of fastener do not cover the other. MHM, short for the German Massiv-Holz-Mauer, is one mechanically joined system. The boards are dried, each layer is laid at 90 degrees to the one beneath, and profiled aluminium dowels are driven in where the boards cross, one layer at a time. Each is far shorter than the wall is thick, so no fastener runs from one face to the other.

Glued cross-laminated timber arrives at a panel that looks much the same from the outside. Its layers are bonded face to face with structural adhesive. Both are solid timber walls. Both are engineered in a factory. They answer different priorities, and the structural stage is where that shows.

The mistake here is ranking them by ingredient list. Drop CLT because it contains adhesive, or MHM because it contains metal, and you have made a structural decision from a label. Timber frame belongs in the same conversation, and our guide to which wall system suits which project sets the three side by side. For now, keep both solid timber options open until an engineer has sketched the structure.

Structural design: how a dowelled timber wall carries load

Vertical load goes down mainly through the boards whose grain runs upright. The crossing layers brace those boards against buckling and help the wall resist forces acting along its own plane. The dowels tie each layer to the next, so the layers share the work instead of acting as a stack of loose boards.

Each dowel is a grooved aluminium pin. Its profile bites into the surrounding wood fibres, which resists pull-out and lets shear pass from one layer to the next. The boards are dried before assembly, so they start closer to the moisture content they will settle at in a heated house. That limits the shrinkage that could otherwise slacken the grip.

A mechanical joint slips very slightly under load. A continuous glue line is far stiffer. The engineer designs the dowelled wall as a connected element with that slip built into the calculation, and taking floors and roof down to the foundation is exactly the job the system is made for.

Where people go wrong: a panel that looks like a solid block gets treated as if it behaves like a glued plate in every direction. It does not. Openings, the lintels over them and bracing against wind are designed for the specific house, whichever system you choose.

Where glued CLT does the job better

When a panel has to span, cantilever or brace hard in its own plane, glued CLT has the edge. The continuous bond gives it higher in-plane shear stiffness. Where the supports and connections are designed for it, a floor or roof panel can also carry load in two directions. Both matter for long floors, overhangs and larger or taller buildings. CLT has its own European product standard, EN 16351, and the adhesive families used include polyurethane (PUR), melamine-urea-formaldehyde (MUF) and emulsion polymer isocyanate (EPI).

That is why one house can sensibly combine dowelled walls with glued panels or glulam in the floors and roof. The choice is made element by element, by the engineer with the loads in front of them.

Reading the offer: what the European Technical Assessment actually covers

A European Technical Assessment sets down the declared characteristics of a specific product, such as its load-bearing and fire performance, assessed against a European Assessment Document. For a mechanically joined wall this document carries real weight. These walls fall outside the product standard written for bonded CLT, and the assessment is the route by which the fasteners and the wall system get values an engineer can design with. The MHM system has two such assessments, one for the wall element and one for the aluminium fastener, and both are the system's documents, not ours.

What you are deciding now is whether the wall you have been offered actually sits inside that assessment. Check these on the document itself:

• the issuing body and the date
• the product it names
• the wall thicknesses it covers
• the layer build-ups it covers
• the dowel type, and the separate assessment for the dowel where the system has one

A wall thicker than the assessed range, or with a different layer pattern, needs the engineer's confirmation before anyone puts a price on it.

The assessment stops at the product. It does not sign off a house, a floor plan or a complete build-up. The engineer still has to design the walls, the openings, and the connections to floors and to the wall base for your project, using the values the assessment declares.

Treat fire figures with the same suspicion. A classification only means something alongside the tested thickness and load behind a fire class. Ask for the report and hold it up against the wall on your drawings.

Choosing finishes: what the missing glue line does for indoor air

A glue-free core takes adhesive layers out of the wall. The timber, and whatever you put on it, becomes what the room air meets. That simplifies one part of the picture and leaves the rest to the interior specification: which face stays exposed, how it is finished, and what flooring, sealants, paints and furniture come into the room.

Wood has its own natural volatile compounds. They give fresh softwood its smell, and they fade as the timber ages. Oils, varnishes and flooring adhesives bring their own emissions, and furnishings add more. Ventilation then decides how fast any of it leaves.

A glue-free core does not make an emission-free room. A glue line does not, by itself, make a worse one. In CLT the bond lines sit behind the surface layer, and PUR and EPI adhesives are formulated without added formaldehyde. What the finished panel emits is a separate question, answered by its emission declaration. Ask the maker of any panel for one, and ask the same of every finish you plan to put on it.

Years later: alterations, dismantling and recovery

You can alter a dowelled wall later. At end of life its wood and aluminium can be separated in processing, if the facility that receives it is equipped for that. Both depend on conditions set long after handover, and on records kept at the start.

Aluminium is used for the dowels partly because suitable woodworking tooling can cut through it, which a hardened steel fastener would not allow. So a new opening is physically possible. It is still a structural change: the cut removes boards and dowels, and an engineer has to check it against the original design.

At end of life one workable route is shredding followed by metal separation. Eddy-current sorting can pull non-ferrous metal such as aluminium out of a wood stream, which opens the way to recycling both. Whether that actually happens depends on the facility, and practice varies between countries and operators. Glued CLT goes down the recovery routes for bonded wood products, which vary in the same way.

People misjudge this stage in two directions. Some picture the panels coming apart board by board like flat-pack furniture, which is precisely what the grooved dowels are there to prevent. Others wave end of life away as irrelevant, when the decision that counts is made now: keep the as-built drawings and the wall specification, so whoever alters or takes down the house knows what is inside the walls.

Before you compare prices: questions for every supplier

If you have two offers in hand, you are at the offer review. Settle these with each supplier before the prices mean anything:

• Name the wall system on each offer and, for a dowelled timber wall, the European Technical Assessment behind it, with its issuing body and date.
• Confirm that the assessed range covers the offered wall thickness, layer build-up and dowel type.
• Ask the engineer which elements are dowelled, which are glued CLT or glulam, and why.
• Ask how openings, lintels and wind bracing in the long walls are designed for your house.
• Request the fire classification report and compare its tested thickness and load with your drawings.
• Ask for emission declarations for the panel and for every interior finish on the specification.
• Make the as-built drawings and the wall specification part of the handover.
• Ask which end-of-life recovery route the offer assumes, and treat the answer as a scenario that depends on local facilities.

Put the questions to every supplier in writing, and lay the answers side by side, line by line.