Choosing adhesive for pergolas and other outdoor furniture

In summer we all want to be outside in nature, surrounded by as much natural greenery as possible, and those who have space in their backyard are thinking about setting up a place like this. It's the time of year when orders for gazebos, pergolas and other wooden decorations go through the roof. About pergolas and gazebos we've already talked about, but without insisting on gluing the wood that sits outside.

The elements of these constructions are not always made of a single piece of wood. Those light beams that form the 'roof' of pergolas or support posts are made of layers of wood glued together. These elements are sometimes impregnated in autoclave before the work is carried out to increase their resistance to damp, insects or mould. For the beams to remain intact the solder must not be affected by the aggressive conditions in the autoclave. Polyurethane adhesives are most often used as they are highly resistant to boiling or pressure impregnation. But they are not the only adhesives that can be used in aggressive conditions, and certainly not the only ones that can withstand the high humidity outside and the action of weathering and UV radiation.

High-strength adhesives are needed for building pergolas and other outdoor installations

Pergolas, unlike gazebos, do not have a roof. They are used in the garden to create an alley with walls and a roof of greenery, to enclose a space or to support climbing plants. Basically, they are frames attached to lightweight beams that plants can climb on. Instead of a roof, there are cross beams, which the plants cling to, shading and sheltering the place.

In order for the pergola to withstand the humidity of the plants and the weather, the wood needs to be protected with oil or outdoor varnish. Compared to other outdoor constructions, moisture protection is very important because the presence of moisture is permanent and is maintained by plants.

There are other types of landscaping that we find in the garden. Although they are called differently, they are very similar. Often the pergola can be confused with the canopy, the difference being in the roof, where the pergola is straight and the canopy is curved. Terraces are another construction similar to pergolas, the difference being that the terrace is always connected to the house.

For all these constructions the joints made by gluing, especially those forming beams or poles, must be very strong and durable. The glue line must be elastic and waterproof, and the adhesive must not 'age' under the sun's rays.

I said there are several types of adhesives that can be used outdoors. Let's take a look at the most commonly used.

Polyvinylacetate PVA adhesives

They are the most commonly used wood adhesives, both in industry and as DIY adhesives. They have very good adhesive properties, are non-toxic (they do not emit formaldehyde) and are easy to clean with water before curing.

There is a perception that such adhesives are only for indoors and that moisture resistance is low. Not so. There are also versions with very good outdoor and even boiling resistance (strength classes D3 and D4). For example, Szolvegy Târgu Mureș has a one-component polyvinyl acetate-based adhesive-Tiszabond 1kD4 - with very good resistance to moisture and outdoor conditions. The adhesive is much more elastic because it contains no filler and has reactive groups that allow advanced cross-linking giving the resulting film much better strength. If all the steps in the product data sheet are followed to obtain the bonded elements, they can be autoclaved with very good results.

Despite being a very strong adhesive for its class it is not recommended for obtaining support beams. In these cases polyurethane adhesives are recommended.

To increase their resistance to moisture and outdoor conditions, some PVA adhesives can be transformed into two-component adhesives by adding a catalyst. This considerably increases cross-linking and the adhesive moves from a lower strength class to class D4. This is also the case for TISZABOND 2kD4 who was awarded the quality certificate byInstitut fur Fenstetechnik Rosenheim, Germany (Institute for Window Technology Rosenheim). By adding hardener, the Class D1 adhesive is transformed from a very low moisture resistant adhesive to a strong and durable Class D4 adhesive, making it a very good choice for window frame manufacturing.

Polyurethane adhesives

They are the most durable and strongest wood adhesives. They are very versatile and can be used for bonding many types of materials: wood, glass, metal, ceramics, polyurethane foam, textiles, plastic, stone. They are very resistant to humidity and are very suitable for use both indoors, in places with high humidity (bathrooms, kitchens), and outdoors (fences, gazebos, pergolas). They are used for gluing solid wood and wood derivatives.

Polyurethane adhesives can be single or two-component. In the case of one-component adhesives, the reaction (drying of the adhesive - bonding) is determined by the humidity in the atmosphere. This is why they bond well to materials that contain moisture inside. When gluing different materials (wood-stone, wood-concrete) at least one of the materials should contain moisture.

For the production of laminated beams to be autoclaved and impregnated with substances that make them resistant to fungi, mould and insects, the following can be usedTiszapur D4 4401 Speciala one-component polyurethane adhesive that is highly resistant to extreme conditions. The strength of the glued wood is very good provided that the gluing is done according to the instructions and the beams have a resting time after gluing of 7 days before autoclaving.

To test the adhesive's capabilities the Solvegy they spliced a beam and left it for only 4 days before autoclaving, not 7 as recommended in the data sheet. Then the beam was left out continuously for 2 years. The glue behaved very well and the impregnated wood did not change colour over time.

Cyanoacrylate-based adhesives, so-called Super Glue

Super Glue is a brand of glue that has become synonymous with glue that sticks anything very quickly. This type of adhesive - cyanoacrylates - bonds very hard surfaces together and has a very short reaction time. It's the best choice for quick and easy repairs, but not the best for big jobs like a pergola.

It is very suitable for gluing small parts.

Epoxy adhesives

They are bicomponent adhesives that react after mixing the 2 parts. The reaction is quite slow and the parts that stick together must be held together (clamped) until the reaction is complete. The result is a very durable solder. There are variants that can only be used for indoor soldering and variants that can also be used for outdoor soldering.

If you decide to do such work in your garden, don't forget to choose an adhesive with very good resistance to moisture, weather and solar radiation. If you buy laminated beams or posts, don't forget to make sure that the glue used for gluing is outdoor resistant. And always protect the wood with oil or exterior-resistant varnishes to prolong the life of the work as long as possible.