Timber Cladding: Concept Design: RIBA Plan of Work Stage 2 - Swedish Wood

Sustainability Aspirations: Timber Cladding

Timber is the most sustainable mainstream construction material. It's naturally renewable low carbon and can be recycled or used as biomass energy at the end of its life. Under forests where our construction timber comes from, in Scandinavia, Europe and North America are growing more extensive year by year.

The quality of timber starts with the sort of slow growth achieved in cold northern climates such as Sweden, in forests certified as sustainably managed, and it continues in the mill through the care taken with grading, machining and moisture content, to produce quality timber like this.

Login to record your CPD points

Concept Design - Outline Specification

There are two important things to get right with cladding, you want to ensure that it can dry out easily, so you don't want to create anything through bad design that traps water. You can also add a coating to it to make it more aesthetically pleasing but of course that doesn't actually necessary mean you're going to add to the longevity of the product of the durability of the cladding.

Getting the detailing the fixing of the cladding right is as important to the life of the cladding as the choice of the species itself. If we're using softwood, then his choice of spruce which is commonly known commercially as white wood or pine which is commonly known as Redwood.

Slow Growth Pine and Spruce

We're looking for good quality for material which is slow grown which typically come from the Nordic countries and in particular Sweden.

The growth rate is important because the tighter the growth rings the better the finish you can get when you machine it. Ideally the timber that we use for the cladding should be even more she content between about 16 and 19%. Slow grown spruce and pine is ideal for cladding but really for the UK climate it really needs to be preservative, pre-treated to ensure its durability.

Dried After Preservative Treatment

After preservative pre-treatment in whatever form that’s taken and we need to ensure the surface of the cladding is dry enough to accept a coating if you're going to coat it. Ideally, we actually want to get the cladding back down to the 16 to 19% moisture content that we talked about earlier.

Then there are other softwoods to consider for cladding that are naturally durable these typically species such as Siberian larch and Western Red Cedar. When you specify these sorts of naturally durable claddings in any timber actually, we need to ensure that the sapwood of the tree is excluded.

Specify Sapwood Excluded

The sapwood is the live part of the tree which is naturally perishable in any timber species. It's not possible to exclude it commercially completely but if you get that into specification then we ensure that we get as little sapwood in the passes as possible.

So, then you can also consider hardwoods for cladding as long as they're naturally durable and that would be species typically like Oak or Chestnut. And then there are the new generation Timbers, known as modified wood. Wood that is made more durable through a heat treatment process or chemical modifications such as Acetylisation. 

16mm and 25mm thickness

The thickness of the boards can be typically between 16mm and 25mm.

75mm and 150mm in width

Ideally 19mm would be the minimum in terms of the width of cladding typically you're looking at between 75mm and 150mm in width.  If you want to go beyond that then you need to choose a species with low movement and ensure wide expansion gaps are used in the system.

10mm to 12mm Tongue

When using tongue and groove boards specify a 10mm tongue ideally 12mm. This is so that the tongue and groove don't get disengaged when the timber cladding moves.

3mm Radius Edge

In order to achieve a consistent even coating you need to avoid sharp edges so specify a minimum 3 mile radius on any edge. The radiused edge on the bottom of the cladding is very important. What it does is ensure water is shed away from the elevation rather than potentially tracking up the back of the cladding. Fine sawn finish will hold a coating much better than smooth planed finish and sawn finish is actually less likely to check and split on exposure.  Fine sawn finishes appear to perform better than smooth finishes whether they're painted or not.

21mm Air Gap

Ensure adequate ventilation a 21mm gap between the back of the cladding and the wall.

Protect vunerable end grain, design out water traps stop cladding 150 millimetres above ground level. Ideally cladding should be fixed with a stainless steel proprietary clip system such as that offered by Brooks brothers and this avoids nailing through the surface of the cladding.

Nail Flush, Treat Exposed Grain

If you're nailing through the surface the cladding is sure the nail head is flush with the surface. If you punch it through the surface, then you're inviting water to get in.

Stainless Steel Annular Ringshank Nails

For most softwoods use small headed stainless steel annular ring Shank nails.

For lower density species such as western red Cedar you use a bigger headed nail to stop it pulling through.

Secret nailing so that the adjacent board covers the nail hole is an elegant solution.

Drill at Angle

High density species such as Siberian larch in hardwoods should be drilled and screwed to avoid splitting.

For high tanning species such as Oak use stainless steel fixings to avoid staining.

Oversized Holes and Washers

If you're using green timber such as Oak drill oversize holes and use washers to allow for the movement.

Nail and screw length should be a minimum 2 1/2 times the thickness of the board.

Clear End-Grain Sealer

So, your timber will arrive on site preservative treated. Preservatives alone will not stop moisture ingress. You need to treat it with a clear end grain sealer even if you decide not to paint it. To apply the end grain sealer is a simple process brush application along the full length of the engrained making sure you get a liberal amount on there, so every part is covered.

Any access on the face of the board you need to remove that with a cloth or a tissue. You may find you have to do some trimming during your fitting, prior to putting the end grain sealer on you will need to retreat with preservative treatment.

Treat Cuts and Notches with Preservative

You may also find that you have shakes or cuts in the coating system, again before recoating bring it back to its former glory you will need to reapply the preservative treatment.

The key to recoat in the cladding is not to wait until the paint actually breaks down. If you catch it early enough, preparation and recoating can be very easy.

Inspect and Refresh Coating after First Year

It’s always advisable after the first year to do a proper inspection, a wash down and inspect for any open joints and movements of the timber and do a full recoat of the system.

Translucent coatings should be washed down and recoated every two to three years of fake coatings every five. And if you're doing a semi translation somewhere in between the two.

The key to UV protection is basically the amount of pigment in the coating system. Generally, the rule is the darker the colours are, the more pigment the better. If you've got darker translucent colours, you're always going to get a lot more UV protection.

When you get to opaque colours, they do reflect the UV a lot better and in fact white is probably the best colour to have because it reflects it the most. Weathered timber can be cleaned down using the mall detergent solution and rinsed off with clean water.

There are many cladding profiles and styles and here are some that are used horizontally, feather edge cladding both sawn in this situation the first one is treated spruce. It's just a simple feather edge simply overlaps one another each board, as opposed to this black painted feather edge, which is again spruce, but it's rebated to allow the feather edge cladding to sit flatter on the elevation. So, here are examples of machine cladding. The first one is a simple rebated shiplap again it loosely connects to give you that finish as opposed to one that can be tongue and groove. This being a larch.

Tongue and Groove

The tongue is extended so that you can hide any fixings. Some cladding profiles aren't suitable in a horizontal situation particularly tongue and groove varieties, which can cause that capillary action of the water getting in behind the cladding. Here seeing examples of vertical claddings that do use a tongue and groove. Simple connection, this style would allow you to have very smooth from finish to the cladding and this is identical but with a little tiny V joint, so it gives you just a different feature at the front.

Vertical Cladding

Here are examples of vertical cladding both tongue and grooved and both with the added benefit of being able to hide the fixing in this groove here. So, these are secret fix vertical claddings. The first one is a secret fix shiplap again which connects like that. Again, smooth front, the fixings hidden in that little V groove there by the adjacent tongue. Just a different profile, same type of fixing is this channel profile. Just gives you a very different detail at the front a bit like a shadow gap, you can see there. Here’s a simple but popular board on board style cladding that can be smooth as this is smoothly machined or in sawn. It is used vertically in this board since on the adjacent board to give a sort of stepped feel to the elevation of the building. 

Rainscreen Cladding

Here's an example of a timber rainscreen made of treated Swedish pine. Many architects like the look of natural timber. Most unfinished timber will weather to silver grey colour because of the natural effect of ultraviolet light. You can expect some differential weathering if some sections of facade are more subject effects of light and water. That's a good reason to consider one of the many coating options available.

Coatings can be translucent to bring out the beauty of the wood itself or semi translucent which adds colour to the cladding, while still showing the beauty of the wood grain. Both provide a very natural way of reducing the visual impact of differential weathering of the timber surface. And of course, there are opaque colours such as his brush applied water based wax or finishes which can be easily maintained every year or so for a fresh fashionable solid finish. For a more durable finish specify a water based micro porous coating to minimum dry thickness of 100 microns. Or two brush coats have a water based water boil for an easily maintained system.

Pre-painted | Exterior cladding

Pre-painted exterior cladding is becoming increasingly popular. It provides a uniform controlled coating finish. It's more durable and quicker to install on site. This is typically available as a fine sawn Scandinavian spruce. Pre-treated for use in class 3.1 with either 1 coat or 2 coats of a microporous water borne coating system already applied in the factory. One of the main benefits of factory pre-painting is that the coating can be applied shortly after the boards are produced, getting the best possible adhesion.

Here's an example of fine sawn feather edge black cladding with a single coat, when installed it is advisable to add a second coat. This is rebated white-feather-edge cladding with 2 coats and secret fix finds on shiplap with 2 coats of white.

Procurement - Certified Sustainable Wood

Wood is the greenest building material known to man and the great thing about it is its ability to absorb carbon dioxide from the atmosphere to store the carbon in the wood that we used to build buildings with. But that's only true of course if the wood has been grown sustainably. The good news for us in the UK is that 83% of all wood we use comes from a certified sustainable source.

Certified that is to the FSC or the PEFC standard and nearly 100% of softwood, the principle building material is certified as sustainable. For wood coming in which hasn't got a certificate from FSC or PEFC the good news there is that most of that will be coming from parts of the world, where sustainable forest management is the norm, but certification just isn't very popular.

Due Diligence

On top of that the United Kingdom is subject to the EU timber regulations which places an obligation on those bringing timber in from outside the EU to conduct a form of due diligence on the product to ensure that it is unlikely that that has come from an illegal source, and depending on the level of risk to take necessary measures to minimise that risk. Those specifying wood should to always ask for proof that it has come from a sustainable source.

Proof | Chain of Custody

In most cases that will be via a chain of custody certificate from either FSC or PEFC. In those cases where that's not available the specifier, should ask the supplier to give evidence that due diligence under the EU timber regulation has been performed on the product, to ensure that it is come from a legal source. Both FSC and PEFC now offer the opportunity for a developer to get their development certified as FSC or PEFC on a cite wide basis. You would need to make sure that all your suppliers are supplying material which is either coming from FSC or what they call controlled wood sources or vice versa.