Designing with glulam – top detailing tips for external glulam designs

1.    Internal to external glulam beam detailing

Designs which include glulam beams extending through an external wall from the inside to the outside mean one end of the beam will be exposed to the elements.  

This is most common when you have roof beams or rafters that extend outside. If the roof stops right at the edge or end of the beam, or just after, what tends to happen is when the rain and the wind blows the water usually lands directly onto the main bottom corner of the sticking out beam. If this gets continually wet and cold, over time this corner will begin to rot before the rest of the beam, because it is being exposed. 

You can of course choose a hardier timber for these beams, but depending on what you want your whole structure to be made of and what aesthetic you’re going for, you’re still going to have glulam externally which could perish. 

With a small tweak to your design, you can minimise this problem. By setting back the end of the beam so that it would be sheltered from rain falling at a 45 degree angle, the glulam will have much less exposure.  Also if you cut a chamfer on the end of the beam you can minimise the area which is likely to be affected by wind-blown rain. (Unless your rain comes in sideways 😉) 

Adjusting your design will help protect your glulam, but the end grain of a glulam beam will always crack and weather more quickly than the rest of the beam. While not a design tip as such, we recommend you seal end grain with an end grain sealer, which helps a lot. It’s Best to avoid externally exposed end grain, continuing roof flashing over the end of the beam and having a robust drip detail to prevent any water from tracking along the bottom of the beam)

In a recent experiment we tested the (end grain) absorbency of different timbers and the effectiveness of end grain sealants.

The end result, even our basic water-based varnish drastically decreases water absorption compared to the untreated sample. Combining finishes with design details is a belt and braces approach to getting the best from your glulam.

(More on how to protect your glulam here, and stains and varnishes here.)

2.  External glulam post detailing

With detailing tip one, the key point is to try and reduce the area where your glulam will be exposed to the elements to protect it as much as possible.

In the same way, if your project design has external glulam vertical posts which are fixed to the ground, you’ll find rain will hit the post sides and run down the post to the bottom. If this has plenty of drainage you might be okay, but generally anywhere where water can collect and sit without drying out quickly will cause the timber to rot.

The current recommendation is your glulam design should be 200mm from the level of the ground to the bottom of the wood, and the base of the post should be galvanised steel.

This is the only detail where we would not be willing to go against the grain (pun intended). Especially for posts sitting under overhangs (because most of what we do is nearly always under some sort of roof overhang) if the base is likely to get wet and stay damp for weeks or even months, all sorts of creatures will start to grow and eat away at the base of your glulam post.

For external timber a small amount of wetting is not of concern if it’s for a short period of time and the timber can air dry soon after. Posts in ground contact are often subject to prolonged periods of damp. Once the timber moisture content rises above 20% it is susceptible to fungal growth and decay. Lifting the post 200mm off the ground protects the end grain from splash back.

(If you’re worried about glulam column damage or repair, we can also help restore your glulam, even if we were not part of the initial design or installation, see our blog on glulam repair here.)

3.    Internal to external glulam column detailing

Quite often people want glulam columns in their design, and they also want the columns to go from the inside to the outside.

Something people sometimes worry about is cold bridging, which can happen with steel columns. If you’ve got a piece of steel that goes from your inside to the outside when the outside gets cold the inside will also get cold. If the inside gets cold this is where condensation can form.

Cold bridging is not generally as much of a concern for glulam columns, as timber is not a very good transmitter of heat relative to steel or concrete columns. However, the main concern with a glulam column is when you’ve got a structural column in a building. When the outside part starts to deteriorate (which eventually it will) you’re potentially going to have to remove a whole column and possibly lots of windows etc, which is a lot of hassle and expense.

Even if you choose a more durable timber for your glulam, such as larch instead of spruce, it’s still not got any huge long-term durability when exposed to weather – you either must keep treating it or make sure it doesn’t get wet at all. It will also push your budget up if you choose to use a more costly timber throughout.

In this scenario, we recommend you change your design to put the column completely inside and then have the windows going to the outside face of the glulam with a fascia piece which you can screw into the column and hide the join. This way you can insulate behind the fascia and simply replace that part if it does start to wear. Visually you get almost the same effect but with a much easier and cost-effective maintenance option should you need it.

4.  Completely external glulam structure detailing

Finally we do get asked to create structures that are for external use only – bandstands, pergolas, bridges etc.

Usually, if we create a garden structure it is understood that it will eventually be weathered so detailing isn’t as critical, however, should it be an external structure which still needs to be structurally sound and last, such as a bridge, we do offer advice on what you can do to help.

Any sort of glulam fully outside is going to weather and crack (see what you need to know about cracks in glulam here). It will crack a lot less than big green timbers, but it will still crack as with the changing of the seasons it will get wet and dry out continually, so it’s almost inevitable

As mentioned above if a beam gets wet, we want it to dry out as quickly and efficiently as possible. Large horizontal surfaces can be an issue. The tops of large externally exposed structural beams e.g. a bridge, have potential to have standing water on the tops of the beams leading to accelerated weathering of the beam. This issue can be intensified if there are any cracks or checking on the top surface allowing moisture to sink deeper into the beam making it harder for the beam to dry out after a period of rain.

As a bare minimum, the design should have a slant on it to help the water drain off, rather than having a flat area where water can pool. The best practice is, where possible, to fit capping pieces to the tops of beams that will brunt the worst of the weathering and throw off any water. This can be incorporated into your design and means your main structure will be protected as much as possible.

Committing to a maintenance program is important to keep your external glulam in as good a shape as possible, but there are a couple of details in your design you can consider which will help.

Details like these in your design can make a big difference to your project. If you’d like to discuss your next project, or have questions about design, do get in touch and one of the team will be happy to help.