How to build a terrace roof in six steps24. March 2020
Anyone can install a modern, stable and high-quality terrace roof, but it does require a bit of preparation. Stefan, a master carpenter and fischer’s Technical Trainer, gives a step-by-step guide on how to build a high-quality roof and what to be aware of.
What are the pros of a terrace roof and the cons of a marquee?
“I recently built my own terrace roof”, Stefan reports. “When I first started thinking about a terrace roof I didn’t even consider a marquee. Marquees retract when there is wind because they have a wind sensor. This might protect the marquee, but it means that I can no longer sit out on the terrace underneath my marquee. To me, this is an important advantage of the wood terrace roof: I can sit underneath it come rain or shine.”
“As a master carpenter, constructive wood protection was an important aspect of my decision for a high-quality terrace roof and against a marquee”, Stefan argues. “I want the roof to be as large and wide as possible to protect my facade in the lower storey from rain and snow. I was also thinking in the long term: If you want to sell the house then a high-quality terrace roof will increase its value”.
Do you need planning permission for a terrace roof?
How big do you want the terrace roof to be? This will affect any potential planning permissions. Because Stefan’s roof is smaller than 30 square metres he doesn’t require planning permission according to the State Building Code of Baden-Württemberg. Other federal states and countries may have different regulations. If you want your roof to be bigger you may need to seek planning permission, which could take between six to eight weeks depending on the planning authority.
Take local foundation challenges into account
The foundation has the task of evenly distributing the construction’s load across the ground. Local wind and weather conditions should be taken into account in order to determine the loads and correct dimensions of the foundation. “As a carpenter or construction worker you’ll know this”, Stefan states. “If a house is located in the Black Forest, for instance, then you will have a snow load of approximately 260 kilogrammes per square metre, which is higher than by the North Sea, for example. But the North Sea is exposed to higher wind drag loads than the Black Forest. These kinds of factors will affect the foundation.”
What type of wood should you use?
“As professionals you will be aware that durability class 2 wood is used outdoors”, Stefan explains. Those who don’t want to treat the wood should opt for Douglas fir wood or larch wood. A tip from the professional: “I matched the colour of the terrace roof to the colour of the windows and rafters, which is why you can’t see any of the wood grain on our roof. But the wood grade is crucial when it comes to timber”, which is why Stefan’s first choice was GL24H glue-laminated timber. Glue-laminated timber is truer to size, has a compliance certificate and guarantees fewer splits than construction timber. It also remains truer to size because the lamella, which have a width of 3.5 – 4 cm, are laminated using a weather-resistant adhesive. Stefan didn’t want the wood to warp towards the wall purlin should it get wet through humidity or rain, which is why he opted for GL24H glue-laminated timber. If you want dimensional accuracy, then glulam is the only option.
What are the advantages of glass roofing?
There are numerous cladding options for the upper surface of the roof. Glass is one of the more high-quality options, but it is also more costly. Those who opt for laminated glass have the advantage that light still shines into the rooms behind the roof, so that the sun can heat up the rooms. “If you choose a 12 mm glass sheet then there won’t be any noise in case of rain or hail”, Stefan states. This isn’t the case when it comes to cheaper alternatives such as corrugated metal, acrylic panels, plexiglass or tiles.
Calculating loads and installing connecting parts
Connecting main beams and/or secondary beams
“You might usually connect the main beam and/or secondary beam with a dovetail or an integral connector. But what you might not know is that you can also connect the entire thing using full thread screws”, Stefan says. This is done as follows: If the rafter connects to the front side of the eaves purlin as a secondary beam, then insert the full thread screw at a 45-degree angle. This will give you a self-supporting connection that guarantees tensile strength under full load. You can calculate these types of connections and many others using Woodfix. Woodfix is a free design tool that is part of our FIXPERIENCE software and allows you to calculate the most common types of wood connections.
Connecting wooden posts to the foot of the post
Wood posts are generally connected to the foot of the post with a conventional DIN 571 hex head screw, which is inserted into the foot of the post using an impact wrench. But Stefan recommends a fixing method that saves time and doesn’t require any predrilling: the fischer steel plate wood construction screw. It is fairly similar to a hex head screw and was developed in order to directly connect steel components to wood. This approved screw allows fixings to be placed close to the edge without splitting the wood.
Connecting the wall purlin to the main rafter
Another connection point lies between the wall purlin and the main rafter. Nails were used for this purpose in the past, but flat head screws have since become a common solution. The advantage of a flat head screw is that it provides the rafter with more contract pressure and offers more resistance to wind loads.
One of the most important main connections of this project is the main wall purlin, which is attached to the main frame work through the wooden frame and the external wall insulation. In the past, the wall purlin was aligned and fixed in place with two countersunk screws per fastening point, but this puts too much pressure on the screws. Stefan therefore recommends: “Use a flat head screw to align the wall purlin and then use a full thread screw. You can find the recommended spacing in the ETA. Mark the distance for each fastening point and then insert the full thread screw at an angle of 45 degrees. When load is applied, it will now be subject to tensile loads.” Stefan knows from experience that “a screw’s steel tensile load-bearing capacity is always higher than the wood itself, which means that the construction will be defective before the screw fails.”