Useful Tips

Wooden trusses - strong and lightweight structures for covering large spans


In the construction industry, the word "farm" (a bunch of rafters) means a system of rods, usually straightforward. Connecting among themselves in knots, it forms an invariable geometric design.

Farms are often used to create a strong structural roofing system for a wide variety of building types. This article is an instruction that describes in stages the creation of simple basic farms. It is suitable for the construction of a small barn, warehouse building or any other structure that does not require special technical requirements.

Consider the sequence of constructing this core system

Determine if your homework is suitable for your project. Remember that the farm supports the roof of your building. Its construction must not violate building codes, including the requirements of local wind and snow loads and other structural requirements.

At a minimum, you should choose the look and design of your farm. The basis of this rafter system is the most rigid and economical design - a triangle. It is formed from the following elements: the upper belt (two rafter legs, rafters), the lower belt (tightening) and diagonal struts in this closed loop - braces that connect two nodes of the truss (frame) and provide structural rigidity.

A farm consisting only of belts can withstand only a very light roof. Heavier roofing materials: corrugated board, metal, need solid support. Therefore, most farms are equipped with such internal supports as braces, struts, contractions and racks.

The quality of the farm depends on the quality of the wood. For this kind of construction, a high density of conifers is desirable. Southern yellow pine or fir is perfect, spruce, northern pine and Weimutova pine (white eastern pine) are less preferred.

Also, when choosing wood, pay attention:

Lumber size: with well-designed and manufactured mounts for light trusses, 5 x 10 cm (2 x 4 in) is suitable,

Timber quality: for this project, straight-layered, dense boards with a few knots are more preferable. Try to choose without an obvious split or with a peeled bark, with a "pickle".

A few facts: in the construction the word “obzol” is used - not sawn side surface of logs located on the edges of lumber. There are two types.

1. A dull overview, in the case when the upper parts of the edge of the board (from the outside) are not sawn, and the lower ones are sawn along the entire length on both sides.

2. Acute scaling - when the edge of the board is sawn to a part of the length and some of its sections are not sawn across the entire thickness.

Brawl significantly reduces the grade, and, consequently, the cost of edged lumber. In Russia, GOST 8486-86 strictly regulates its presence on the surface of boards,

Straightness of wood: twisted, deformed or curved boards are not suitable for truss construction. Lumber with a slight curvature can be used in construction, they are installed "crown" up,

Sawn timber must be dried in an oven; if this is not done, then the raw wood is pulled together and / or deformed after installation and assembly of the truss.

Be sure to set up goats or any other flat, stable place to work. If possible, extend the full-size truss on the floor to precisely set the correct length and the various angular cuts required for the process. If this does not work out for you, it is recommended to lay the bottom boards of the truss on the goats to mark the parts according to the sample.

Place the board of the lower girdle face up (the gusset is the wide part of the board), put the boards of the upper girdle on it. Subsequently, they will be cut off along the border, as each part of the overlap will intersect.

Mark the cut line in the places where the support nodes overlap, cutting off the excess, if necessary, the tightening (lower belt) is cut first.

Mark the ridge knot where the tops of the boards of the upper girdle are connected and cut. If you know the angle of inclination (slope) of the roof, then the angle of the cut can be determined with a winkel (a tool for checking right angles, a square of a square rafter). As a rule, the slope of the roof slope ranges from 10 to 60 degrees.

Mark the common rafter node, immediately cutting off both upper belts or sequentially, to mark most accurately, from the central point of the perpendicular of the lower belt through the two upper strapping to the area where they overlap.

Determine which belt, top or bottom, will hang over the ledge of your building, and then select the appropriate bottom and cut off the top of the board. To plumb the cornice line, use the angle that you set for the top of the rafters. If you want to tilt the plank panel inward (if you think you can do it alone), use a sharper angle.

To install a truss, you must determine the positions of the intermediate diagonal braces. There are certain technical recommendations and even online resources on which you can calculate the supports and span declared in your project, but this article considers simple farms (bundles) where the general rule of the thumb applies: one bracket from the point of one third of the lower belt to the middle upper belts.

Superintendent's council:

1. Divide the length of the lower belt into three, then measure the resulting distance through the farm belt, mark the two points where the braces cross the belt.

2. Mark the center point in each upper harness, then connect the spacers so that they overlap each belt so that the intersection angles are marked each in its place.

Use all the unique details of your rafters as a model to mark the subsequent details for each farm that you plan to build. Do everything possible to make sure that the upper ends of the belts are placed on top when the ligament is finally built. Take care of maximum accuracy by marking and trimming every detail, as their installation will be difficult with the wrong length or angles.

Determine if the nailed belts will hold fast enough. You will probably want to use reinforcing plates, a “construction gun for ligaments”, or angles for all communication nodes of structural elements. The illustrations show 16 patterns of galvanized iron plates (from scrap scrap metal waste). If diverse atmospheric effects are not considered during construction, then 12.7 mm (1/2 in) plywood plates may be used.

Each part of the first ligament is nailed together, supporting it at intersections with additional spacers or pillars. This is done in order to keep the assembly on a flat surface while laying on the wall. Use the clamps to hold the fitted parts together, securing them and handling any displacements in the wood you are using.

For metal or sheet plates designed to strengthen plywood, when hammering nails, do not pierce the board that you are strengthening. Otherwise, bend the protruding ends of these nails before placing the plates on the opposite side of the ligament.

Using the first bunch - template, collect the remaining bundles. Make sure all joints are supported by temporary supports if you are working on a goat or a table that is not large enough to set up a complete farm.

Lay the finished trusses on a “compact stacker,” stacked on top of each other so that they lie on the most flat surface. This will allow you to align them and avoid deformation of the ligaments before installation. Placing the ligaments folded in this way will reveal the correspondence of the ligaments to each other. Keep in mind that poorly measured and straight material results in slight differences between one bundle and another.

Set the bundles on the building under construction. Securely attach them to the main flat surface in accordance with building codes.

Types of Wooden Farms

Such structures can be used not only in the construction of roofs, before, for example, they were used in the construction of bridges. But, given the susceptibility of the tree to moisture and the effects of insects (despite all the protective impregnations), recently they have been used mainly for roofing, or as floors between floors in private construction.

As for the classification, according to the purpose, it is possible to distinguish:

  • wooden truss trusses - they can be assembled on the ground, and then simply lifted and fixed on an almost finished house. It is very convenient and reduces construction time,
  • overlapping trusses - in this case they are used as overlapping between floors.

In the photo - blanks for overlapping

There are certain differences in the form of construction, we can distinguish such types as:

  • with parallel belts - used for the device of overlapping between floors,

Note! This type can also be used in the construction of a roof with a slight slope.

  • triangular - in the construction of private houses, the roof frame consists of several triangular trusses connected on top by a wooden beam,
  • rectangular trusses can be used for roofing with a slight slope,
  • options with a trapezoidal outline, as well as with a curved upper belt are possible.

The diagram shows different options for the outline of the upper belt

As a rule, a wooden farm is made either from a wooden beam, or from sufficiently wide boards (if the load is mainly vertical).

But other options are possible:

  • if necessary, the perception of large loads can be used combined design. In them, for example, the lower belt can be made of steel, and the rest of the elements can be made of wood. This will reduce weight as much as possible, and the metal can withstand the operational load without problems,
  • there are also farms made of plywood pipes. The main advantage of this type can be considered low weight (even compared to the analogue of a bar or board). A small price can also be considered an advantage. Of course, the strength of such structures is slightly less than that of analogues made of thick bars, therefore, they are mainly used not as supporting structures,

Plywood pipe assembly diagram

  • farms in which fiberglass and wood are combined can be considered exotic, but they exist. Fiberglass is quite durable, and also has a low weight, so that the design is quite strong and lightweight,
  • but most often there are ordinary structures made of solid or glued beams, as well as boards.

More about wood farms

To a person far from construction, the use of wooden trusses may seem counterintuitive - why waste time assembling such a structure if you can simply cover the span with a wooden beam (read also the article “Wooden panels for walls: selection and installation”).

In fact, this is nothing more than a delusion, and in some cases these constructions are simply irreplaceable. For example, a beam with a span of 12 m only under the influence of its own weight will give a considerable deflection and cannot be used without additional supports.

Advantages and disadvantages of using lumber trusses

The use of such designs has several advantages:

  • it is possible to easily cover large spans without installing additional supports. No beam can compete with them for a span of several tens of meters,

The length of this structure exceeds 30 meters

  • due to their rigidity and low weight, their assembly can be arranged on the ground, and then delivered to the installation site,
  • in comparison with deformation beams, there is much less deformation, the deflections are so small that this does not affect the condition of the ceilings of the lower floors in cases when wooden ceilings of the truss are used
  • communications can be hidden in the gaps between the elements, which will positively affect the interior of the room,
  • with proper dexterity, such designs can also be used as a design element.

Of course, there were some shortcomings:

  • compared with beams, the thickness of the trusses is still much larger, respectively, and the thickness of the ceilings between floors is large,
  • purchased structures cost a lot,
  • if you try to make such a design yourself, you will have to pay special attention to the nodes and the quality of the assembly as a whole. It is desirable that even a simple wooden farm of 8 meters should be assembled perfectly, without gaps and crevices.

Knots of wooden trusses - the junction of 2 or more elements, it can be considered as the weak point of the whole structure. With careless assembly, poor-quality connection, structural rigidity can be forgotten.

The connection of individual elements into a single whole can be performed in several ways, consider the simplest rafter truss:

  • in the ridge node of the rafters are connected in half the tree and bolted. It is allowed to use metal or wooden linings for greater rigidity,

Skate and support units

  • for rigidity, a tightening is required connecting both rafters. In this case, a groove is made in the rafters according to the size of the tightening, and it is fixed with bolts,
  • a wooden truss of 6 meters and a longer length (up to 12) requires the installation of additional ties (vertical and inclined bars) to provide rigidity. The connection diagram will look like this - in a vertical connection, small grooves are made into which the inclined connections abut. Additionally, the unit is strengthened by a metal plate,
  • also the roof truss structure made of wood will require a reliable connection to the wall. The easiest way to do this is to simply lay the wooden bars on the wall and install the rafters in the grooves, securing them with bolts.

On the diagram - various options for trusses

Note! There are options for the support node using ceiling beams, in which case they will also fulfill the role of tightening. True, the design of the node is somewhat more complicated.

Designs with parallel belts are usually created using special metal plates. Such plates for connecting wooden trusses on one side have a series of teeth that securely fix the assembly.

Ceiling Beam Mounting Scheme

Homemade wood farms

Classification of the design and calculation of wooden trusses is rather the concern of specialized organizations. When the need arises for its construction on its own, most often they just use ready-made drawings, especially since private houses are usually built as standard ones.

Most often, farms in private construction are used in the construction of the roof. In this case, you can advise the classic design in the form of a triangle with a puff located in the middle or at the bottom.

More precisely, you will need not 1, but several identical structures, which you can assemble on the ground with your own hands, and then lift it up. And you will need a minimum of tools - a saw, a hammer, pads for knots, bolts or nails.

Metal plate for knots

Note! An exception can be considered roofs of complex configuration with an abundance of slopes. In this case, you need an individual approach and it is better to contact the design organization.

In the case of such a roof, it is better not to engage in amateur activity

In principle, you can make and do-it-yourself overlapping. It is enough to know the simplest dependence - for belts, the dangerous section is in the middle of the span, and for inclined connections - at the supports. Accurate layout and trimming of bars and boards may present some difficulties, but this is a solvable problem.

In conclusion

Farms made of wood can significantly expand the scope of lumber. If ordinary beams can hardly be used to cover large spans, then trusses are devoid of this drawback. It is also important that with a little dexterity it is possible to make such a structure on your own (see also the article "How to make an aged tree at home").

The video in this article shows the assembly and installation of roof trusses.

Types of roof trusses ↑

Visually, they look like a lattice in which the main part is several times greater than the height. They are polygons in shape, usually a triangle, or a hemisphere. The choice of the triangular shape is not accidental - it provides the necessary rigidity and immutability for the design. Made in the form of a set of bar triangles in a trellised ligament, they are especially effective for overlapping spans with a large width.

A hanging type truss has only two, without intermediate, stop points located at the edges on the walls. The upper ends converge on the ridge of the roof. All ends, both upper and lower, work on compression and bending. They are practically designed to span between external walls without additional support on the internal walls. Это дает возможность после их монтажа использовать пространство под крышей в качестве одной большой сборочной площадки.

Таким образом, внутренние перегородки можно устанавливать, не учитывая местонахождение несущих стен, то есть появляется полная свобода в планировке внутренней части дома. However, the horizontal burst reinforcement, which is transmitted to the walls, is ultimately large enough. To eliminate the deflections and lighten the belt (overlap of 6–9 m), a crossbar is additionally used. For large spans, they are usually equipped with a headstock and struts.

Calculation of a triangular wooden truss

Specify dimensions in millimeters

X - truss length
Y - farm height

Z - beam thickness
W - beam width

S - number of uprights
P - struts
N - show the numbering of parts

The wooden truss truss consists of elements forming rigid triangular structures.
The recommended height of the wooden truss is at least 20 percent of the span.
The program will calculate the required amount of materials, show the farm drawings and the necessary dimensions of the parts.