Houten kappen in Nederland 1000-1940


auteur: Herman Janse


bron: Herman Janse, Houten kappen in Nederland 1000-1940. Delftse Universitaire Pers, Delft / Rijksdienst voor de Monumentenzorg, Zeist 1989


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Summary
Wooden roof framing in the Netherlands

Roof framing in the Dutch-speaking region have their origin in the northwest European common rafter roof, in which the roof is supported by common rafters which are connected in pairs by collars. The pairs of common rafters are at right angles to the longitudinal timbers of the roof. Originally, the pairs of rafters were not connected at all, with the result that the roofs often became lopsided. The oldest examples of which traces remain date from the 11th and 12 centuries.

In the second half of the 13th century, constructions were developed in the Netherlands to make the frame more rigid. These took the form of trusses, which were also placed laterally across the building, to carry the plate purlins which in turn supported the rafters.

The craftmen's guilds played an important part in the process whereby these constructions spread. Because knowledge was communicated in the guilds largely by word of mouth, these roof constructions became current almost exclusively in the area where Dutch was spoken, which in the middle Ages also included parts of northern France and western and northern Germany. In German-speaking areas, the northern part of France and in England other forms of roof support developed from the original common rafter roof. Carpenters travelled less than stonemasons and even though roof frames may have been built relatively far from the construction site, they were always, as far as we know, still prepared within the Netherlands or Belgium.

There were no guilds in rural areas and here the original simple methods of construction sometimes remained in use up until the beginning of the 20th century.

The purlin roof, which is of southern European origin, influenced Dutch constructions in the Late Middle Ages. Elements of this variant were in general use after about 1500.

In the extreme Southeast of the Netherlands the influence may be seen of the post roof, which was particularly common to Central Europe.

An important development in the study of roof constructions in the Netherlands was the rediscovery of the system of assembly marks which featured on all the elements of medieval roof constructions. This was a decimal system of marks scored in the wood, which, after 1300, included a symbol to distinguish elements on the left side of the roof from those on the right side. This was extremely important given that the constituent parts were often made by hand miles from the construction site and then transported by boat or cart. From 1500 onwards the marks were cut in the wood with a chisel.

he use of marks cut in the wood heralded the beginning of the period in which assembly marks were used less systematically. A variety of factors led eventually to carpenters virtually abandoning the use of marks with figures indicating numbers over 10.

The assembly marks often provide an indication of the time of construction and even, in some cases, of the part of the Netherlands in which the different elements were constructed. Using the assembly marks it is possible to discover the order in which the different elements of the roof of a building were assembled. Assembly marks are thus extremely important for studying the history of a building's construction.

For centuries, roofs with free-standing coupled trusses and supported rafters remained the most common type in the Netherlands. Since the second half of the 13th century a number of variants have been developed, for example using a central support and king-posts, but ultimately a single principal form remained, in which the pairs of rafters are supported by longitudinal timbers (plate purlins) resting on trusses placed laterally. The upper face of the purlins is horizontal. The rafters are in principle not fixed to the purlins and are not supported at the apex of the roof. The uppermost beam of the truss rests on the principals.

Numerous variants of this type of truss are to be found; they are explained with the help of the illustrations. The number of parts comprising a roof truss depends, among other things, on the width of the building. There are no essential differences between churches and secular buildings. In the case of private houses the parapet is important, because space is thereby created under the collar beams in the attic for people to walk upright and for goods to be stored. For this reason trusses were usually made using curved oak principals.

An unusual variant, found mostly in the southeast of the Netherlands, is the truss with an upper beam placed between the principals. This is perhaps related to the tie-beam truss found in rural architecture.

A form which originated in the south was the purlin roof, a roof supported by longitudinal purlins, which rest on trusses and whose upper surface is parallel to the pitch of the roof. There is also a ridge purlin at the apex and rafters fixed to the purlins. The pure form of purlin roof was combined in the course of the 15th century and early 16th century with the Dutch common rafter roof with trusses.

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The aim of architects in the Gothic period was to raise churches to the greatest possible height using a minimum of building materials. If a flat ceiling is placed against the underside of the truss beams the interior space can only be as high as the walls on which the roof rests. Equally, if stone vaulting is used, the Height of the interior space cannot greatly exceed that of the walls, and extensive supporting structures are also needed. In a large area of western Europe, bordering the North Sea and the English Channel, a style of architecture developed in the 13th century which made it possible to create a high interior space with relatively low external walls. Builders incorporated the lower section of roof frame into the interior of the building by adding thin wooden panelling to the elements of the roof frame. This gave a semicircular or pointed arch barrel ceiling that was integrated into the roof frame. Using this technique the interior could be as high as or higher than the width of the building. As a result, the windows in the gables could be higher, enabling the interior space to be properly lit.

The area in which the wooden barrel-vaulted wagon ceiling was used as a specific architectural style for churches in the Netherlands extends to the west of a line running from the Frisian coast to the Zuiderzee. To the east of this line only stone vaulting was used in the Middle Ages. To the south, the eastern border of the barrel-vaulted wagon ceiling runs from roughly forty kilometers east of the city of Utrecht down to the Belgian border and thence into Brittany in France.

The Dutch barrel-vaulted wagon ceiling is not specifically a product of the marshy terrain in the Netherlands. It is also to be found in areas of firm clay and sandy soil. Indeed, the early examples are found precisely in land where firm foundations could be laid. Nor is there any connection with shipbuilding. The style seems to have developed in the northern France around the middle of the 13th century.

Many variants of the roof construction of which the barrel-vaulted wagon ceiling is an inseparable part were also to be found in the Netherlands between 1300 and 1800. In the Late Middle Ages the structure in cross section usually had the form of a pointed arch. In the southwest of the country tie-beams were not generally used, whereas elsewhere the space was spanned by tie-beams at the base of the vault. The purlin roof is distinguished by the use of a horizontal rob on each side of the vault. The thin layer of panelling was only added a few decades later, and in some cases was omitted altogether. In the western part of the Netherlands the undersides of wooden barrel-vaulted ceilings were sometimes decorated with wood carving. The custom of painting such ceilings was common in a slightly larger area.

In South Limburg and the adjoining regions of the Eifel and the northern Ardennes, houses and farm buildings are to be found in which the characteristic features are roof posts, purlins and foot plates. The distinguishing features of the post roof are the central posts and the ridge purlin which rests on them. An interesting variant of such a roof is found in Maastricht, where the houses are often built with the narrow side facing the street and extending a long way back. The roof was built by successively reducing by one the number of beams at front and back.

In the 13th century in Germany a method of supporting the simple common rafter roof was developed, consisting of one or more longitudinal beams under the collar beams, resting on posts, which were connected together in the longitudinal axis of the roof. In effect wooden walls were built which were connected together across the tie-beams at right angles to the longitudinal axis of the buildings. The trusses were then erected transversely on the top of the longitudinal ‘walls’. The construction thus alternated between the transverse and the longitudinal axes, in contrast to the method used in the Netherlands, where the structure was built only transversely. The roof ridge is not supported, which distinguishes this form of construction from the post roof. This type of roof is known in Germany as a ‘stehende Stuhl’. It was almost never used in the netherlands. The derivative form, the ‘liegende Stuhl’, was likewise used only very occasionally in the Netherlands, in the east and far south of the country.

In the 17th and 18th century, too, most buildings had a roof with Dutch trusses, ridge piece and rafters. A type of purlin roof developed from this which had no rafters and virtually watertight roofboarding. Roofs with wooden barrel-vaulted ceilings continued to be built in this period too. Because the vault came to have an elliptical cross-section, it was often necessary to add a second horizontal beam to the frame. The Burgerzaal of the Amsterdam Town Hall (now the Royal Palace), construction of which began in 1648, had a barrel-vaulted ceiling of this type, but without tie-beams. problems arose and the ceiling had to be replaced.

The groundplan of churches built for Protestant worship was more rational and focused on a central point. They were cruciform, polygonal or circular in shape. Several were built with domed roofs. Because there was no tradition of this sort of construction the quality of the roofs varied.

The southern part of Limburg shows the influence of many different cultures and principles of construction. The confused politics of the area in the period prior to 1815 no doubt contributed to the diversity of the methods of roof construction in South Limburg. Almost no two are alike, whereas elsewhere in the Netherlands the principles of roof construction were much more uniform. In church architecture the purlin roof and the Dutch truss roof vied for supremacy; in secular architecture the influence of the post roof, together with that of the common rafter roof and the purlin roof is evident. The influence of the German ‘leaning truss’ may also be detected.

In the early years of the 19th century, roof construction tended largely to follow the traditional methods from previous periods, as these had been used by generations of carpenters. The guild system was abolished at the end of the 18th century by the French invaders, with major consequences, particularly for the building trade in the Netherlands. The system of full-time training which the guilds had used for centuries was swept away. The systematic training of apprentices and the strict rules governing such apprenticeships all disappeared. The situation in which each craft existed in relative isolation as a self-

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contained entity made way for a more open system. Building work ceased to be contracted out by craft; instead the entire project was consigned to a general building contractor. The chief contractor was no longer an outstanding craftsman but primarily an organizer and entrepreneur, who naturally had a good understanding of the building trade as a whole but was not himself a master of each individual trade.

The development of roofing materials which mad it possible even in the damp Dutch climate to use only a gentle roof pitch had a major influence on the development of roof frames in the 19th century. With roof tiles and slates it was impossible to make a roof reasonably watertight with a pitch of less than 35 degrees. The use of manufactured tiles allowed a slightly gentler incline. Lead would in principle have served the purpose, but its weight and the high cost involved meant that its use was restricted to small surface areas and spans. Copper was rarely used because it was expensive in the Netherlands and because with horizontal seams on sloping roofs it was difficult to make it watertight. When sheet zinc became available in the Netherlands around 1818 it became possible to make large roof surfaces with a gentle pitch watertight at a reasonable price. his also enabled architects to satisfy the neoclassical ideal of e roof pitch of 22,5 degrees.

In earlier centuries some roofs had also been built using the principles of the hanging truss, as applied in southern Europe as early as Roman times. In the 19th century this type was widely used. It fell into disuse because the attic area could not be used. Strut frames and false-work were often also used to span large areas. Trusses were also constructed for this purpose based on the principle of timber-framing.

For spanning wide areas trusses were favored in which the loads were transmitted in a flowing line, as it were from the top downwards. The pointed arch continued to dominate church architecture. A form with an elliptical cross section was commonly used for large halls.

For all of these variants trusses were developed which made use of new techniques. Examples include the Philibert truss which consists of braces made of pieces of wood joined together with their broad side perpendicular to the surface of the roof. Because of their lack of rigidity these trusses ceased to be used after 1900.

Several domed roofs were also built in the 19th century, using a load-bearing construction of straight parts and curved surfaces, supported by braces It was rare for curved trusses to be built on the principle developed by Emy using timbers with their broad side parallel to the surface of the roof. From 1921 onwards trusses were constructed according to the principle developed by Hetzer and consisting of pieces of wood glued together, broad side parallel to the roof surface. This could be used to span an area up to 75m wide. Finally, some roofs were built using trusses made of wood and iron.

The Second World War and period of reconstruction were followed by the era of prefabricated building, nailed trusses, synthetic materials, pre-stressed concrete and space frames. Impressive timber constructions became a thing of the past. Engineers have always found it difficult to work with an unpredictable natural building material such as wood. Architects and carpenters who could handle the material with consummate skill became increasingly scarce. The building trade concentrated on new materials and methods of construction, designed with the help of calculators and computers. Nowadays wood is mainly used to cover surfaces and for partitions, almost never for major load-bearing constructions. We have therefore reached a point where we can draw up the balance of at least ten centuries of wooden load-bearing constructions in the Low Countries.

Given that it is conceivable that no new wooden roof frames of any importance will ever be built again, it is essential that we take greater care than we have in the past to preserve the interesting products of the carpenter's craft. We face the prospect of the gradual disappearance of these old wooden roofs.

For centuries the Netherlands was a country where wood was widely used in building. The preservation of this part of our cultural heritage calls for a collective effort on the part of owners and interested parties, and administrators and conservationists concerned with historic buildings and monuments to ensure that in years to come our descendants will still be able to see, study and admire the craft skills of earlier generations.