Wall lath
The wall lath rests on the masonry and serves as a support for the ceiling beams or the rafters of the roof structure . In the latter case it is also called an ankle purlin .
The use of a wall lath makes it easier to fasten the ceiling joists to secure the position and to compensate for the height of uneven masonry.
The connection of ceiling beams and wall battens was traditionally done by combing . Today, bolts or long wooden construction screws are predominantly used. In particular, if the masonry was raised between the beams, it is also possible to dispense with a connection between the beams and the battens.
Since the wall batten is usually in full contact, it only needs to be strong enough to allow bolts, nails or screws to be held securely. Their width should be measured so that the wood fibers of the wood on top are not squashed by excessive compressive stress . A width of 10 cm is usually sufficient.
Danger from moisture
In the past, the roof cladding was often not made as tight as it is today, so that under certain circumstances certain amounts of rainwater could run into the outer wall. In the case of roofs with a snow guard and, due to the reduced pitch of the rafter roofs, in the area of the pegs, the area near the eaves was particularly at risk. The pack of snow that collected here was able to hold back the condensation that ran off in the spring and let it drain through the roof.
Small amounts of penetrating water can often spread through the masonry without being noticed. However, the increased moisture in the wall enables the growth of wood-decomposing fungi in wooden components that come into contact with the masonry, especially in beam heads and wall battens.
According to DIN 68800-3, hazard class 2 must be assumed in this installation situation, at least in old buildings , which suggests treatment with a wood preservative if it cannot be ensured that the masonry is kept dry over the long term. In addition to pressure impregnation, the local application of relatively harmless boron salts is suitable for this .
Due to the experience with unplanned increased masonry moisture, it is occasionally advised to use more resistant oak wood or to do without the masonry lath and to place the beam heads directly on the masonry. Since the beam heads have less contact surface with the masonry, they are a little less prone to rot. It is also easier to isolate the beam heads from the masonry with a capillary-breaking layer and to ensure that the wood is flushed with air in order to allow any increased masonry moisture to evaporate and to keep the beam heads dry.
For these reasons, it makes sense to also provide a barrier layer under the wall batten. If a foil or roofing felt is used, care should be taken to ensure that it is never made wider than the wall batten or even raised to the side of it. Otherwise, penetrating rainwater and condensation can run off poorly and the wall batten will be soaked through. The wall lath should not have any side contact with the surrounding masonry or with building materials, which can lead moisture from the outside of the outer wall to the wood by capillary action.
In parts 2 and 4 of DIN 68800, an air gap of at least 2 cm as well as an insulation layer between the air layer and the outer masonry is provided. The insulation layer, which is at least 2 cm thick, is intended to prevent the formation of condensation when warm and humid room air from the room below reaches the ceiling level and rises through the air gap.
The risk that masonry moisture from the inside of the house will get to the wall batten is much lower than that as a result of driving rain from outside or in the event of water damage from above.
The free airflow around the beam heads for drying off any moisture that may have penetrated, as provided for in DIN 68800-4, can hardly be implemented across floors in most buildings, as a closed and airtight lower ceiling is required for various reasons:
- the sound insulation requirements according to DIN 4109 cannot be met,
- the required fire resistance class according to DIN 4102 (mostly F 30-B) cannot be achieved,
- EnEV and DIN 4108 require an airtight building outer skin for reasons of thermal insulation ,
- the air rising through a leaky ceiling structure can cause an often undesirable draft,
- If the building does not have a closed insulation layer on the outside of the exterior walls, an insulation layer inserted at the height of the ceiling structure cannot completely exclude the formation of condensation in cold outside temperatures.
As an alternative, it is a good idea to surround the batten with insulating material. Waterproof insulation made of foam glass , neoprene or rigid foam should be used on the underside , although the latter could have negative effects on sound insulation.
Vapor diffusion-open insulation material should be used on the outside and top. If moisture penetration of the outer wall by driving rain can be ruled out (e.g. under the roof overhang, on the (south and) east side facing away from the weather, with a double-shell, insulated or clad facade), natural insulation material made of wood fibers , cellulose , hemp or similar can be used which, due to their capillary conductivity, also ensure that liquid water is transported away quickly in the event of an unplanned entry of moisture.
Layers of air in the wall level should either be avoided, as they hinder the capillary removal of the condensation that occurs, or are in contact with a larger volume of air (such as that of the space between the ceiling) so that any condensate can dry off.
Individual evidence
- ↑ a b Lexicon of technical terms from the construction sector, entry Mauerlatte ( Memento of the original from May 18, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Engineering office LKG
- ↑ a b c Dr. André Peylo: The beam head - how do you do it correctly? , "Protect & Receive" magazine, September 2007
