When supplying internal doors to a newly constructed building, it is advisable to instruct your customer in good practice that will ensure the good performance of the doors. A major issue with new build is the amount of moisture that can be present due to all the wet elements such as floor screed and plaster. It would be best to hang the doors late in the build programme, but often they are taken by the customer and hung in the drying building. Best practice is to supply joinery where the timber has a moisture content close to the expected in service conditions.
BS EN 942: 2007 gives recommended moisture contents in Annex B:
External: 19%
Internal – unheated: 16%
Internal – heated from 120C to 210C: 13%
Internal – heated to above 210C: 10%
These conditions are for the finished building, but the conditions in a building under construction can be quite different. Initially, if the doors have been hung very early, the building will be unheated and damp; conditions will be similar to external conditions but without the issue of direct wetting from rain. If doors are hung and propped open against a wall the timber, which has been dried to suit dry internal conditions, will be in almost direct contact with the dampness of the drying plaster and will absorb moisture. The changes in moisture content can lead to adverse movement. Heating installation is often commissioned by running at high temperature or used to force dry the building.
The combination of a lot of moisture and high temperatures will cause the atmosphere to take up a large amount of water. Warm air can hold more moisture than cooler air and, if the heating is switched off over night when all the external doors and windows are closed, the damp, trapped air cools and looses some of the moisture, which can then condense. The doors rapidly go through cycles of hot and cold, dry and damp. These rapid cycles will have an adverse effect on the doors. Worse still, if the heating isn't working and stand-alone gas heaters are brought in these create water as part of the combustion process generating more damp and warm air. It is always better to dry buildings using good ventilation. The affects of the internal conditions will be exacerbated if the doors are not fully finished.
The finishing system, for example painting or staining, will fully seal the timber or at least inhibit the movement of moisture reducing the affects of rapid changes in moisture content. Fully finishing applies both to the faces of the door and also to the four edges and any cut-outs that have been prepared for iron mongery. The top and bottom edges of the door are particularly vulnerable as often end-grain is exposed and this absorbs more moisture than the side faces. If the timber in a door does absorb moisture then this can lead to movement. Timber will generally change dimension by about 0.5% radially (across the annual growth rings) and 1% tangentially (along the annual growth rings) for each 3% change in moisture content. Distortion can occur if the grain is not regular. Some movement, however, is acceptable. For tolerances for bow, cup and twist and changes in height, width and squareness we have tended to refer to BS 4787:1980 however, parts of this Standard will be superseded by European Classifications and it is likely that the following classes will apply: BS EN 1530:2000 Door leaves General and local flatness tolerance classes. Class 3, Bow: 4mm, Cup: 2mm, Twist: 4mm. BS EN 952:1999 Door leaves General and local flatness measurement method; and tolerance classes Class 2, Height and width: 1.5mm, Thickness: 1.0mm, Out of square*: 1.5mm.
* measured by placing one leg of a 500mm carpenters square flat against one edge of a door and measuring the gap at the end of the other leg.
Further advice can be found in the BWF Guide Number 11: Care of timber doors on site