The second in our series on getting the best out of your buildings.
Windows should keep rain out.

It would seem simple enough to design and install a rectangular fame into a rectangular opening in a wall, but in buildings, wherever there are junctions between different structural elements, there are potential problem areas.

The greater the exposure of the building, the more critical the issue of water penetration becomes. In other words, detailing and execution will be more demanding for a house on a Scottish hillside than a flat in central Exeter. The solutions for a low-exposure site will not be adequate for a high-exposure site.

The problem
Putting windows into masonry walls involves forming junctions between different materials such as timber, steel, insulation, plastics, bricks and blocks as well as the cavity.

The position, sealing and protection of all the components concerned needs to be just right. Where these issues are not properly resolved, water penetration and the premature deterioration of materials or components are likely.

It's best to rely on tried and tested solutions. Good design should keep out the water without having to rely on sealants applied on the building site.

What to look for in designs

  • Window frames should be set back at least 50 mm from the external face of the wall. Ideally, they should be set closer to the internal face of the wall or, even better, the window frame should be set behind brickwork. This increases protection to the window and makes it easier to finish the junction between window and wall.

  • Sills should include a drip groove to prevent water backtracking onto the wall. The sill should project 40-50 mm beyond the face of the wall to help shed water. Avoid wide and thin wooden sills; these have a tendency to distort, leaving gaps between sill and window, and back-falls that expose the window frame edges to water penetration.

  • A separate secondary sill, called a sub-sill, should always be installed where the exposure is severe. A sub-sill is better than a very wide primary sill which is an integral part of the window frame. Between the two sills, there should be a water bar.

    What to look out for on site

  • Timber window frames should be protected from moisture. A plastic strip known as a damp-proof course and sealant to maintain water exclusion should be included.

  • If windows are built-in, the damp-proof course should be attached to the frame and folded back ready for the wall.

  • There must be a gap of between 5 and 15 mm between the window frame and the masonry, to keep the window separate from the wall where windows are installed into preformed openings.

  • The gap between the window and wall provides a convenient space to apply a sealant joint or foam joint. The damp-proof course should be extend into the cavity of the wall, thus keeping the inner and outer parts of the cavity wall separate.

  • The performance of sealants relies heavily on the way they are applied. Triangular seals should have a minimum face width of 10 mm and the exposed surface should ideally have a convex finish. A rectangular seal should have a face width of at least 6 mm. In each case the depth should be no less than the width.

    All these tips are pointers to good practice, but they are not necessarily the only correct solution. The detailing at the top of the window is also important, for example, and there are many different types of window.

    • Source

    Housing Today

    Postscript

    For more information please contact Jeff Maxted, head of Risk Management Services, on 020 7204 2466 or by email at jeff.maxted@buildinglifeplans.com