Contrary to popular belief, fitting windows in oak frame buildings need not be difficult. Because oak frames inevitably shrink over time, there is a widely held belief that this will eventually put stress on glazing. If glazed elements were mounted directly into the oak frame, this would certainly be the case. Movement of the frame could also cause gaps, allowing air and water to penetrate the building envelope. This will compromise the thermal integrity of the building by allowing heat to escape, driving up heating costs. Any water entering could lead to damp issues, encouraging the growth of mould and potentially damaging the frame itself.
At Hardwoods Group we recommend the use of a weatherproof capping system for all glazing systems to counter these risks. This consists of two layers of air dried oak boards which are secured to the external face of the structural oak frame. The glass is then fitted inside this, surrounded by flexible weatherproof tape to prevent ingress of water. This allows movement without any stress being placed on the glass or gaps appearing around it, keeping the structure weatherproof.
Installing Weatherproof Capping
The capping system is supplied in three parts – firstly the capping boards that are planed all round to the correct width and depth. These will need to be cut to the correct length on site. As the first stage boards will be concealed, they may be supplied in two or three pieces for larger lengths. The capping boards are numbered to correspond with the supplied plan showing their positions. Also supplied are 80mm flashing strips that form the tray beneath the glass and pre machined drip bead for the drip system.
1. Assemble the Oak Frame
Fit the oak frame down to foundations and ensure it is pegged together securely. The frame must be set back 40mm from the face of the masonry beneath. As the two layers of capping boards have a combined thickness of 60mm, this ensures a 20mm drip after installation.
2. Fit First Stage Capping Boards
The first stage boards are 34mm thick and form the rebate that the glass panels will sit in. Following the supplied plan, make sure the correctly numbered boards are used for each glazed section of the build. Working from the ground up, screw the capping boards to the frame every 600mm with 80mm screws. All horizontal first stage boards should be fitted at this stage. As these will be concealed beneath the second stage boards, the quality of screwing is not important provided they are securely attached. Where vertical members meet a horizontal with drip detail, allow a 10mm gap for the aluminium flashing. The sides and top of the rebate will be set in 20mm from the frame behind to allow a 15mm overlap for the glass units and a 5mm allowance for fitting. The bottom of the rebate is set down 35mm from the frame to give space for the drip bead and flashing system.
3. Fit Second Stage Horizontal Capping and Flashing
With installation of the first stage capping boards complete, the horizontal second stage capping boards are fitted. The 26mm thick second stage boards are fitted flush with the first stage boards and attached with 80mm facing screws. These should be set into the face of the boards with a 12mm hole, passing through the first boards and into the frame beneath. The aluminium flashing strip should be cut to the required length, fitted in place and hammered into shape. This should give a 10mm upstand against the oak frame and a 10mm overhang on the second stage board. This allows water to flow off the flashing and prevents backflow seeping behind the capping boards.
4. Apply Tape and Install Glass
Apply 7.5mm glazing tape all round the interior of the rebate, taking care to keep it neatly flush with the edge. This will cushion the glass against the oak frame and form a weatherproof and airtight seal. The glass units should be installed immediately after this, as if the tape is exposed to rain it will peel away from the frame. Peel the white backing away from the tape to expose the sticky surface and sit 22mm thick oak or softwood packers on the aluminium. Lift the glass units into the opening, ensuring a 5mm gap between the edge of the glass and the vertical capping boards. Remove any protective film from the glass ensuring the correct side faces out and use plastic packers to secure it in place. Temporary timber stays can be screwed into the upright boards to hold the glass units in place and prevent them falling out.
5. Fit Drip Bead and Remaining Capping boards
Measure the length of drip bead required and cut to the correct length. if more than one piece is needed, neatly mitre the pieces together where they meet. Place the drip bead into position and mark where they line up with vertical frame members. Use a 12mm plug drill bit to predrill the drip bead every 400mm, making sure a screw hole lines up with each vertical member. These will fix the drip bead to the frame itself, ensuring a strong connection. Attach tape to the inside of the drip bead where it faces the glass, making sure to trim it away where the bead meets a vertical member. Place 4mm packers on the aluminium to support the bead and allow water to escape. Screw the drip bead into place, ensuring at least 3/4mm clearance between screws and the base of the glass unit. Where drip bead pieces meet at corners of the building they should be mitred together, and oak coloured silicone applied to the join. Second stage vertical capping boards should now be screwed into place with tape also applied to the inside edges. The capping system is now fully installed, and any holes should be plugged with oak plugs glued into place. When the glue has dried cut the plugs flush with the boards and sand them smooth.
Hardwoods Group are experts in the design and manufacture of oak framed buildings. In addition to a wide range of pre designed oak frame kits, we also offer a bespoke design service. We also make and supply complete capping systems designed for easy installation for all types of oak frames. Contact us to discuss your requirements, we will be happy to help.
Need to Know Basics for Oak Framed Buildings
Is A Timber Frame Home Energy Efficient?
Timber Frame Construction Methods