Fitting Windows In Oak Framed Buildings

Fitting windows in Oak Framed Buildings requires specific planning and capping systems.


Because Oak Frames inevitably shrink over time, there is the 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 causes gaps, which would allow air and water to penetrate the building. This would increase heating costs as the thermal integrity of the building would be compromised.


Any water entering could lead to damp issues, encouraging the growth of mould and potentially damaging the frame itself.


When preparing a frame for glazing, we manufacture our Oak Capping System right into the frame.


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.

Installing Weatherproof Capping

The Oak Capping System for fitting windows in Oak Framed Buildings is supplied in three parts.


Firstly the capping boards 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.


To display their positions, the capping boards are numbered to correspond with the supplied plan.


Along with the boards, 80mm flashing trips are supplied which will form the tray beneath the glass. As well as 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.

Fitting Frame to the Foundation

2. Fit First Stage Capping Boards

The first stage boards are 34mm thick and form the rebate for the glass panels.


Make sure the correctly numbered boards are used for each glazed section of the build according to the supplied plan.


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 isn’t important. Despite this they should of course be 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. This will benefit the glass units and provide 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 complete, the horizontal second stage capping boards are fitted.


The 26mm thick second stage boards are fitted flush with the first stage boards. They’re also 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 into place and hammered into shape. Consequently this should give a 10mm up-stand against the Oak Frame and a 10mm overhang on the second stage board. This allows water to flow off the flashing and prevents back flow.


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 to form a weatherproof and airtight seal.


Then the glass units should be installed immediately after this. If not, tape exposed to the rain 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.


Then 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 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 from falling.

5. Fit Drip Bead and Remaining Capping boards

Next you’ll need to 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. Then use a 12mm plug drill bit to pre-drill the drip bead every 400mm. Finally ensure your screw hole lines up with each vertical member.


These will fix the drip bead to the frame itself, guaranteeing a strong connection.


Next, attach tape to the inside of the drip bead where it faces the glass. Then trim away where the bead meets a vertical member.


Place 4mm packers on the aluminium to support the bead. And screw the drip bead into place, ensuring at least 3/4mm clearance between screws and the base of the glass unit.


Where the drip bead pieces meet at corners of the building they should be mitred together with 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.


Finally the capping system is fully installed.


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.


Here at Hardwoods Group, our craftsmen have mastered the Oak Capping System for fitting windows in Oak Framed Buildings. Book a meeting with our Design Team to go over your specific needs for your Oak Frame.


Or send us an email with any questions you may have!