Garden room extension
The custodians of this grade II listed building were keen to create a unique contemporary extension which embraces traditional techniques and materials. The oak frame will be ‘floating’ within the building envelope, this means there will be a visible gap around all of the inner timbers. With a design that does not comply with the traditional rule-set this posed a few challenges. Design by Verve Architects of Cambridge.
The first of which was how to get hold of timbers for the rafters which have a large curve and a straight section within the same piece! It was critical that the grain followed the profile as best as possible to maintain strength and reduce splitting effects once the oak begins to dry. A template was left with our timber supplier and over a period of nearly a year, six suitable timbers were found. These come to us as slabs, from which the profile is carefully marked and cut. This picture shows one of the slabs (not the best one!) partially cut.
From a framing perspective, the lay-up process for an open truss design like this was challenging. We had to resort to a full ‘plumb scribe’ technique, marking out a 1:1 design on the workshop floor and used a plumb bob to position the timbers before scribing the joints.
We opted for a large 2″ tenon at the rafter foot, into a diminished housed mortice at the top of the post. The picture here is of the gable closest to the house.
The other main challenge with this design is the lack of tying timbers, critically at the middle cross-frame. This has been overcome by the use of engineered glue-lam beams within the outer building envelope and rather heavy duty steel shoes, which have been flitched into the main oak posts and resin bolted to the footings. The following picture from the mature gardens.
Please note that all the completed pictures show softwood spanning & bracing timbers. These will be removed, but are temporarily needed to support the frame.
The gable end will be direct glazed using matched curved cover boards (currently drying in our solar timber kiln!)
One of the most challenging features of this design, which you may have noticed is the profiled wall plates. The inner face has two facets, the lower one matches the profile of the back on the curved rafter. Another quirky, but interesting feature.
We also decided to use a face-halved scarf joint over the post to help resist outward thrust, this can be just made out in the above picture.
From the inside, the gable end facing the mature gardens.
Further back, note the fixings will be hidden against the adjacent wall.
And finally, our client had a camera spying on our every move (apologies for the quality, we are yet to receive better resolution video):