We are having an extension which is an oak frame with some full glazing panels and some cavity brick walls.  The oak frame company has recommended not using mortar to bed the dpm in.  The sole plates just sit directly on the dpm.  We are doing some of the block work ourselves and feel a bit confused with the idea of not bedding the dpm in.  Has anyone come across this before or got any advice?  Many thanks.

 I also put it under the sole plates when building stud walls on the screed, again no bed of course. There are some exceptions though listed by NHBC

 

• be fully bedded on mortar, and masonry bedded on mortar on the DPC where required by the design, or where the building is over three storeys in height

 

 

https://nhbc-standards.co.uk/6-superstructure-excluding-roofs/6-1-external-masonry-walls/6-1-17-dpcs-and-cavity-trays/

 

 

I think if the surface is nice and flat you don't need mortar but if it is less even, like brickwork frog up, you do.

You may find the oak frame company wish to set the frame down on packers, on top of the DPM. It is unlikely the oak is dead straight and flat and in time will distort anyway. I would suggest taking their advice and pointing up between sole plate and DPM as late as possible in the build. For air-tightness, you could consider using and expanding foam tape, such as EXP6 to keep a tight seal until the frame has fully dried.