The four-year framework covers NHS building projects in Wales with construction values of above £10m.
But procurement chiefs have steered clear of setting an expected workload under the framework because of spending cuts.
One source said: “It’s a very difficult environment with individual projects having to go forward on a case by case basis for Welsh Government funding.
“As it stands nobody is prepared to say what this framework will be worth over the next four years, because nobody knows.”
Both Bam and Interserve won places along with Kier on the £4m-£10m regional framework last year.
Interserve divisional director Health, Andrew Jowett said: “This appointment is great news for all our team, enabling us to build upon our strong track record in healthcare infrastructure across the UK.
“It also represents a significant opportunity for Interserve to support the NHS in its challenge to deliver high quality, equitable and sustainable health services for Wales.”
The national and regional frameworks cover the whole of NHS Wales including Abertawe Bro Morgannwg University Health Board, Aneurin Bevan Health Board, Betsi Cadwaladr University Health Board, Cardiff and Vale University Health Board, Cwm Taf Health Board, Hywel Dda Health Board, Powys Teaching Health Board and Velindre NHS Trust.
The Welsh Ambulance Services NHS Trust and Public Health Wales, which are organisations operating on a national basis, will also be able to draw down services for their projects.