The move was revealed in a trading update ahead of its interim results in September.
The company said profits since then were “in-line with management’s expectations.”
But it added: “Given current economic uncertainty and sufficient brick inventory levels, the Board has reviewed the current production plan and has decided to maximise the utilisation of our most efficient brick plants and effectively manage our cost base.”
Forterra said the mothballing plans were a “prudent action that will allow us to continue to meet customers’ needs while effectively managing our costs and cash position until we are able to more effectively forecast demand.”
Both the plants were mothballed between 2010 and 2014 and Forterra said it can “bring production back online in a short period of time.”
The company added: “Forterra has sufficient brick inventory and production capacity to service customer demand should housing output continue to grow as previously expected.”
Steve Kemp, GMB National Officer, said “Imported bricks are still coming into the UK at an unprecedented and alarming rate.
“In 2014 brick imports accounted for 25% of sales in the UK representing £80 million per year to the UK economy and these imports have meant that future investment is becoming difficult to forge and is increasingly unsteady.
“We call on the UK Government to act in defence of the UK brick industry to give both stability and continuity to this important industry.”
But one brick supply specialist said: “House builders started panic ordering and stock piling back in 2014 so of course importers filled some gaps.
“It’s a bit rich to turn round and start blaming them now for decisions like this.”