It also fails badly if it is rendered the wrong way. Blueboard is a sheet product, sheets move independently, and standard cement render laid over the top will crack along every single joint. The system matters more here than on any other substrate.
The system, and the step people skip
A correct blueboard render is a base coat of acrylic modified render, with alkali resistant fibreglass mesh embedded into it while it is still wet, across the entire wall and with extra strips over every sheet joint and every window and door corner. Then a second coat over the mesh, then the texture and colour.
The mesh is the whole point. It ties adjacent sheets together so they behave as one surface instead of individual panels. Skip it, or run it only over the joints instead of the full wall, and the wall will crack in a grid pattern that traces every sheet edge. It usually shows up in the second summer.
We mesh the full wall. It is not the cheapest way to do it and it is the only way we do it.
Why builders specify blueboard
On a first floor addition the reason is almost always weight. A brick skin adds serious load to footings that were designed for a single storey house, and upgrading those footings is expensive. Blueboard on a timber or steel frame weighs a fraction of that.
On new builds it is speed and thermal performance. A framed and sheeted wall goes up in a day where brickwork takes a week, and the cavity can be insulated to a standard a brick veneer wall struggles to reach.
Getting the sheeting right before we arrive
Render can only be as flat as what is behind it. If the sheets have been fixed to a frame that is out of plane, or the screws are proud, or the joints are not tight, the render will follow all of it.
We check the sheeting before we start and we will tell you if something needs fixing. It is a much shorter conversation before there is render on the wall than after.
- Sheets fixed at the correct centres with the specified fasteners
- Screw heads set just below the surface, not proud and not broken through
- Joints tight and consistent, sheets landing on framing
- Control joints at the spacings the sheet manufacturer requires
- Flashings and weatherproofing complete before render goes on