Dormer external skin - block or renderboard

[Omi]

Hello,

 

The dormers in our timber frame build are designed to sit flush with the external wall and are currently specified as using blocks for the front face elevations -- presumably just continuing up from the blockwork wall below:

 

 

However, an alternative option may be to stop the blockwork at/just above the wall level and use render board as the external skin for the dormers.
 

 

What are the pros & cons of each method? The render board approach seems simpler but would it be less robust? Would the change in materials (blocks -> render board) cause cracks to appear in the render over time?

 

@ProDave - I see here that you were not that keen on the blockwork skin in your own build. What other alternatives would you consider? The "pillars" on the sides are around 260 mm each in our design.

 

 

 

 

 

 

 

Edited August 22, 2021 by Omi

[Mr Punter]

It is only a small amount of masonry extending up and I think with blockwork there is less chance of cracking.  You will probably want to have this meshed and rendered.  What are you considering for the cheeks?

[ProDave]

19 minutes ago, Omi said:

@ProDave - I see here that you were not that keen on the blockwork skin in your own build. What other alternatives would you consider? The "pillars" on the sides are around 260 mm each in our design.

 

In our new build, I set out to achieve a "good" house and my original mantra was I didn't want a ventilated cavity and blockwork outer as it was just a big rain screen and added almost nothing to the insulation.  So I ended up with a house design clad in wood fibre insulation and render onto that, the idea being all elements of the build were adding to the insulation.

 

My view has changed in light of recent issues and I am afraid I could not recommend that system at the moment and in particular could not recommend the Baumit render that we used as the system has proven to be "less than robust" and is presently undergoing some rework and the supplier and manufacturer have not been interested in helping determine what went wrong.  Only time will tell after that if the issues have been fixed by the rework or re occur.

 

So although the blockwork is little more that a rain screen, it is a proven and robust rain screen.

 

Our previous house had dormers just like yours, the blockwork was continued up either side of the dormer, a concrete lintel and blockwork above the dormer.  The side cheeks were clad in plywood, galvanised wire mesh fixed on and the same cement render on the side cheeks as the rest of the wall, and it has proved reliable.

[nod]

I always prefer Block or brick over TF

We have three parapet type upstand a on our next I definitely wouldn’t use brick or block 

Render-board and Render or Render-board and cladding 

[Omi]

Thanks everyone. Perhaps we should just stick with the originally specified design which is blockwork + lintel over the window openings as a tried and tested method.

 

The cheeks will be renderboard + render.

[tonyshouse]

Interested in their U-value, can you share that number please 

[Omi]

4 hours ago, tonyshouse said:

Interested in their U-value, can you share that number please 

 

Don't have it I'm afraid. Our SAP calculation has assumed a value of of 0.35 for the dormer walls on the basis of 140mm insulation between studs and 12.5mm plasterboard. I would read this as meaning negligible insulation contribution from the external skin.