hi
actually i was testing some lighting and i get this behavior:
i simply open a new third person blueprint project and used the ground mesh and divide it by three in order to make a ceiling for the example map. every thing was okay until i got my lightmass built and this is the result:
lightmass settings are shown in the right and the left of images.
do why is this happening? is it related for lightmass grid or i have lost some principle or settings?
sorry for my wierd english. 
thx
Yes it does increase build time!! …if you don’t need hyper resolution lightmaps (=archviz) then just use 1 mesh / wall and don’t use modular pieces!
Hi!
Meshes are calculated separately during light build! What you see here is shading differences! You can tune your lightmass settings to get rid of those: set 0.15 for scale and 0.6 for smoothess and they will be gone! 
thx it worked, but this will increase lightmass calculating time dramatically, is there any other way to keep the calculate time low and avoid the effect?