How to create half a circle?

[info25]

I want to create a circle cut in half like this without errors: http://i.imgur.com/obiwomG.jpg

[waffl3head]

A general advise to avoid BSP holes is to stick the vertices of a brush to the grid.
Try going to edit mode (shift+5), selecting vertices in a 2D viewport and rightclicking on them. They should shift to the nearest gridpoint.

[NATO_chrisjm]

Best way to create a semi circle in BSP is to use the Brush Clip too to slice it in half. Sometimes it'll create a hole like you showed; if this happens you can use the create tool too add the face back in. Select each vertex of the face you want to create in a clockwise pattern then press create. You can see a few more tricks in my BSP tutorial

[~SPECTRE~]

Try the clip in open space, with out any other geometry around.

I'm curious if that would avoid a BSP hole. :/

[Dementiurge]

One could take a box brush and vertex edit it into a concave arch, using a cylinder as a guide. That way you can use an additive brush instead of a subtractive one.

A general advise to avoid BSP holes is to stick the vertices of a brush to the grid.
Try going to edit mode (shift+5), selecting vertices in a 2D viewport and rightclicking on them. They should shift to the nearest gridpoint.

There is also a button to 'Align Vertices to Grid', in the brush's details. Sadly it can't be hotkeyed.