DomPDF (version 0.51, also version 0.6 beta 2) doesn’t display table rows properly when colspan is being used. There is no solution or bug fix for it from DOMPDF right now, cellmap­.cls.php which is responsible for table generation is buggy and calculating or rendering the table rows wrong.

I suggest you to use mPDF instead. mPDF is also a PHP class which generates PDF files from HTML files. It is based on FPDF and HTML2FPDF, with a number of enhancements. mPDF is more flexible compared to DomPDF when it comes to sophisticated table structures and layouts.