iTextSharp Header is merging with document content

I need is header table on all pages and page content/table which will be different every page. I am using PdfPageEventHelper for header. I have below code in OnOpenDocument

 header.WriteSelectedRows(0, -1, document.Left+20, writer.PageSize.GetTop(document.TopMargin + 10), writer.DirectContent); 

In Main I have

 Rectangle r = new Rectangle(PageSize.A4); Document doc = new Document(r); doc.SetMargins(15f, 15f, 35f, 35f); PdfWriter writer = PdfWriter.GetInstance(doc, new FileStream(file_name, FileMode.Create)); writer.PageEvent = new PDFHeaderAndFooter(this._configuration, companyViewModel, doctorDetails); doc.Open();

and after this my page table is adding as below

 document.add(table). 

But this is overlapping with my header table. What could be the reason?