Expert: Bruce E. Johnson - 2/11/2009

Question
We are adding an extension onto our home.  Current home has a basement - so first floor has joists.  The extension will not have a basement (due to new egress code restrictions) and my builder is telling me a slab foundation is cheaper than a joist crawlspace due to labor costs.  We have about a 30" cinder-block wall that will now be filled with gravel prior to pouring the concrete.   Just curious if this is indeed cheaper or just easier for the builder.  Seems like joists (even with the labor) would be cheaper than all that gravel.  The room being added is about 36' x '28'.

Answer
Hi Sean, I have found over the years that a slab on grade is always cheaper than a foundation with floor joists. The slab on grade involves form setup concrete pour and finish, boom, done.  A stem wall foundation involves, digging the footing, pouring the footing, forming and pouring the stem wall, installing a mudsill, installing floor joists, insulation in the floor joists, Sheathing and then an underlayment.  Now you tell me which is the cheapest. Materials may cost the about the same but the joist system involves three times as much labor.   I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com