Expert: Bruce E. Johnson - 4/23/2011

Question
QUESTION: Greetings Bruce, I am adding a new living room to an existing pier and beam cabin.  The room will extend out 12' and span the length of the house which is 38'.  The existing house has spot piers around the perimeter at 5' intervals.  Can I tie into the existing floor headers without installing additional piers or will the weight of the addition cause the existing wall to sag?  It already requires releveleing every 5-8 years.  Also what is the most common combination of spacing and thickness for my joists, beams and piers.  I am considering augering 3' deep holes and filling them with concrete for piers. However, the existing house is only built on 18"X18" concrete pads.  Any advice or recommendations you can give would be greatly appreciated.

ANSWER: HI Randy, if you have to relevel every so often then it wouldn't hurt to add some additional supports along the rim beam along the outside wall of your existing cabin where you plan to attach your addition.  You don't mention what exactly your existing beams are.  Generally with five foot spacing they are probably 4x6 or 4x8.  Augering holes for footings isn't the best way to support a floor system in my opinion unless the holes are part of a monolithic concrete floor and the holes are used to anchor a house on a hillside.  The wider your footing pads the better since the larger pads distribute the weight over more of an area.  Think of it this way, if your ground is capable of supporting 1500 pounds a square foot and your column pad is 2 foot by 2 foot by a foot thick you have 4 square feet of area X 1500 giving you a bearing capability of 6000 pounds per pad. A figure of 45-60 pounds per square foot is used to determine how much weight your new addition will weigh depending on height, type of roof system, etc.  As far as floor joists go..If you are looking for joists to span the entire twelve feet from existing house to outside of new addition then you will need minimum 2x10 joists at 24" spacing.  If you are doing a post and beam floor system then 4x6 beams set at 32" spacing with tongue and groove 1 1/4" plywood decking makes a good floor system.  The 4x6's would need an intermediate support in the 12' span.  Rule of thumb for beams is one foot of span per inch of thickness on a 4" beam..in other words a 4x6 will span 6', 4x8 will span 8' etc.  Same with floor joists one foot of span per inch thickness of 2x joist..i.e. 2x12 for a 12' span  however by spacing your joists at 24" you can get a little more span, that's why I gave you a minimum joist size of 2x10 for your 12' span..For cabins or houses with limited accessability I like to use premade cardboard footing forms.  They are sized properly and you just fill them with concrete and level them out for instant concrete pads.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: Thanks for your reply Bruce. I appreciate your input. I have a follow up question. Would I need to set the 2x2 pads below the surface.(the house sits on tightly packed sandy soil) or can I just place them on grade. Also the existing floor system is triple 2x6's beams with 2x6 joists set on 24" centers.

Answer
Hi again Randy, yes you can set the pads on grade as long as any grass and other degradable materials are removed.  A tightly packed sandy soil is great for load bearing so I am not sure why you have had settling issues in the past.  It sounds like your floor system is properly sized if the beams are less than 8' apart.  If your addition is going to have 2x6 joists you will need an intermediate beam to cut the span down to 6'. I hope this info helps feel free to write again regarding this or other matters..sincerely Bruce E Johnson ..bejohnsonconsulting.com